draft-ietf-httpbis-semantics-14.txt   draft-ietf-httpbis-semantics-15.txt 
HTTP Working Group R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235, M. Nottingham, Ed. Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235, M. Nottingham, Ed.
7538, 7615, 7694 (if approved) Fastly 7538, 7615, 7694 (if approved) Fastly
Updates: 3864 (if approved) J. Reschke, Ed. Updates: 3864 (if approved) J. Reschke, Ed.
Intended status: Standards Track greenbytes Intended status: Standards Track greenbytes
Expires: July 17, 2021 January 13, 2021 Expires: 1 October 2021 30 March 2021
HTTP Semantics HTTP Semantics
draft-ietf-httpbis-semantics-14 draft-ietf-httpbis-semantics-15
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document describes the overall architecture of HTTP, systems. This document describes the overall architecture of HTTP,
establishes common terminology, and defines aspects of the protocol establishes common terminology, and defines aspects of the protocol
that are shared by all versions. In this definition are core that are shared by all versions. In this definition are core
protocol elements, extensibility mechanisms, and the "http" and protocol elements, extensibility mechanisms, and the "http" and
"https" Uniform Resource Identifier (URI) schemes. "https" Uniform Resource Identifier (URI) schemes.
skipping to change at page 1, line 39 skipping to change at page 1, line 39
This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<https://lists.w3.org/Archives/Public/ietf-http-wg/>. <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <https://httpwg.org/>; Working Group information can be found at <https://httpwg.org/>;
source code and issues list for this draft can be found at source code and issues list for this draft can be found at
<https://github.com/httpwg/http-core>. <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix C.15. The changes in this draft are summarized in Appendix C.16.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2. History and Evolution . . . . . . . . . . . . . . . . . . 10 1.2. History and Evolution . . . . . . . . . . . . . . . . . . 9
1.3. Core Semantics . . . . . . . . . . . . . . . . . . . . . 10 1.3. Core Semantics . . . . . . . . . . . . . . . . . . . . . 10
1.4. Specifications Obsoleted by this Document . . . . . . . . 11 1.4. Specifications Obsoleted by this Document . . . . . . . . 11
2. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 12 2. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 12 2.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 12
2.2. Requirements Notation . . . . . . . . . . . . . . . . . . 12 2.2. Requirements Notation . . . . . . . . . . . . . . . . . . 12
2.3. Length Requirements . . . . . . . . . . . . . . . . . . . 13 2.3. Length Requirements . . . . . . . . . . . . . . . . . . . 13
2.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 14 2.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 14
2.5. Protocol Version . . . . . . . . . . . . . . . . . . . . 14 2.5. Protocol Version . . . . . . . . . . . . . . . . . . . . 15
3. Terminology and Core Concepts . . . . . . . . . . . . . . . . 15 3. Terminology and Core Concepts . . . . . . . . . . . . . . . . 15
3.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Representations . . . . . . . . . . . . . . . . . . . . . 16 3.2. Representations . . . . . . . . . . . . . . . . . . . . . 16
3.3. Connections, Clients and Servers . . . . . . . . . . . . 16 3.3. Connections, Clients and Servers . . . . . . . . . . . . 17
3.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . 17 3.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . 17
3.5. User Agents . . . . . . . . . . . . . . . . . . . . . . . 17 3.5. User Agents . . . . . . . . . . . . . . . . . . . . . . . 18
3.6. Origin Server . . . . . . . . . . . . . . . . . . . . . . 18 3.6. Origin Server . . . . . . . . . . . . . . . . . . . . . . 19
3.7. Intermediaries . . . . . . . . . . . . . . . . . . . . . 18 3.7. Intermediaries . . . . . . . . . . . . . . . . . . . . . 19
3.8. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.8. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.9. Example Message Exchange . . . . . . . . . . . . . . . . 21 3.9. Example Message Exchange . . . . . . . . . . . . . . . . 22
4. Identifiers in HTTP . . . . . . . . . . . . . . . . . . . . . 22 4. Identifiers in HTTP . . . . . . . . . . . . . . . . . . . . . 22
4.1. URI References . . . . . . . . . . . . . . . . . . . . . 22 4.1. URI References . . . . . . . . . . . . . . . . . . . . . 22
4.2. HTTP-Related URI Schemes . . . . . . . . . . . . . . . . 23 4.2. HTTP-Related URI Schemes . . . . . . . . . . . . . . . . 23
4.2.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 23 4.2.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 24
4.2.2. https URI Scheme . . . . . . . . . . . . . . . . . . 24 4.2.2. https URI Scheme . . . . . . . . . . . . . . . . . . 25
4.2.3. http(s) Normalization and Comparison . . . . . . . . 25 4.2.3. http(s) Normalization and Comparison . . . . . . . . 25
4.2.4. Deprecation of userinfo in http(s) URIs . . . . . . . 25 4.2.4. Deprecation of userinfo in http(s) URIs . . . . . . . 26
4.2.5. http(s) References with Fragment Identifiers . . . . 26 4.2.5. http(s) References with Fragment Identifiers . . . . 27
4.3. Authoritative Access . . . . . . . . . . . . . . . . . . 26 4.3. Authoritative Access . . . . . . . . . . . . . . . . . . 27
4.3.1. URI Origin . . . . . . . . . . . . . . . . . . . . . 26 4.3.1. URI Origin . . . . . . . . . . . . . . . . . . . . . 27
4.3.2. http origins . . . . . . . . . . . . . . . . . . . . 27 4.3.2. http origins . . . . . . . . . . . . . . . . . . . . 28
4.3.3. https origins . . . . . . . . . . . . . . . . . . . . 28 4.3.3. https origins . . . . . . . . . . . . . . . . . . . . 29
4.3.4. https certificate verification . . . . . . . . . . . 29 4.3.4. https certificate verification . . . . . . . . . . . 30
5. Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.3.5. IP-ID reference identity . . . . . . . . . . . . . . 31
5.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 30 5. Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.2. Field Lines and Combined Field Value . . . . . . . . . . 31 5.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 32
5.3. Field Order . . . . . . . . . . . . . . . . . . . . . . . 31 5.2. Field Lines and Combined Field Value . . . . . . . . . . 32
5.4. Field Limits . . . . . . . . . . . . . . . . . . . . . . 32 5.3. Field Order . . . . . . . . . . . . . . . . . . . . . . . 33
5.5. Field Values . . . . . . . . . . . . . . . . . . . . . . 33 5.4. Field Limits . . . . . . . . . . . . . . . . . . . . . . 34
5.6. Common Rules for Defining Field Values . . . . . . . . . 35 5.5. Field Values . . . . . . . . . . . . . . . . . . . . . . 34
5.6.1. Lists (#rule ABNF Extension) . . . . . . . . . . . . 35 5.6. Common Rules for Defining Field Values . . . . . . . . . 36
5.6.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 36 5.6.1. Lists (#rule ABNF Extension) . . . . . . . . . . . . 36
5.6.3. Whitespace . . . . . . . . . . . . . . . . . . . . . 36 5.6.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 37
5.6.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 37 5.6.3. Whitespace . . . . . . . . . . . . . . . . . . . . . 38
5.6.5. Comments . . . . . . . . . . . . . . . . . . . . . . 38 5.6.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 39
5.6.6. Parameters . . . . . . . . . . . . . . . . . . . . . 38 5.6.5. Comments . . . . . . . . . . . . . . . . . . . . . . 39
5.6.7. Date/Time Formats . . . . . . . . . . . . . . . . . . 38 5.6.6. Parameters . . . . . . . . . . . . . . . . . . . . . 39
6. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 40 5.6.7. Date/Time Formats . . . . . . . . . . . . . . . . . . 40
6.1. Framing and Completeness . . . . . . . . . . . . . . . . 41 6. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 42
6.2. Control Data . . . . . . . . . . . . . . . . . . . . . . 42 6.1. Framing and Completeness . . . . . . . . . . . . . . . . 43
6.3. Header Fields . . . . . . . . . . . . . . . . . . . . . . 43 6.2. Control Data . . . . . . . . . . . . . . . . . . . . . . 44
6.4. Content . . . . . . . . . . . . . . . . . . . . . . . . . 43 6.3. Header Fields . . . . . . . . . . . . . . . . . . . . . . 45
6.4.1. Content Semantics . . . . . . . . . . . . . . . . . . 44 6.4. Content . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.4.2. Identifying Content . . . . . . . . . . . . . . . . . 45 6.4.1. Content Semantics . . . . . . . . . . . . . . . . . . 45
6.5. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 46 6.4.2. Identifying Content . . . . . . . . . . . . . . . . . 46
6.5.1. Limitations on use of Trailers . . . . . . . . . . . 46 6.5. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 47
6.5.2. Processing Trailer Fields . . . . . . . . . . . . . . 47 6.5.1. Limitations on use of Trailers . . . . . . . . . . . 48
7. Routing HTTP Messages . . . . . . . . . . . . . . . . . . . . 48 6.5.2. Processing Trailer Fields . . . . . . . . . . . . . . 49
7.1. Determining the Target Resource . . . . . . . . . . . . . 48 7. Routing HTTP Messages . . . . . . . . . . . . . . . . . . . . 49
7.2. Host and :authority . . . . . . . . . . . . . . . . . . . 49 7.1. Determining the Target Resource . . . . . . . . . . . . . 49
7.3. Routing Inbound Requests . . . . . . . . . . . . . . . . 50 7.2. Host and :authority . . . . . . . . . . . . . . . . . . . 50
7.3.1. To a Cache . . . . . . . . . . . . . . . . . . . . . 50 7.3. Routing Inbound Requests . . . . . . . . . . . . . . . . 51
7.3.2. To a Proxy . . . . . . . . . . . . . . . . . . . . . 50 7.3.1. To a Cache . . . . . . . . . . . . . . . . . . . . . 51
7.3.3. To the Origin . . . . . . . . . . . . . . . . . . . . 50 7.3.2. To a Proxy . . . . . . . . . . . . . . . . . . . . . 51
7.4. Rejecting Misdirected Requests . . . . . . . . . . . . . 50 7.3.3. To the Origin . . . . . . . . . . . . . . . . . . . . 51
7.5. Response Correlation . . . . . . . . . . . . . . . . . . 51 7.4. Rejecting Misdirected Requests . . . . . . . . . . . . . 52
7.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 51 7.5. Response Correlation . . . . . . . . . . . . . . . . . . 52
7.6.1. Connection . . . . . . . . . . . . . . . . . . . . . 51 7.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 53
7.6.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 53 7.6.1. Connection . . . . . . . . . . . . . . . . . . . . . 53
7.6.3. Via . . . . . . . . . . . . . . . . . . . . . . . . . 54 7.6.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 55
7.7. Message Transformations . . . . . . . . . . . . . . . . . 55 7.6.3. Via . . . . . . . . . . . . . . . . . . . . . . . . . 56
7.8. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 56 7.7. Message Transformations . . . . . . . . . . . . . . . . . 57
8. Representation Data and Metadata . . . . . . . . . . . . . . 59 7.8. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 58
8.1. Representation Data . . . . . . . . . . . . . . . . . . . 59 8. Representation Data and Metadata . . . . . . . . . . . . . . 61
8.2. Representation Metadata . . . . . . . . . . . . . . . . . 59 8.1. Representation Data . . . . . . . . . . . . . . . . . . . 61
8.3. Content-Type . . . . . . . . . . . . . . . . . . . . . . 59 8.2. Representation Metadata . . . . . . . . . . . . . . . . . 61
8.3.1. Media Type . . . . . . . . . . . . . . . . . . . . . 60 8.3. Content-Type . . . . . . . . . . . . . . . . . . . . . . 61
8.3.2. Charset . . . . . . . . . . . . . . . . . . . . . . . 61 8.3.1. Media Type . . . . . . . . . . . . . . . . . . . . . 62
8.3.3. Canonicalization and Text Defaults . . . . . . . . . 61 8.3.2. Charset . . . . . . . . . . . . . . . . . . . . . . . 63
8.3.4. Multipart Types . . . . . . . . . . . . . . . . . . . 62 8.3.3. Multipart Types . . . . . . . . . . . . . . . . . . . 63
8.4. Content-Encoding . . . . . . . . . . . . . . . . . . . . 62 8.4. Content-Encoding . . . . . . . . . . . . . . . . . . . . 64
8.4.1. Content Codings . . . . . . . . . . . . . . . . . . . 63 8.4.1. Content Codings . . . . . . . . . . . . . . . . . . . 65
8.5. Content-Language . . . . . . . . . . . . . . . . . . . . 64 8.5. Content-Language . . . . . . . . . . . . . . . . . . . . 66
8.5.1. Language Tags . . . . . . . . . . . . . . . . . . . . 65 8.5.1. Language Tags . . . . . . . . . . . . . . . . . . . . 67
8.6. Content-Length . . . . . . . . . . . . . . . . . . . . . 66 8.6. Content-Length . . . . . . . . . . . . . . . . . . . . . 67
8.7. Content-Location . . . . . . . . . . . . . . . . . . . . 67 8.7. Content-Location . . . . . . . . . . . . . . . . . . . . 69
8.8. Validator Fields . . . . . . . . . . . . . . . . . . . . 69 8.8. Validator Fields . . . . . . . . . . . . . . . . . . . . 71
8.8.1. Weak versus Strong . . . . . . . . . . . . . . . . . 70 8.8.1. Weak versus Strong . . . . . . . . . . . . . . . . . 71
8.8.2. Last-Modified . . . . . . . . . . . . . . . . . . . . 71 8.8.2. Last-Modified . . . . . . . . . . . . . . . . . . . . 73
8.8.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 73 8.8.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 75
8.8.4. When to Use Entity-Tags and Last-Modified Dates . . . 77 8.8.4. When to Use Entity-Tags and Last-Modified Dates . . . 78
9. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 9. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 77 9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 79
9.2. Common Method Properties . . . . . . . . . . . . . . . . 79 9.2. Common Method Properties . . . . . . . . . . . . . . . . 81
9.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 80 9.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 81
9.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 81 9.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 82
9.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 82 9.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 83
9.3. Method Definitions . . . . . . . . . . . . . . . . . . . 82 9.3. Method Definitions . . . . . . . . . . . . . . . . . . . 83
9.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 82 9.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 83
9.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 83 9.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 84
9.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 84 9.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 85
9.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 85 9.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 86
9.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 88 9.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 89
9.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 89 9.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 90
9.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 90 9.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 91
9.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 91 9.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 92
10. Message Context . . . . . . . . . . . . . . . . . . . . . . . 92 10. Message Context . . . . . . . . . . . . . . . . . . . . . . . 93
10.1. Request Context Fields . . . . . . . . . . . . . . . . . 92 10.1. Request Context Fields . . . . . . . . . . . . . . . . . 93
10.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 92 10.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 93
10.1.2. From . . . . . . . . . . . . . . . . . . . . . . . . 94 10.1.2. From . . . . . . . . . . . . . . . . . . . . . . . . 95
10.1.3. Referer . . . . . . . . . . . . . . . . . . . . . . 95 10.1.3. Referer . . . . . . . . . . . . . . . . . . . . . . 96
10.1.4. TE . . . . . . . . . . . . . . . . . . . . . . . . . 96 10.1.4. TE . . . . . . . . . . . . . . . . . . . . . . . . . 97
10.1.5. Trailer . . . . . . . . . . . . . . . . . . . . . . 97 10.1.5. Trailer . . . . . . . . . . . . . . . . . . . . . . 98
10.1.6. User-Agent . . . . . . . . . . . . . . . . . . . . . 97 10.1.6. User-Agent . . . . . . . . . . . . . . . . . . . . . 98
10.2. Response Context Fields . . . . . . . . . . . . . . . . 98 10.2. Response Context Fields . . . . . . . . . . . . . . . . 99
10.2.1. Allow . . . . . . . . . . . . . . . . . . . . . . . 99 10.2.1. Allow . . . . . . . . . . . . . . . . . . . . . . . 100
10.2.2. Date . . . . . . . . . . . . . . . . . . . . . . . . 99 10.2.2. Date . . . . . . . . . . . . . . . . . . . . . . . . 100
10.2.3. Location . . . . . . . . . . . . . . . . . . . . . . 100 10.2.3. Location . . . . . . . . . . . . . . . . . . . . . . 101
10.2.4. Retry-After . . . . . . . . . . . . . . . . . . . . 102 10.2.4. Retry-After . . . . . . . . . . . . . . . . . . . . 103
10.2.5. Server . . . . . . . . . . . . . . . . . . . . . . . 102 10.2.5. Server . . . . . . . . . . . . . . . . . . . . . . . 103
11. HTTP Authentication . . . . . . . . . . . . . . . . . . . . . 103 11. HTTP Authentication . . . . . . . . . . . . . . . . . . . . . 104
11.1. Authentication Scheme . . . . . . . . . . . . . . . . . 103 11.1. Authentication Scheme . . . . . . . . . . . . . . . . . 104
11.2. Authentication Parameters . . . . . . . . . . . . . . . 103 11.2. Authentication Parameters . . . . . . . . . . . . . . . 104
11.3. Challenge and Response . . . . . . . . . . . . . . . . . 104 11.3. Challenge and Response . . . . . . . . . . . . . . . . . 105
11.4. Credentials . . . . . . . . . . . . . . . . . . . . . . 105 11.4. Credentials . . . . . . . . . . . . . . . . . . . . . . 106
11.5. Establishing a Protection Space (Realm) . . . . . . . . 105 11.5. Establishing a Protection Space (Realm) . . . . . . . . 106
11.6. Authenticating Users to Origin Servers . . . . . . . . . 106 11.6. Authenticating Users to Origin Servers . . . . . . . . . 107
11.6.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 106 11.6.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 107
11.6.2. Authorization . . . . . . . . . . . . . . . . . . . 107 11.6.2. Authorization . . . . . . . . . . . . . . . . . . . 108
11.6.3. Authentication-Info . . . . . . . . . . . . . . . . 108 11.6.3. Authentication-Info . . . . . . . . . . . . . . . . 109
11.7. Authenticating Clients to Proxies . . . . . . . . . . . 108 11.7. Authenticating Clients to Proxies . . . . . . . . . . . 109
11.7.1. Proxy-Authenticate . . . . . . . . . . . . . . . . . 108 11.7.1. Proxy-Authenticate . . . . . . . . . . . . . . . . . 109
11.7.2. Proxy-Authorization . . . . . . . . . . . . . . . . 109 11.7.2. Proxy-Authorization . . . . . . . . . . . . . . . . 110
11.7.3. Proxy-Authentication-Info . . . . . . . . . . . . . 109 11.7.3. Proxy-Authentication-Info . . . . . . . . . . . . . 110
12. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 110 12. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 111
12.1. Proactive Negotiation . . . . . . . . . . . . . . . . . 111 12.1. Proactive Negotiation . . . . . . . . . . . . . . . . . 112
12.2. Reactive Negotiation . . . . . . . . . . . . . . . . . . 112 12.2. Reactive Negotiation . . . . . . . . . . . . . . . . . . 113
12.3. Request Content Negotiation . . . . . . . . . . . . . . 113 12.3. Request Content Negotiation . . . . . . . . . . . . . . 114
12.4. Content Negotiation Field Features . . . . . . . . . . . 113 12.4. Content Negotiation Field Features . . . . . . . . . . . 114
12.4.1. Absence . . . . . . . . . . . . . . . . . . . . . . 113 12.4.1. Absence . . . . . . . . . . . . . . . . . . . . . . 114
12.4.2. Quality Values . . . . . . . . . . . . . . . . . . . 114 12.4.2. Quality Values . . . . . . . . . . . . . . . . . . . 114
12.4.3. Wildcard Values . . . . . . . . . . . . . . . . . . 114 12.4.3. Wildcard Values . . . . . . . . . . . . . . . . . . 115
12.5. Content Negotiation Fields . . . . . . . . . . . . . . . 114 12.5. Content Negotiation Fields . . . . . . . . . . . . . . . 115
12.5.1. Accept . . . . . . . . . . . . . . . . . . . . . . . 115 12.5.1. Accept . . . . . . . . . . . . . . . . . . . . . . . 115
12.5.2. Accept-Charset . . . . . . . . . . . . . . . . . . . 117 12.5.2. Accept-Charset . . . . . . . . . . . . . . . . . . . 118
12.5.3. Accept-Encoding . . . . . . . . . . . . . . . . . . 118 12.5.3. Accept-Encoding . . . . . . . . . . . . . . . . . . 118
12.5.4. Accept-Language . . . . . . . . . . . . . . . . . . 119 12.5.4. Accept-Language . . . . . . . . . . . . . . . . . . 120
12.5.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . 121 12.5.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . 121
13. Conditional Requests . . . . . . . . . . . . . . . . . . . . 122 13. Conditional Requests . . . . . . . . . . . . . . . . . . . . 123
13.1. Preconditions . . . . . . . . . . . . . . . . . . . . . 122 13.1. Preconditions . . . . . . . . . . . . . . . . . . . . . 123
13.1.1. If-Match . . . . . . . . . . . . . . . . . . . . . . 123 13.1.1. If-Match . . . . . . . . . . . . . . . . . . . . . . 124
13.1.2. If-None-Match . . . . . . . . . . . . . . . . . . . 124 13.1.2. If-None-Match . . . . . . . . . . . . . . . . . . . 125
13.1.3. If-Modified-Since . . . . . . . . . . . . . . . . . 126 13.1.3. If-Modified-Since . . . . . . . . . . . . . . . . . 127
13.1.4. If-Unmodified-Since . . . . . . . . . . . . . . . . 128 13.1.4. If-Unmodified-Since . . . . . . . . . . . . . . . . 129
13.1.5. If-Range . . . . . . . . . . . . . . . . . . . . . . 129 13.1.5. If-Range . . . . . . . . . . . . . . . . . . . . . . 130
13.2. Evaluation of Preconditions . . . . . . . . . . . . . . 130 13.2. Evaluation of Preconditions . . . . . . . . . . . . . . 131
13.2.1. When to Evaluate . . . . . . . . . . . . . . . . . . 131 13.2.1. When to Evaluate . . . . . . . . . . . . . . . . . . 132
13.2.2. Precedence of Preconditions . . . . . . . . . . . . 132 13.2.2. Precedence of Preconditions . . . . . . . . . . . . 132
14. Range Requests . . . . . . . . . . . . . . . . . . . . . . . 133 14. Range Requests . . . . . . . . . . . . . . . . . . . . . . . 134
14.1. Range Units . . . . . . . . . . . . . . . . . . . . . . 133 14.1. Range Units . . . . . . . . . . . . . . . . . . . . . . 134
14.1.1. Range Specifiers . . . . . . . . . . . . . . . . . . 134 14.1.1. Range Specifiers . . . . . . . . . . . . . . . . . . 135
14.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . 135 14.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . 136
14.2. Range . . . . . . . . . . . . . . . . . . . . . . . . . 137 14.2. Range . . . . . . . . . . . . . . . . . . . . . . . . . 137
14.3. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 138 14.3. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 139
14.4. Content-Range . . . . . . . . . . . . . . . . . . . . . 139 14.4. Content-Range . . . . . . . . . . . . . . . . . . . . . 139
14.5. Partial PUT . . . . . . . . . . . . . . . . . . . . . . 141 14.5. Partial PUT . . . . . . . . . . . . . . . . . . . . . . 141
14.6. Media Type multipart/byteranges . . . . . . . . . . . . 141 14.6. Media Type multipart/byteranges . . . . . . . . . . . . 142
15. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 143 15. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 144
15.1. Overview of Status Codes . . . . . . . . . . . . . . . . 144 15.1. Overview of Status Codes . . . . . . . . . . . . . . . . 145
15.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 144 15.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 145
15.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 145 15.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 146
15.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 145 15.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 146
15.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 145 15.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 146
15.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 146 15.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 147
15.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 146 15.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 147
15.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 147 15.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 148
15.3.4. 203 Non-Authoritative Information . . . . . . . . . 147 15.3.4. 203 Non-Authoritative Information . . . . . . . . . 148
15.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 147 15.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 148
15.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 148 15.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 149
15.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 148 15.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 149
15.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 152 15.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 153
15.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 154 15.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 155
15.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 155 15.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 156
15.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 155 15.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 156
15.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 156 15.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 157
15.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 156 15.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 158
15.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 157 15.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 158
15.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 157 15.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 158
15.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 157 15.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 159
15.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 158 15.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 159
15.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 158 15.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 159
15.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 158 15.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 160
15.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 159 15.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 160
15.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 159 15.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 160
15.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 159 15.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 160
15.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 159 15.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 161
15.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 160 15.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 161
15.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 160 15.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 161
15.5.8. 407 Proxy Authentication Required . . . . . . . . . 160 15.5.8. 407 Proxy Authentication Required . . . . . . . . . 162
15.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 160 15.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 162
15.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 161 15.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 162
15.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 161 15.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 162
15.5.12. 411 Length Required . . . . . . . . . . . . . . . . 162 15.5.12. 411 Length Required . . . . . . . . . . . . . . . . 163
15.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 162 15.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 163
15.5.14. 413 Content Too Large . . . . . . . . . . . . . . . 162 15.5.14. 413 Content Too Large . . . . . . . . . . . . . . . 163
15.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 162 15.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 164
15.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 163 15.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 164
15.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 163 15.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 164
15.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 164 15.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 165
15.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 164 15.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 165
15.5.20. 421 Misdirected Request . . . . . . . . . . . . . . 164 15.5.20. 421 Misdirected Request . . . . . . . . . . . . . . 166
15.5.21. 422 Unprocessable Content . . . . . . . . . . . . . 165 15.5.21. 422 Unprocessable Content . . . . . . . . . . . . . 166
15.5.22. 426 Upgrade Required . . . . . . . . . . . . . . . . 165 15.5.22. 426 Upgrade Required . . . . . . . . . . . . . . . . 166
15.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 165 15.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 167
15.6.1. 500 Internal Server Error . . . . . . . . . . . . . 165 15.6.1. 500 Internal Server Error . . . . . . . . . . . . . 167
15.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 166 15.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 167
15.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 166 15.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 167
15.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 166 15.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 167
15.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 166 15.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 168
15.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 166 15.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 168
16. Extending HTTP . . . . . . . . . . . . . . . . . . . . . . . 167 16. Extending HTTP . . . . . . . . . . . . . . . . . . . . . . . 168
16.1. Method Extensibility . . . . . . . . . . . . . . . . . . 167 16.1. Method Extensibility . . . . . . . . . . . . . . . . . . 169
16.1.1. Method Registry . . . . . . . . . . . . . . . . . . 167 16.1.1. Method Registry . . . . . . . . . . . . . . . . . . 169
16.1.2. Considerations for New Methods . . . . . . . . . . . 168 16.1.2. Considerations for New Methods . . . . . . . . . . . 169
16.2. Status Code Extensibility . . . . . . . . . . . . . . . 168 16.2. Status Code Extensibility . . . . . . . . . . . . . . . 170
16.2.1. Status Code Registry . . . . . . . . . . . . . . . . 169 16.2.1. Status Code Registry . . . . . . . . . . . . . . . . 170
16.2.2. Considerations for New Status Codes . . . . . . . . 169 16.2.2. Considerations for New Status Codes . . . . . . . . 170
16.3. Field Extensibility . . . . . . . . . . . . . . . . . . 170 16.3. Field Extensibility . . . . . . . . . . . . . . . . . . 171
16.3.1. Field Name Registry . . . . . . . . . . . . . . . . 170 16.3.1. Field Name Registry . . . . . . . . . . . . . . . . 172
16.3.2. Considerations for New Fields . . . . . . . . . . . 172 16.3.2. Considerations for New Fields . . . . . . . . . . . 173
16.4. Authentication Scheme Extensibility . . . . . . . . . . 174 16.4. Authentication Scheme Extensibility . . . . . . . . . . 175
16.4.1. Authentication Scheme Registry . . . . . . . . . . . 174 16.4.1. Authentication Scheme Registry . . . . . . . . . . . 176
16.4.2. Considerations for New Authentication Schemes . . . 175 16.4.2. Considerations for New Authentication Schemes . . . 176
16.5. Range Unit Extensibility . . . . . . . . . . . . . . . . 176 16.5. Range Unit Extensibility . . . . . . . . . . . . . . . . 177
16.5.1. Range Unit Registry . . . . . . . . . . . . . . . . 176 16.5.1. Range Unit Registry . . . . . . . . . . . . . . . . 178
16.5.2. Considerations for New Range Units . . . . . . . . . 176 16.5.2. Considerations for New Range Units . . . . . . . . . 178
16.6. Content Coding Extensibility . . . . . . . . . . . . . . 176 16.6. Content Coding Extensibility . . . . . . . . . . . . . . 178
16.6.1. Content Coding Registry . . . . . . . . . . . . . . 177 16.6.1. Content Coding Registry . . . . . . . . . . . . . . 178
16.6.2. Considerations for New Content Codings . . . . . . . 177 16.6.2. Considerations for New Content Codings . . . . . . . 179
16.7. Upgrade Token Registry . . . . . . . . . . . . . . . . . 177 16.7. Upgrade Token Registry . . . . . . . . . . . . . . . . . 179
17. Security Considerations . . . . . . . . . . . . . . . . . . . 178 17. Security Considerations . . . . . . . . . . . . . . . . . . . 180
17.1. Establishing Authority . . . . . . . . . . . . . . . . . 178 17.1. Establishing Authority . . . . . . . . . . . . . . . . . 180
17.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 179 17.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 181
17.3. Attacks Based on File and Path Names . . . . . . . . . . 180 17.3. Attacks Based on File and Path Names . . . . . . . . . . 182
17.4. Attacks Based on Command, Code, or Query Injection . . . 181 17.4. Attacks Based on Command, Code, or Query Injection . . . 182
17.5. Attacks via Protocol Element Length . . . . . . . . . . 181 17.5. Attacks via Protocol Element Length . . . . . . . . . . 183
17.6. Attacks using Shared-dictionary Compression . . . . . . 182 17.6. Attacks using Shared-dictionary Compression . . . . . . 183
17.7. Disclosure of Personal Information . . . . . . . . . . . 182 17.7. Disclosure of Personal Information . . . . . . . . . . . 184
17.8. Privacy of Server Log Information . . . . . . . . . . . 182 17.8. Privacy of Server Log Information . . . . . . . . . . . 184
17.9. Disclosure of Sensitive Information in URIs . . . . . . 183 17.9. Disclosure of Sensitive Information in URIs . . . . . . 185
17.10. Disclosure of Fragment after Redirects . . . . . . . . . 184 17.10. Disclosure of Fragment after Redirects . . . . . . . . . 185
17.11. Disclosure of Product Information . . . . . . . . . . . 184 17.11. Disclosure of Product Information . . . . . . . . . . . 186
17.12. Browser Fingerprinting . . . . . . . . . . . . . . . . . 184 17.12. Browser Fingerprinting . . . . . . . . . . . . . . . . . 186
17.13. Validator Retention . . . . . . . . . . . . . . . . . . 185 17.13. Validator Retention . . . . . . . . . . . . . . . . . . 187
17.14. Denial-of-Service Attacks Using Range . . . . . . . . . 186 17.14. Denial-of-Service Attacks Using Range . . . . . . . . . 187
17.15. Authentication Considerations . . . . . . . . . . . . . 186 17.15. Authentication Considerations . . . . . . . . . . . . . 188
17.15.1. Confidentiality of Credentials . . . . . . . . . . 186 17.15.1. Confidentiality of Credentials . . . . . . . . . . 188
17.15.2. Credentials and Idle Clients . . . . . . . . . . . 187 17.15.2. Credentials and Idle Clients . . . . . . . . . . . 188
17.15.3. Protection Spaces . . . . . . . . . . . . . . . . . 187 17.15.3. Protection Spaces . . . . . . . . . . . . . . . . . 189
17.15.4. Additional Response Fields . . . . . . . . . . . . 188 17.15.4. Additional Response Fields . . . . . . . . . . . . 189
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 188 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 189
18.1. URI Scheme Registration . . . . . . . . . . . . . . . . 188 18.1. URI Scheme Registration . . . . . . . . . . . . . . . . 190
18.2. Method Registration . . . . . . . . . . . . . . . . . . 188 18.2. Method Registration . . . . . . . . . . . . . . . . . . 190
18.3. Status Code Registration . . . . . . . . . . . . . . . . 189 18.3. Status Code Registration . . . . . . . . . . . . . . . . 190
18.4. Field Name Registration . . . . . . . . . . . . . . . . 190 18.4. Field Name Registration . . . . . . . . . . . . . . . . 193
18.5. Authentication Scheme Registration . . . . . . . . . . . 192 18.5. Authentication Scheme Registration . . . . . . . . . . . 195
18.6. Content Coding Registration . . . . . . . . . . . . . . 192 18.6. Content Coding Registration . . . . . . . . . . . . . . 196
18.7. Range Unit Registration . . . . . . . . . . . . . . . . 193 18.7. Range Unit Registration . . . . . . . . . . . . . . . . 196
18.8. Media Type Registration . . . . . . . . . . . . . . . . 193 18.8. Media Type Registration . . . . . . . . . . . . . . . . 197
18.9. Port Registration . . . . . . . . . . . . . . . . . . . 193 18.9. Port Registration . . . . . . . . . . . . . . . . . . . 197
18.10. Upgrade Token Registration . . . . . . . . . . . . . . . 194 18.10. Upgrade Token Registration . . . . . . . . . . . . . . . 197
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 194 19. References . . . . . . . . . . . . . . . . . . . . . . . . . 197
19.1. Normative References . . . . . . . . . . . . . . . . . . 194 19.1. Normative References . . . . . . . . . . . . . . . . . . 197
19.2. Informative References . . . . . . . . . . . . . . . . . 196 19.2. Informative References . . . . . . . . . . . . . . . . . 199
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 202 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 205
Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 207 Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 210
B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 207 B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 210
B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 207 B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 210
B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 208 B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 211
B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 210 B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 213
B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 210 B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 213
B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 210 B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 214
B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 210 B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 214
B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 211 B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 214
B.9. Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 211 B.9. Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 214
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 211 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 214
C.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 211 C.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 214
C.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 211 C.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 215
C.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 212 C.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 215
C.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 213 C.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 216
C.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 214 C.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 217
C.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 215 C.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 218
C.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 215 C.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 219
C.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 217 C.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 220
C.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 218 C.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 221
C.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 219 C.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 223
C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . . . . . 221 C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . . . . . 224
C.12. Since draft-ietf-httpbis-semantics-10 . . . . . . . . . . 221 C.12. Since draft-ietf-httpbis-semantics-10 . . . . . . . . . . 224
C.13. Since draft-ietf-httpbis-semantics-11 . . . . . . . . . . 222 C.13. Since draft-ietf-httpbis-semantics-11 . . . . . . . . . . 226
C.14. Since draft-ietf-httpbis-semantics-12 . . . . . . . . . . 223 C.14. Since draft-ietf-httpbis-semantics-12 . . . . . . . . . . 226
C.15. Since draft-ietf-httpbis-semantics-13 . . . . . . . . . . 225 C.15. Since draft-ietf-httpbis-semantics-13 . . . . . . . . . . 228
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 225 C.16. Since draft-ietf-httpbis-semantics-14 . . . . . . . . . . 229
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 226 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 231
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 232
1. Introduction 1. Introduction
1.1. Purpose 1.1. Purpose
The Hypertext Transfer Protocol (HTTP) is a family of stateless, The Hypertext Transfer Protocol (HTTP) is a family of stateless,
application-level, request/response protocols that share a generic application-level, request/response protocols that share a generic
interface, extensible semantics, and self-descriptive messages to interface, extensible semantics, and self-descriptive messages to
enable flexible interaction with network-based hypertext information enable flexible interaction with network-based hypertext information
systems. systems.
skipping to change at page 11, line 31 skipping to change at page 11, line 21
Semantics also include representation metadata that describe how Semantics also include representation metadata that describe how
content is intended to be interpreted by a recipient, request header content is intended to be interpreted by a recipient, request header
fields that might influence content selection, and the various fields that might influence content selection, and the various
selection algorithms that are collectively referred to as _content selection algorithms that are collectively referred to as _content
negotiation_ (Section 12). negotiation_ (Section 12).
1.4. Specifications Obsoleted by this Document 1.4. Specifications Obsoleted by this Document
This document obsoletes the following specifications: This document obsoletes the following specifications:
-------------------------------------------- ----------- --------- +============================================+===========+=========+
Title Reference Changes | Title | Reference | Changes |
-------------------------------------------- ----------- --------- +============================================+===========+=========+
HTTP Over TLS [RFC2818] B.1 | HTTP Over TLS | [RFC2818] | B.1 |
HTTP/1.1 Message Syntax and Routing [*] [RFC7230] B.2 +--------------------------------------------+-----------+---------+
HTTP/1.1 Semantics and Content [RFC7231] B.3 | HTTP/1.1 Message Syntax and Routing [*] | [RFC7230] | B.2 |
HTTP/1.1 Conditional Requests [RFC7232] B.4 +--------------------------------------------+-----------+---------+
HTTP/1.1 Range Requests [RFC7233] B.5 | HTTP/1.1 Semantics and Content | [RFC7231] | B.3 |
HTTP/1.1 Authentication [RFC7235] B.6 +--------------------------------------------+-----------+---------+
HTTP Status Code 308 (Permanent Redirect) [RFC7538] B.7 | HTTP/1.1 Conditional Requests | [RFC7232] | B.4 |
HTTP Authentication-Info and Proxy- [RFC7615] B.8 +--------------------------------------------+-----------+---------+
Authentication-Info Response Header Fields | HTTP/1.1 Range Requests | [RFC7233] | B.5 |
HTTP Client-Initiated Content-Encoding [RFC7694] B.9 +--------------------------------------------+-----------+---------+
-------------------------------------------- ----------- --------- | HTTP/1.1 Authentication | [RFC7235] | B.6 |
+--------------------------------------------+-----------+---------+
| HTTP Status Code 308 (Permanent Redirect) | [RFC7538] | B.7 |
+--------------------------------------------+-----------+---------+
| HTTP Authentication-Info and Proxy- | [RFC7615] | B.8 |
| Authentication-Info Response Header Fields | | |
+--------------------------------------------+-----------+---------+
| HTTP Client-Initiated Content-Encoding | [RFC7694] | B.9 |
+--------------------------------------------+-----------+---------+
Table 1 Table 1
[*] This document only obsoletes the portions of RFC 7230 that are [*] This document only obsoletes the portions of RFC 7230 that are
independent of the HTTP/1.1 messaging syntax and connection independent of the HTTP/1.1 messaging syntax and connection
management; the remaining bits of RFC 7230 are obsoleted by management; the remaining bits of RFC 7230 are obsoleted by
"HTTP/1.1" [Messaging]. "HTTP/1.1" [Messaging].
2. Conformance 2. Conformance
2.1. Syntax Notation 2.1. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234], extended with the notation for case- notation of [RFC5234], extended with the notation for case-
sensitivity in strings defined in [RFC7405]. sensitivity in strings defined in [RFC7405].
It also uses a list extension, defined in Section 5.6.1, that allows It also uses a list extension, defined in Section 5.6.1, that allows
for compact definition of comma-separated lists using a "#" operator for compact definition of comma-separated lists using a "#" operator
(similar to how the "*" operator indicates repetition). Appendix A (similar to how the "*" operator indicates repetition). Appendix A
shows the collected grammar with all list operators expanded to shows the collected grammar with all list operators expanded to
skipping to change at page 13, line 13 skipping to change at page 13, line 13
beyond the scope of a single communication. beyond the scope of a single communication.
The verb "generate" is used instead of "send" where a requirement The verb "generate" is used instead of "send" where a requirement
applies only to implementations that create the protocol element, applies only to implementations that create the protocol element,
rather than an implementation that forwards a received element rather than an implementation that forwards a received element
downstream. downstream.
An implementation is considered conformant if it complies with all of An implementation is considered conformant if it complies with all of
the requirements associated with the roles it partakes in HTTP. the requirements associated with the roles it partakes in HTTP.
Conformance includes both the syntax and semantics of protocol A sender MUST NOT generate protocol elements that do not match the
elements. A sender MUST NOT generate protocol elements that convey a grammar defined by the corresponding ABNF rules. Within a given
meaning that is known by that sender to be false. A sender MUST NOT message, a sender MUST NOT generate protocol elements or syntax
generate protocol elements that do not match the grammar defined by alternatives that are only allowed to be generated by participants in
the corresponding ABNF rules. Within a given message, a sender MUST other roles (i.e., a role that the sender does not have for that
NOT generate protocol elements or syntax alternatives that are only message).
allowed to be generated by participants in other roles (i.e., a role
that the sender does not have for that message). Conformance to HTTP includes both conformance to the particular
messaging syntax of the protocol version in use and conformance to
the semantics of protocol elements sent. For example, a client that
claims conformance to HTTP/1.1 but fails to recognize the features
required of HTTP/1.1 recipients will fail to interoperate with
servers that adjust their responses in accordance with those claims.
Features that reflect user choices, such as content negotiation and
user-selected extensions, can impact application behavior beyond the
protocol stream; sending protocol elements that inaccurately reflect
a user's choices will confuse the user and inhibit choice.
When an implementation fails semantic conformance, recipients of that
implementation's messages will eventually develop workarounds to
adjust their behavior accordingly. A recipient MAY employ such
workarounds while remaining conformant to this protocol if the
workarounds are limited to the implementations at fault. For
example, servers often scan portions of the User-Agent field value,
and user agents often scan the Server field value, to adjust their
own behavior with respect to known bugs or poorly chosen defaults.
2.3. Length Requirements 2.3. Length Requirements
A recipient SHOULD parse a received protocol element defensively, A recipient SHOULD parse a received protocol element defensively,
with only marginal expectations that the element will conform to its with only marginal expectations that the element will conform to its
ABNF grammar and fit within a reasonable buffer size. ABNF grammar and fit within a reasonable buffer size.
HTTP does not have specific length limitations for many of its HTTP does not have specific length limitations for many of its
protocol elements because the lengths that might be appropriate will protocol elements because the lengths that might be appropriate will
vary widely, depending on the deployment context and purpose of the vary widely, depending on the deployment context and purpose of the
skipping to change at page 15, line 39 skipping to change at page 16, line 16
The target of an HTTP request is called a _resource_. HTTP does not The target of an HTTP request is called a _resource_. HTTP does not
limit the nature of a resource; it merely defines an interface that limit the nature of a resource; it merely defines an interface that
might be used to interact with resources. Most resources are might be used to interact with resources. Most resources are
identified by a Uniform Resource Identifier (URI), as described in identified by a Uniform Resource Identifier (URI), as described in
Section 4. Section 4.
One design goal of HTTP is to separate resource identification from One design goal of HTTP is to separate resource identification from
request semantics, which is made possible by vesting the request request semantics, which is made possible by vesting the request
semantics in the request method (Section 9) and a few request- semantics in the request method (Section 9) and a few request-
modifying header fields. If there is a conflict between the method modifying header fields. A resource cannot treat a request in a
semantics and any semantic implied by the URI itself, as described in manner inconsistent with the semantics of the method of the request.
Section 9.2.1, the method semantics take precedence. For example, though the URI of a resource might imply semantics that
are not safe, a client can expect the resource to avoid actions that
are unsafe when processing a request with a safe method (see
Section 9.2.1).
HTTP relies upon the Uniform Resource Identifier (URI) standard HTTP relies upon the Uniform Resource Identifier (URI) standard
[RFC3986] to indicate the target resource (Section 7.1) and [RFC3986] to indicate the target resource (Section 7.1) and
relationships between resources. relationships between resources.
3.2. Representations 3.2. Representations
A _representation_ is information that is intended to reflect a past, A _representation_ is information that is intended to reflect a past,
current, or desired state of a given resource, in a format that can current, or desired state of a given resource, in a format that can
be readily communicated via the protocol. A representation consists be readily communicated via the protocol. A representation consists
skipping to change at page 17, line 5 skipping to change at page 17, line 29
An HTTP _client_ is a program that establishes a connection to a 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 for the purpose of sending one or more HTTP requests. An HTTP
_server_ is a program that accepts connections in order to service _server_ is a program that accepts connections in order to service
HTTP requests by sending HTTP responses. HTTP requests by sending HTTP responses.
The terms "client" and "server" refer only to the roles that these The terms "client" and "server" refer only to the roles that these
programs perform for a particular connection. The same program might programs perform for a particular connection. The same program might
act as a client on some connections and a server on others. act as a client on some connections and a server on others.
HTTP is defined as a stateless protocol, meaning that each request
message's semantics can be understood in isolation, and that the
relationship between connections and messages on them has no impact
on the interpretation of those messages. For example, a CONNECT
request (Section 9.3.6) or a request with the Upgrade header field
(Section 7.8) can occur at any time, not just in the first message on
a connection. Many implementations depend on HTTP's stateless design
in order to reuse proxied connections or dynamically load balance
requests across multiple servers.
As a result, 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.
3.4. Messages 3.4. Messages
HTTP is a stateless request/response protocol for exchanging HTTP is a stateless request/response protocol for exchanging
_messages_ across a connection. The terms _sender_ and _recipient_ _messages_ across a connection. The terms _sender_ and _recipient_
refer to any implementation that sends or receives a given message, refer to any implementation that sends or receives a given message,
respectively. respectively.
A client sends requests to a server in the form of a _request_ A client sends requests to a server in the form of a _request_
message with a method (Section 9) and request target (Section 7.1). message with a method (Section 9) and request target (Section 7.1).
The request might also contain header fields (Section 6.3) for The request might also contain header fields (Section 6.3) for
skipping to change at page 19, line 48 skipping to change at page 20, line 41
Gateways are often used to encapsulate legacy or untrusted Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through information services, to improve server performance through
_accelerator_ caching, and to enable partitioning or load balancing _accelerator_ caching, and to enable partitioning or load balancing
of HTTP services across multiple machines. of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server also apply to All HTTP requirements applicable to an origin server also apply to
the outbound communication of a gateway. A gateway communicates with the outbound communication of a gateway. A gateway communicates with
inbound servers using any protocol that it desires, including private inbound servers using any protocol that it desires, including private
extensions to HTTP that are outside the scope of this specification. extensions to HTTP that are outside the scope of this specification.
However, an HTTP-to-HTTP gateway that wishes to interoperate with However, an HTTP-to-HTTP gateway that wishes to interoperate with
third-party HTTP servers ought to conform to user agent requirements third-party HTTP servers needs to conform to user agent requirements
on the gateway's inbound connection. on the gateway's inbound connection.
A _tunnel_ acts as a blind relay between two connections without A _tunnel_ acts as a blind relay between two connections without
changing the messages. Once active, a tunnel is not considered a changing the messages. Once active, a tunnel is not considered a
party to the HTTP communication, though the tunnel might have been party to the HTTP communication, though the tunnel might have been
initiated by an HTTP request. A tunnel ceases to exist when both initiated by an HTTP request. A tunnel ceases to exist when both
ends of the relayed connection are closed. Tunnels are used to ends of the relayed connection are closed. Tunnels are used to
extend a virtual connection through an intermediary, such as when extend a virtual connection through an intermediary, such as when
Transport Layer Security (TLS, [RFC8446]) is used to establish Transport Layer Security (TLS, [RFC8446]) is used to establish
confidential communication through a shared firewall proxy. confidential communication through a shared firewall proxy.
skipping to change at page 20, line 33 skipping to change at page 21, line 24
For example, an _interception proxy_ [RFC3040] (also commonly known For example, an _interception proxy_ [RFC3040] (also commonly known
as a _transparent proxy_ [RFC1919]) differs from an HTTP proxy as a _transparent proxy_ [RFC1919]) differs from an HTTP proxy
because it is not chosen by the client. Instead, an interception because it is not chosen by the client. Instead, an interception
proxy filters or redirects outgoing TCP port 80 packets (and proxy filters or redirects outgoing TCP port 80 packets (and
occasionally other common port traffic). Interception proxies are occasionally other common port traffic). Interception proxies are
commonly found on public network access points, as a means of commonly found on public network access points, as a means of
enforcing account subscription prior to allowing use of non-local enforcing account subscription prior to allowing use of non-local
Internet services, and within corporate firewalls to enforce network Internet services, and within corporate firewalls to enforce network
usage policies. 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.
3.8. Caches 3.8. Caches
A _cache_ is a local store of previous response messages and the A _cache_ is a local store of previous response messages and the
subsystem that controls its message storage, retrieval, and deletion. subsystem that controls its message storage, retrieval, and deletion.
A cache stores cacheable responses in order to reduce the response A cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent time and network bandwidth consumption on future, equivalent
requests. Any client or server MAY employ a cache, though a cache requests. Any client or server MAY employ a cache, though a cache
cannot be used while acting as a tunnel. cannot be used while acting as a tunnel.
The effect of a cache is that the request/response chain is shortened The effect of a cache is that the request/response chain is shortened
skipping to change at page 21, line 22 skipping to change at page 21, line 50
UA =========== A =========== B - - - - - - C - - - - - - O UA =========== A =========== B - - - - - - C - - - - - - O
< < < <
Figure 3 Figure 3
A response is _cacheable_ if a cache is allowed to store a copy of A response is _cacheable_ if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. Even the response message for use in answering subsequent requests. Even
when a response is cacheable, there might be additional constraints when a response is cacheable, there might be additional constraints
placed by the client or by the origin server on when that cached placed by the client or by the origin server on when that cached
response can be used for a particular request. HTTP requirements for response can be used for a particular request. HTTP requirements for
cache behavior and cacheable responses are defined in Section 2 of cache behavior and cacheable responses are defined in [Caching].
[Caching].
There is a wide variety of architectures and configurations of caches There is a wide variety of architectures and configurations of caches
deployed across the World Wide Web and inside large organizations. deployed across the World Wide Web and inside large organizations.
These include national hierarchies of proxy caches to save bandwidth These include national hierarchies of proxy caches to save bandwidth
and reduce latency, Content Delivery Networks that use gateway and reduce latency, Content Delivery Networks that use gateway
caching to optimise regional and global distribution of popular caching to optimise regional and global distribution of popular
sites, collaborative systems that broadcast or multicast cache sites, collaborative systems that broadcast or multicast cache
entries, archives of pre-fetched cache entries for use in off-line or entries, archives of pre-fetched cache entries for use in off-line or
high-latency environments, and so on. high-latency environments, and so on.
3.9. Example Message Exchange 3.9. Example Message Exchange
The following example illustrates a typical HTTP/1.1 message exchange The following example illustrates a typical HTTP/1.1 message exchange
for a GET request (Section 9.3.1) on the URI "http://www.example.com/ for a GET request (Section 9.3.1) on the URI "http://www.example.com/
hello.txt": hello.txt":
Client request: Client request:
GET /hello.txt HTTP/1.1 GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3 User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com Host: www.example.com
Accept-Language: en, mi Accept-Language: en, mi
Server response: Server response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00" ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes Accept-Ranges: bytes
Content-Length: 51 Content-Length: 51
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Hello World! My content includes a trailing CRLF. Hello World! My content includes a trailing CRLF.
4. Identifiers in HTTP 4. Identifiers in HTTP
Uniform Resource Identifiers (URIs) [RFC3986] are used throughout Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
HTTP as the means for identifying resources (Section 3.1). HTTP as the means for identifying resources (Section 3.1).
4.1. URI References 4.1. URI References
URI references are used to target requests, indicate redirects, and URI references are used to target requests, indicate redirects, and
define relationships. define relationships.
skipping to change at page 23, line 21 skipping to change at page 24, line 5
example, the request line in HTTP/1.1) will necessarily be larger in example, the request line in HTTP/1.1) will necessarily be larger in
some cases. some cases.
4.2. HTTP-Related URI Schemes 4.2. HTTP-Related URI Schemes
IANA maintains the registry of URI Schemes [BCP35] at IANA maintains the registry of URI Schemes [BCP35] at
<https://www.iana.org/assignments/uri-schemes/>. Although requests <https://www.iana.org/assignments/uri-schemes/>. Although requests
might target any URI scheme, the following schemes are inherent to might target any URI scheme, the following schemes are inherent to
HTTP servers: HTTP servers:
------------ ------------------------------------ ------- +============+====================================+=======+
URI Scheme Description Ref. | URI Scheme | Description | Ref. |
------------ ------------------------------------ ------- +============+====================================+=======+
http Hypertext Transfer Protocol 4.2.1 | http | Hypertext Transfer Protocol | 4.2.1 |
https Hypertext Transfer Protocol Secure 4.2.2 +------------+------------------------------------+-------+
------------ ------------------------------------ ------- | https | Hypertext Transfer Protocol Secure | 4.2.2 |
+------------+------------------------------------+-------+
Table 2 Table 2
Note that the presence of an "http" or "https" URI does not imply Note that the presence of an "http" or "https" URI does not imply
that there is always an HTTP server at the identified origin that there is always an HTTP server at the identified origin
listening for connections. Anyone can mint a URI, whether or not a listening for connections. Anyone can mint a URI, whether or not a
server exists and whether or not that server currently maps that server exists and whether or not that server currently maps that
identifier to a resource. The delegated nature of registered names identifier to a resource. The delegated nature of registered names
and IP addresses creates a federated namespace whether or not an HTTP and IP addresses creates a federated namespace whether or not an HTTP
server is present. server is present.
skipping to change at page 24, line 29 skipping to change at page 25, line 14
4.2.2. https URI Scheme 4.2.2. https URI Scheme
The "https" URI scheme is hereby defined for minting identifiers The "https" URI scheme is hereby defined for minting identifiers
within the hierarchical namespace governed by a potential origin within the hierarchical namespace governed by a potential origin
server listening for TCP connections on a given port and capable of server listening for TCP connections on a given port and capable of
establishing a TLS ([RFC8446]) connection that has been secured for establishing a TLS ([RFC8446]) connection that has been secured for
HTTP communication. In this context, _secured_ specifically means HTTP communication. In this context, _secured_ specifically means
that the server has been authenticated as acting on behalf of the that the server has been authenticated as acting on behalf of the
identified authority and all HTTP communication with that server has identified authority and all HTTP communication with that server has
been protected for confidentiality and integrity through the use of confidentiality and integrity protection that is acceptable to both
strong encryption. client and server.
https-URI = "https" "://" authority path-abempty [ "?" query ] https-URI = "https" "://" authority path-abempty [ "?" query ]
The origin server for an "https" URI is identified by the authority The origin server for an "https" URI is identified by the authority
component, which includes a host identifier and optional port number component, which includes a host identifier and optional port number
([RFC3986], Section 3.2.2). If the port subcomponent is empty or not ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not
given, TCP port 443 (the reserved port for HTTP over TLS) is the given, TCP port 443 (the reserved port for HTTP over TLS) is the
default. The origin determines who has the right to respond default. The origin determines who has the right to respond
authoritatively to requests that target the identified resource, as authoritatively to requests that target the identified resource, as
defined in Section 4.3.3. defined in Section 4.3.3.
A sender MUST NOT generate an "https" URI with an empty host A sender MUST NOT generate an "https" URI with an empty host
identifier. A recipient that processes such a URI reference MUST identifier. A recipient that processes such a URI reference MUST
reject it as invalid. reject it as invalid.
The hierarchical path component and optional query component identify The hierarchical path component and optional query component identify
the target resource within that origin server's name space. the target resource within that origin server's name space.
A client MUST ensure that its HTTP requests for an "https" resource A client MUST ensure that its HTTP requests for an "https" resource
are secured, prior to being communicated, and that it only accepts are secured, prior to being communicated, and that it only accepts
secured responses to those requests. secured responses to those requests. Note that the definition of
what cryptographic mechanisms are acceptable to client and server are
usually negotiated and can change over time.
Resources made available via the "https" scheme have no shared Resources made available via the "https" scheme have no shared
identity with the "http" scheme. They are distinct origins with identity with the "http" scheme. They are distinct origins with
separate namespaces. However, an extension to HTTP that is defined separate namespaces. However, an extension to HTTP that is defined
to apply to all origins with the same host, such as the Cookie to apply to all origins with the same host, such as the Cookie
protocol [RFC6265], can allow information set by one service to protocol [RFC6265], can allow information set by one service to
impact communication with other services within a matching group of impact communication with other services within a matching group of
host domains. host domains.
4.2.3. http(s) Normalization and Comparison 4.2.3. http(s) Normalization and Comparison
Since the "http" and "https" schemes conform to the URI generic The "http" and "https" URI are normalized and compared according to
syntax, such URIs are normalized and compared according to the the methods defined in Section 6 of [RFC3986], using the defaults
algorithm defined in Section 6 of [RFC3986], using the defaults
described above for each scheme. described above for each scheme.
If the port is equal to the default port for a scheme, the normal HTTP does not require use of a specific method for determining
form is to omit the port subcomponent. When not being used as the equivalence. For example, a cache key might be compared as a simple
target of an OPTIONS request, an empty path component is equivalent string, after syntax-based normalization, or after scheme-based
to an absolute path of "/", so the normal form is to provide a path normalization.
of "/" instead. The scheme and host are case-insensitive and
normally provided in lowercase; all other components are compared in Two HTTP URIs that are equivalent after normalization (using any
a case-sensitive manner. Characters other than those in the method) can be assumed to identify the same resource, and any HTTP
"reserved" set are equivalent to their percent-encoded octets: the component MAY perform normalization. As a result, distinct resources
normal form is to not encode them (see Sections 2.1 and 2.2 of SHOULD NOT be identified by HTTP URIs that are equivalent after
[RFC3986]). normalization (using any method defined in Section 6.2 of [RFC3986]).
Scheme-based normalization (Section 6.2.3 of [RFC3986]) of "http" and
"https" URIs involves the following additional rules:
* 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 as the 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: For example, the following three URIs are equivalent:
http://example.com:80/~smith/home.html http://example.com:80/~smith/home.html
http://EXAMPLE.com/%7Esmith/home.html http://EXAMPLE.com/%7Esmith/home.html
http://EXAMPLE.com:/%7esmith/home.html http://EXAMPLE.com:/%7esmith/home.html
4.2.4. Deprecation of userinfo in http(s) URIs 4.2.4. Deprecation of userinfo in http(s) URIs
The URI generic syntax for authority also includes a userinfo The URI generic syntax for authority also includes a userinfo
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4.3.4. https certificate verification 4.3.4. https certificate verification
To establish a secured connection to dereference a URI, a client MUST To establish a secured connection to dereference a URI, a client MUST
verify that the service's identity is an acceptable match for the verify that the service's identity is an acceptable match for the
URI's origin server. Certificate verification is used to prevent URI's origin server. Certificate verification is used to prevent
server impersonation by an on-path attacker or by an attacker that server impersonation by an on-path attacker or by an attacker that
controls name resolution. This process requires that a client be controls name resolution. This process requires that a client be
configured with a set of trust anchors. configured with a set of trust anchors.
In general, a client MUST verify the service identity using the In general, a client MUST verify the service identity using the
verification process defined in Section 6 of [RFC6125] (for a verification process defined in Section 6 of [RFC6125]. The client
reference identifier of type URI-ID) unless the client has been MUST construct a reference identity from the service's host: if the
specifically configured to accept some other form of verification. host is a literal IP address (Section 4.3.5), the reference identity
For example, a client might be connecting to a server whose address is an IP-ID, otherwise the host is a name and the reference identity
and hostname are dynamic, with an expectation that the service will is a DNS-ID.
present a specific certificate (or a certificate matching some
externally defined reference identity) rather than one matching the A reference identity of type CN-ID MUST NOT be used by clients. As
dynamic URI's origin server identifier. noted in Section 6.2.1 of [RFC6125] a reference identity of type CN-
ID might be used by older clients.
A client might be specially configured to accept an alternative form
of server identity verification. For example, a client might be
connecting to a server whose address and hostname are dynamic, with
an expectation that the service will present a specific certificate
(or a certificate matching some externally defined reference
identity) rather than one matching the dynamic URI's origin server
identifier.
In special cases, it might be appropriate for a client to simply In special cases, it might be appropriate for a client to simply
ignore the server's identity, but it must be understood that this ignore the server's identity, but it must be understood that this
leaves a connection open to active attack. leaves a connection open to active attack.
If the certificate is not valid for the URI's origin server, a user If the certificate is not valid for the URI's origin server, a user
agent MUST either notify the user (user agents MAY give the user an agent MUST either notify the user (user agents MAY give the user an
option to continue with the connection in any case) or terminate the option to continue with the connection in any case) or terminate the
connection with a bad certificate error. Automated clients MUST log connection with a bad certificate error. Automated clients MUST log
the error to an appropriate audit log (if available) and SHOULD the error to an appropriate audit log (if available) and SHOULD
terminate the connection (with a bad certificate error). Automated terminate the connection (with a bad certificate error). Automated
clients MAY provide a configuration setting that disables this check, clients MAY provide a configuration setting that disables this check,
but MUST provide a setting which enables it. but MUST provide a setting which enables it.
4.3.5. IP-ID reference identity
A server that is identified using an IP address literal in the "host"
field of an "https" URI has a reference identity of type IP-ID. An
IP version 4 address uses the "IPv4address" ABNF rule and an IP
version 6 address uses the "IP-literal" production with the
"IPv6address" option; see Section 3.2.2 of [RFC3986]. A reference
identity of IP-ID contains the decoded bytes of the IP address.
An IP version 4 address is 4 octets and an IP version 6 address is 16
octets. Use of IP-ID is not defined for any other IP version. The
iPAddress choice in the certificate subjectAltName extension does not
explicitly include the IP version and so relies on the length of the
address to distinguish versions; see Section 4.2.1.6 of [RFC5280].
A reference identity of type IP-ID matches if the address is
identical to an iPAddress value of the subjectAltName extension of
the certificate.
5. Fields 5. Fields
HTTP uses _fields_ to provide data in the form of extensible key/ HTTP uses _fields_ to provide data in the form of extensible key/
value pairs with a registered key namespace. Fields are sent and value pairs with a registered key namespace. Fields are sent and
received within the header and trailer sections of messages received within the header and trailer sections of messages
(Section 6). (Section 6).
5.1. Field Names 5.1. Field Names
A field name labels the corresponding field value as having the A field name labels the corresponding field value as having the
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When a field name is only present once in a section, the combined When a field name is only present once in a section, the combined
_field value_ for that field consists of the corresponding field line _field value_ for that field consists of the corresponding field line
value. When a field name is repeated within a section, its combined value. When a field name is repeated within a section, its combined
field value consists of the list of corresponding field line values field value consists of the list of corresponding field line values
within that section, concatenated in order, with each non-empty field within that section, concatenated in order, with each non-empty field
line value separated by a comma. line value separated by a comma.
For example, this section: For example, this section:
Example-Field: Foo, Bar Example-Field: Foo, Bar
Example-Field: Baz Example-Field: Baz
contains two field lines, both with the field name "Example-Field". contains two field lines, both with the field name "Example-Field".
The first field line has a field line value of "Foo, Bar", while the The first field line has a field line value of "Foo, Bar", while the
second field line value is "Baz". The field value for "Example- second field line value is "Baz". The field value for "Example-
Field" is a list with three members: "Foo", "Bar", and "Baz". Field" is a list with three members: "Foo", "Bar", and "Baz".
5.3. Field Order 5.3. Field Order
A recipient MAY combine multiple field lines within a field section A recipient MAY combine multiple field lines within a field section
that have the same field name into one field line, without changing that have the same field name into one field line, without changing
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| processing fields. (See Appendix A.2.3 of [Kri2001] for | processing fields. (See Appendix A.2.3 of [Kri2001] for
| details.) | details.)
The order in which field lines with differing field names are The order in which field lines with differing field names are
received in a section is not significant. However, it is good received in a section is not significant. However, it is good
practice to send header fields that contain additional control data practice to send header fields that contain additional control data
first, such as Host on requests and Date on responses, so that first, such as Host on requests and Date on responses, so that
implementations can decide when not to handle a message as early as implementations can decide when not to handle a message as early as
possible. possible.
A server MUST NOT apply a request to the target resource until the A server MUST NOT apply a request to the target resource until it
entire request header section is received, since later header field receives the entire request header section, since later header field
lines might include conditionals, authentication credentials, or lines might include conditionals, authentication credentials, or
deliberately misleading duplicate header fields that would impact deliberately misleading duplicate header fields that could impact
request processing. request processing.
5.4. Field Limits 5.4. Field Limits
HTTP does not place a predefined limit on the length of each field HTTP does not place a predefined limit on the length of each field
line, field value, or on the length of a header or trailer section as line, field value, or on the length of a header or trailer section as
a whole, as described in Section 2. Various ad hoc limitations on a whole, as described in Section 2. Various ad hoc limitations on
individual lengths are found in practice, often depending on the individual lengths are found in practice, often depending on the
specific field's semantics. specific field's semantics.
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fields would increase the server's vulnerability to request smuggling fields would increase the server's vulnerability to request smuggling
attacks (Section 11.2 of [Messaging]). attacks (Section 11.2 of [Messaging]).
A client MAY discard or truncate received field lines that are larger A client MAY discard or truncate received field lines that are larger
than the client wishes to process if the field semantics are such than the client wishes to process if the field semantics are such
that the dropped value(s) can be safely ignored without changing the that the dropped value(s) can be safely ignored without changing the
message framing or response semantics. message framing or response semantics.
5.5. Field Values 5.5. Field Values
HTTP field values typically have their syntax defined using ABNF HTTP field values consist of a sequence of characters in a format
([RFC5234]), using the extension defined in Section 5.6.1 as defined by the field's grammar. Each field's grammar is usually
necessary, and are usually constrained to the range of US-ASCII defined using ABNF ([RFC5234]).
characters. Fields needing a greater range of characters can use an
encoding such as the one defined in [RFC8187].
field-value = *field-content field-value = *field-content
field-content = field-vchar field-content = field-vchar
[ 1*( SP / HTAB / field-vchar ) field-vchar ] [ 1*( SP / HTAB / field-vchar ) field-vchar ]
field-vchar = VCHAR / obs-text field-vchar = VCHAR / obs-text
A field value does not include leading or trailing whitespace. When
a specific version of HTTP allows such whitespace to appear in a
message, a field parsing implementation MUST exclude such whitespace
prior to evaluating the field value.
Field values are usually constrained to the range of US-ASCII
characters [USASCII]. Fields needing a greater range of characters
can use an encoding, such as the one defined in [RFC8187].
Historically, HTTP allowed field content with text in the ISO-8859-1 Historically, HTTP allowed field content with text in the ISO-8859-1
charset [ISO-8859-1], supporting other charsets only through use of charset [ISO-8859-1], supporting other charsets only through use of
[RFC2047] encoding. In practice, most HTTP field values use only a [RFC2047] encoding. Specifications for newly defined fields SHOULD
subset of the US-ASCII charset [USASCII]. Newly defined fields limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB.
SHOULD limit their values to US-ASCII octets. A recipient SHOULD A recipient SHOULD treat other octets in field content (obs-text) as
treat other octets in field content (obs-text) as opaque data. opaque data.
Field values containing control (CTL) characters such as CR or LF are Field values containing CR or NUL characters are invalid and
invalid; recipients MUST either reject a field value containing dangerous, due to the varying ways that implementations might parse
control characters, or convert them to SP before processing or and interpret those characters; a recipient of CR or NUL within a
forwarding the message. field value MUST either reject the message or replace each of those
characters with SP before further processing or forwarding of that
message. Field values containing other CTL characters are also
invalid; however, recipients MAY retain such characters for the sake
of robustness if they only appear within safe field value contexts
(e.g., opaque data).
Leading and trailing whitespace in raw field values is removed upon Fields that only anticipate a single member as the field value are
field parsing (e.g., Section 5.1 of [Messaging]). Field definitions referred to as _singleton fields_.
where leading or trailing whitespace in values is significant will
have to use a container syntax such as quoted-string (Section 5.6.4).
Commas (",") often are used to separate members in field values. Fields that allow multiple members as the field value are referred to
Fields that allow multiple members are referred to as _list-based as _list-based fields_. The list operator extension of Section 5.6.1
fields_. Fields that only anticipate a single member are referred to is used as a common notation for defining field values that can
as _singleton fields_. contain multiple members.
Because commas are used as a generic delimiter between members, they Because commas (",") are used as the delimiter between members, they
need to be treated with care if they are allowed as data within a need to be treated with care if they are allowed as data within a
member. This is true for both list-based and singleton fields, since member. This is true for both list-based and singleton fields, since
a singleton field might be sent with multiple members erroneously; a singleton field might be erroneously sent with multiple members and
being able to detect this condition improves interoperability. detecting such errors improves interoperability. Fields that expect
Fields that expect to contain a comma within a member, such as an to contain a comma within a member, such as within an HTTP-date or
HTTP-date or URI-reference element, ought to be defined with URI-reference element, ought to be defined with delimiters around
delimiters around that element to distinguish any comma within that that element to distinguish any comma within that data from potential
data from potential list separators. list separators.
For example, a textual date and a URI (either of which might contain For example, a textual date and a URI (either of which might contain
a comma) could be safely carried in list-based field values like a comma) could be safely carried in list-based field values like
these: these:
Example-URI-Field: "http://example.com/a.html,foo", Example-URIs: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/" "http://without-a-comma.example.com/"
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005" Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double-quote delimiters are almost always used with the
Note that double-quote delimiters almost always are used with the
quoted-string production; using a different syntax inside double- quoted-string production; using a different syntax inside double-
quotes will likely cause unnecessary confusion. quotes will likely cause unnecessary confusion.
Many fields (such as Content-Type, defined in Section 8.3) use a Many fields (such as Content-Type, defined in Section 8.3) use a
common syntax for parameters that allows both unquoted (token) and common syntax for parameters that allows both unquoted (token) and
quoted (quoted-string) syntax for a parameter value (Section 5.6.6). quoted (quoted-string) syntax for a parameter value (Section 5.6.6).
Use of common syntax allows recipients to reuse existing parser Use of common syntax allows recipients to reuse existing parser
components. When allowing both forms, the meaning of a parameter components. When allowing both forms, the meaning of a parameter
value ought to be the same whether it was received as a token or a value ought to be the same whether it was received as a token or a
quoted string. quoted string.
Historically, HTTP field values could be extended over multiple lines Historically, HTTP field values could be extended over multiple lines
by preceding each extra line with at least one space or horizontal by preceding each extra line with at least one space or horizontal
tab (obs-fold). This document assumes that any such obsolete line tab (obs-fold). This document assumes that any such obsolete line
folding has been replaced with one or more SP octets prior to folding has been removed prior to interpreting the field value (e.g.,
interpreting the field value, as described in Section 5.2 of as described in Section 5.2 of [Messaging]).
[Messaging].
| *Note:* This specification does not use ABNF rules to define | *Note:* For defining field value syntax, this specification
| each "Field Name: Field Value" pair, as was done in earlier | uses an ABNF rule named after the field name to define the
| editions (published before [RFC7230]). Instead, ABNF rules are | allowed grammar for that field's value (after said value has
| named according to each registered field name, wherein the rule | been extracted from the underlying messaging syntax and
| defines the valid grammar for that field's corresponding field | multiple instances combined into a list).
| values (i.e., after the field value has been extracted by a
| generic field parser).
5.6. Common Rules for Defining Field Values 5.6. Common Rules for Defining Field Values
5.6.1. Lists (#rule ABNF Extension) 5.6.1. Lists (#rule ABNF Extension)
A #rule extension to the ABNF rules of [RFC5234] is used to improve A #rule extension to the ABNF rules of [RFC5234] is used to improve
readability in the definitions of some list-based field values. readability in the definitions of some list-based field values.
A construct "#" is defined, similar to "*", for defining comma- A construct "#" is defined, similar to "*", for defining comma-
delimited lists of elements. The full form is "<n>#<m>element" delimited lists of elements. The full form is "<n>#<m>element"
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In contrast, the following values would be invalid, since at least In contrast, the following values would be invalid, since at least
one non-empty element is required by the example-list production: one non-empty element is required by the example-list production:
"" ""
"," ","
", ," ", ,"
5.6.2. Tokens 5.6.2. Tokens
Many HTTP field values are defined using common syntax components,
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 "(),/:;<=>?@[\]{}").
Tokens are short textual identifiers that do not include whitespace Tokens are short textual identifiers that do not include whitespace
or delimiters. or delimiters.
token = 1*tchar token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA / DIGIT / ALPHA
; any VCHAR, except delimiters ; any VCHAR, except delimiters
Many HTTP field values are defined using common syntax components,
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 "(),/:;<=>?@[\]{}").
5.6.3. Whitespace 5.6.3. Whitespace
This specification uses three rules to denote the use of linear This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace). BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets The OWS rule is used where zero or more linear whitespace octets
might appear. For protocol elements where optional whitespace is might appear. For protocol elements where optional whitespace is
preferred to improve readability, a sender SHOULD generate the preferred to improve readability, a sender SHOULD generate the
optional whitespace as a single SP; otherwise, a sender SHOULD NOT optional whitespace as a single SP; otherwise, a sender SHOULD NOT
generate optional whitespace except as needed to white out invalid or generate optional whitespace except as needed to overwrite invalid or
unwanted protocol elements during in-place message filtering. unwanted protocol elements during in-place message filtering.
The RWS rule is used when at least one linear whitespace octet is 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 required to separate field tokens. A sender SHOULD generate RWS as a
single SP. single SP.
OWS and RWS have the same semantics as a single SP. Any content OWS and RWS have the same semantics as a single SP. Any content
known to be defined as OWS or RWS MAY be replaced with a single SP known to be defined as OWS or RWS MAY be replaced with a single SP
before interpreting it or forwarding the message downstream. before interpreting it or forwarding the message downstream.
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A _message_ consists of control data to describe and route the A _message_ consists of control data to describe and route the
message, a headers lookup table of key/value pairs for extending that message, a headers lookup table of key/value pairs for extending that
control data and conveying additional information about the sender, control data and conveying additional information about the sender,
message, content, or context, a potentially unbounded stream of message, content, or context, a potentially unbounded stream of
content, and a trailers lookup table of key/value pairs for content, and a trailers lookup table of key/value pairs for
communicating information obtained while sending the content. communicating information obtained while sending the content.
Framing and control data is sent first, followed by a header section Framing and control data is sent first, followed by a header section
containing fields for the headers table. When a message includes containing fields for the headers table. When a message includes
content, the content is sent after the header section and potentially content, the content is sent after the header section, potentially
interleaved with zero or more trailer sections containing fields for followed by a trailer section that might contain fields for the
the trailers table. trailers table.
Messages are expected to be processed as a stream, wherein the Messages are expected to be processed as a stream, wherein the
purpose of that stream and its continued processing is revealed while purpose of that stream and its continued processing is revealed while
being read. Hence, control data describes what the recipient needs being read. Hence, control data describes what the recipient needs
to know immediately, header fields describe what needs to be known to know immediately, header fields describe what needs to be known
before receiving content, the content (when present) presumably before receiving content, the content (when present) presumably
contains what the recipient wants or needs to fulfill the message contains what the recipient wants or needs to fulfill the message
semantics, and trailer fields provide additional metadata that can be semantics, and trailer fields provide optional metadata that was
dropped (safely ignored) when not desired. unknown prior to sending the content.
Messages are intended to be _self-descriptive_: everything a Messages are intended to be _self-descriptive_: everything a
recipient needs to know about the message can be determined by recipient needs to know about the message can be determined by
looking at the message itself, after decoding or reconstituting parts looking at the message itself, after decoding or reconstituting parts
that have been compressed or elided in transit, without requiring an that have been compressed or elided in transit, without requiring an
understanding of the sender's current application state (established understanding of the sender's current application state (established
via prior messages). via prior messages). However, a client MUST retain knowledge of the
request when parsing, interpreting, or caching a corresponding
response. For example, responses to the HEAD method look just like
the beginning of a response to GET, but cannot be parsed in the same
manner.
Note that this message abstraction is a generalization across many Note that this message abstraction is a generalization across many
versions of HTTP, including features that might not be found in some versions of HTTP, including features that might not be found in some
versions. For example, trailers were introduced within the HTTP/1.1 versions. For example, trailers were introduced within the HTTP/1.1
chunked transfer coding as a single trailer section after the chunked transfer coding as a trailer section after the content. An
content. An equivalent feature is present in HTTP/2 and HTTP/3 equivalent feature is present in HTTP/2 and HTTP/3 within the header
within the header block that terminates each stream. However, block that terminates each stream.
multiple trailer sections interleaved with content have only been
deployed as frame extensions.
6.1. Framing and Completeness 6.1. Framing and Completeness
Message framing indicates how each message begins and ends, such that Message framing indicates how each message begins and ends, such that
each message can be distinguished from other messages or noise on the each message can be distinguished from other messages or noise on the
same connection. Each major version of HTTP defines its own framing same connection. Each major version of HTTP defines its own framing
mechanism. mechanism.
HTTP/0.9 and early deployments of HTTP/1.0 used closure of the HTTP/0.9 and early deployments of HTTP/1.0 used closure of the
underlying connection to end a response. For backwards underlying connection to end a response. For backwards
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level error indicates that it is not complete. level error indicates that it is not complete.
6.2. Control Data 6.2. Control Data
Messages start with control data that describe its primary purpose. Messages start with control data that describe its primary purpose.
Request message control data includes a request method (Section 9), Request message control data includes a request method (Section 9),
request target (Section 7.1), and protocol version (Section 2.5). request target (Section 7.1), and protocol version (Section 2.5).
Response message control data includes a status code (Section 15), Response message control data includes a status code (Section 15),
optional reason phrase, and protocol version. optional reason phrase, and protocol version.
In HTTP/1.1 [Messaging] and earlier, control data is sent as the In HTTP/1.1 ([Messaging]) and earlier, control data is sent as the
first line of a message. In HTTP/2 ([RFC7540]) and HTTP/3 ([HTTP3]), first line of a message. In HTTP/2 ([RFC7540]) and HTTP/3 ([HTTP3]),
control data is sent as pseudo-header fields with a reserved name control data is sent as pseudo-header fields with a reserved name
prefix (e.g., ":authority"). prefix (e.g., ":authority").
Every HTTP message has a protocol version. Depending on the version Every HTTP message has a protocol version. Depending on the version
in use, it might be identified within the message explicitly or in use, it might be identified within the message explicitly or
inferred by the connection over which the message is received. inferred by the connection over which the message is received.
Recipients use that version information to determine limitations or Recipients use that version information to determine limitations or
potential for later communication with that sender. potential for later communication with that sender.
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from the response status code or header fields (e.g., Server) that from the response status code or header fields (e.g., Server) that
the server improperly handles higher request versions. the server improperly handles higher request versions.
A server SHOULD send a response version equal to the highest version 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 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 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 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, (HTTP Version Not Supported) response if it wishes, for any reason,
to refuse service of the client's major protocol version. to refuse service of the client's major protocol version.
When an HTTP message is received with a major version number that the A recipient that receives a message with a major version number that
recipient implements, but a higher minor version number than what the it implements and a minor version number higher than what it
recipient implements, the recipient SHOULD process the message as if implements SHOULD process the message as if it were in the highest
it were in the highest minor version within that major version to minor version within that major version to which the recipient is
which the recipient is conformant. A recipient can assume that a conformant. A recipient can assume that a message with a higher
message with a higher minor version, when sent to a recipient that minor version, when sent to a recipient that has not yet indicated
has not yet indicated support for that higher version, is support for that higher version, is sufficiently backwards-compatible
sufficiently backwards-compatible to be safely processed by any to be safely processed by any implementation of the same major
implementation of the same major version. version.
6.3. Header Fields 6.3. Header Fields
Fields (Section 5) that are sent/received before the content are Fields (Section 5) that are sent/received before the content are
referred to as "header fields" (or just "headers", colloquially). referred to as "header fields" (or just "headers", colloquially).
The _header section_ of a message consists of a sequence of of header The _header section_ of a message consists of a sequence of header
field lines. Each header field might modify or extend message field lines. Each header field might modify or extend message
semantics, describe the sender, define the content, or provide semantics, describe the sender, define the content, or provide
additional context. additional context.
| *Note:* We refer to named fields specifically as a "header | *Note:* We refer to named fields specifically as a "header
| field" when they are only allowed to be sent in the header | field" when they are only allowed to be sent in the header
| section. | section.
6.4. Content 6.4. Content
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6.4.2. Identifying Content 6.4.2. Identifying Content
When a complete or partial representation is transferred as message When a complete or partial representation is transferred as message
content, it is often desirable for the sender to supply, or the content, it is often desirable for the sender to supply, or the
recipient to determine, an identifier for a resource corresponding to recipient to determine, an identifier for a resource corresponding to
that representation. that representation.
For a request message: For a request message:
o If the request has a Content-Location header field, then the * If the request has a Content-Location header field, then the
sender asserts that the content is a representation of the sender asserts that the content is a representation of the
resource identified by the Content-Location field value. However, resource identified by the Content-Location field value. However,
such an assertion cannot be trusted unless it can be verified by such an assertion cannot be trusted unless it can be verified by
other means (not defined by this specification). The information other means (not defined by this specification). The information
might still be useful for revision history links. might still be useful for revision history links.
o Otherwise, the content is unidentified. * Otherwise, the content is unidentified.
For a response message, the following rules are applied in order For a response message, the following rules are applied in order
until a match is found: until a match is found:
1. If the request method is HEAD or the response status code is 204 1. If the request method is HEAD or the response status code is 204
(No Content) or 304 (Not Modified), there is no content in the (No Content) or 304 (Not Modified), there is no content in the
response. response.
2. If the request method is GET and the response status code is 200 2. If the request method is GET and the response status code is 200
(OK), the content is a representation of the resource identified (OK), the content is a representation of the resource identified
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value is a reference to a URI different from the target URI, then value is a reference to a URI different from the target URI, then
the sender asserts that the content is a representation of the the sender asserts that the content is a representation of the
resource identified by the Content-Location field value. resource identified by the Content-Location field value.
However, such an assertion cannot be trusted unless it can be However, such an assertion cannot be trusted unless it can be
verified by other means (not defined by this specification). verified by other means (not defined by this specification).
7. Otherwise, the content is unidentified. 7. Otherwise, the content is unidentified.
6.5. Trailer Fields 6.5. Trailer Fields
Fields (Section 5) that are sent/received after the header section Fields (Section 5) that are located within a _trailer section_ are
has ended (usually after the content begins to stream) are referred are referred to as "trailer fields" (or just "trailers",
to as "trailer fields" (or just "trailers", colloquially) and located colloquially). Trailer fields can be useful for supplying message
within a _trailer section_. Trailer fields can be useful for integrity checks, digital signatures, delivery metrics, or post-
supplying message integrity checks, digital signatures, delivery processing status information.
metrics, or post-processing status information.
Trailer fields ought to be processed and stored separately from the Trailer fields ought to be processed and stored separately from the
fields in the header section to avoid contradicting message semantics fields in the header section to avoid contradicting message semantics
known at the time the header section was complete. The presence or known at the time the header section was complete. The presence or
absence of certain header fields might impact choices made for the absence of certain header fields might impact choices made for the
routing or processing of the message as a whole before the trailers routing or processing of the message as a whole before the trailers
are received; those choices cannot be unmade by the later discovery are received; those choices cannot be unmade by the later discovery
of trailer fields. of trailer fields.
6.5.1. Limitations on use of Trailers 6.5.1. Limitations on use of Trailers
Trailer sections are only possible when supported by the version of A trailer section is only possible when supported by the version of
HTTP in use and enabled by an explicit framing mechanism. For HTTP in use and enabled by an explicit framing mechanism. For
example, the chunked coding in HTTP/1.1 allows a trailer section to example, the chunked coding in HTTP/1.1 allows a trailer section to
be sent after the content (Section 7.1.2 of [Messaging]). be sent after the content (Section 7.1.2 of [Messaging]).
Many fields cannot be processed outside the header section because Many fields cannot be processed outside the header section because
their evaluation is necessary prior to receiving the content, such as their evaluation is necessary prior to receiving the content, such as
those that describe message framing, routing, authentication, request those that describe message framing, routing, authentication, request
modifiers, response controls, or content format. A sender MUST NOT modifiers, response controls, or content format. A sender MUST NOT
generate a trailer field unless the sender knows the corresponding generate a trailer field unless the sender knows the corresponding
header field name's definition permits the field to be sent in header field name's definition permits the field to be sent in
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forward messages from one protocol version to another. If the entire forward messages from one protocol version to another. If the entire
message can be buffered in transit, some intermediaries could merge message can be buffered in transit, some intermediaries could merge
trailer fields into the header section (as appropriate) before it is trailer fields into the header section (as appropriate) before it is
forwarded. However, in most cases, the trailers are simply forwarded. However, in most cases, the trailers are simply
discarded. A recipient MUST NOT merge a trailer field into a header discarded. A recipient MUST NOT merge a trailer field into a header
section unless the recipient understands the corresponding header section unless the recipient understands the corresponding header
field definition and that definition explicitly permits and defines field definition and that definition explicitly permits and defines
how trailer field values can be safely merged. how trailer field values can be safely merged.
The presence of the keyword "trailers" in the TE header field The presence of the keyword "trailers" in the TE header field
(Section 10.1.4) indicates that the client is willing to accept (Section 10.1.4) of a request indicates that the client is willing to
trailer fields, on behalf of itself and any downstream clients. For accept trailer fields, on behalf of itself and any downstream
requests from an intermediary, this implies that all downstream clients. For requests from an intermediary, this implies that all
clients are willing to accept trailer fields in the forwarded downstream clients are willing to accept trailer fields in the
response. Note that the presence of "trailers" does not mean that forwarded response. Note that the presence of "trailers" does not
the client(s) will process any particular trailer field in the mean that the client(s) will process any particular trailer field in
response; only that the trailer section(s) will not be dropped by any the response; only that the trailer section(s) will not be dropped by
of the clients. any of the clients.
Because of the potential for trailer fields to be discarded in Because of the potential for trailer fields to be discarded in
transit, a server SHOULD NOT generate trailer fields that it believes transit, a server SHOULD NOT generate trailer fields that it believes
are necessary for the user agent to receive. are necessary for the user agent to receive.
6.5.2. Processing Trailer Fields 6.5.2. Processing Trailer Fields
The "Trailer" header field (Section 10.1.5) can be sent to indicate
fields likely to be sent in the trailer section, which allows
recipients to prepare for their receipt before processing the
content. For example, this could be useful if a field name indicates
that a dynamic checksum should be calculated as the content is
received and then immediately checked upon receipt of the trailer
field value.
Like header fields, trailer fields with the same name are processed Like header fields, trailer fields with the same name are processed
in the order received; multiple trailer field lines with the same in the order received; multiple trailer field lines with the same
name have the equivalent semantics as appending the multiple values name have the equivalent semantics as appending the multiple values
as a list of members, even when the field lines are received in as a list of members. Trailer fields that might be generated more
separate trailer sections. Trailer fields that might be generated than once during a message MUST be defined as a list-based field even
more than once during a message MUST be defined as a list-based field if each member value is only processed once per field line received.
even if each member value is only processed once per field line
received.
Trailer fields are expected (but not required) to be processed one
trailer section at a time. That is, for each trailer section
received, a recipient that is looking for trailer fields will parse
the received section into fields, invoke any associated processing
for those fields at that point in the message processing, and then
append those fields to the set of trailer fields received for the
overall message.
This behavior allows for iterative processing of trailer fields that
contain incremental signatures or mid-stream status information, and
fields that might refer to each other's values within the same
section. However, there is no guarantee that trailer sections won't
shift in relation to the content stream, or won't be recombined (or
dropped) in transit. Trailer fields that refer to data outside the
present trailer section need to use self-descriptive references
(i.e., refer to the data by name, ordinal position, or an octet
range) rather than assume it is the data most recently received.
Likewise, at the end of a message, a recipient MAY treat the entire At the end of a message, a recipient MAY treat the set of received
set of received trailer fields as one data structure to be considered trailer fields as a data structure of key/value pairs, similar to
as the message concludes. Additional processing expectations, if (but separate from) the header fields. Additional processing
any, can be defined within the field specification for a field expectations, if any, can be defined within the field specification
intended for use in trailers. for a field intended for use in trailers.
7. Routing HTTP Messages 7. Routing HTTP Messages
HTTP request message routing is determined by each client based on HTTP request message routing is determined by each client based on
the target resource, the client's proxy configuration, and the target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the response routing follows the same connection chain back to the
client. client.
7.1. Determining the Target Resource 7.1. Determining the Target Resource
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To perform an action on a _target resource_, the client sends a To perform an action on a _target resource_, the client sends a
request message containing enough components of its parsed target URI request message containing enough components of its parsed target URI
to enable recipients to identify that same resource. For historical to enable recipients to identify that same resource. For historical
reasons, the parsed target URI components, collectively referred to reasons, the parsed target URI components, collectively referred to
as the _request target_, are sent within the message control data and as the _request target_, are sent within the message control data and
the Host header field (Section 7.2). the Host header field (Section 7.2).
There are two unusual cases for which the request target components There are two unusual cases for which the request target components
are in a method-specific form: are in a method-specific form:
o For CONNECT (Section 9.3.6), the request target is the host name * For CONNECT (Section 9.3.6), the request target is the host name
and port number of the tunnel destination, separated by a colon. and port number of the tunnel destination, separated by a colon.
o For OPTIONS (Section 9.3.7), the request target can be a single * For OPTIONS (Section 9.3.7), the request target can be a single
asterisk ("*"). asterisk ("*").
See the respective method definitions for details. These forms MUST See the respective method definitions for details. These forms MUST
NOT be used with other methods. NOT be used with other methods.
Upon receipt of a client's request, a server reconstructs the target Upon receipt of a client's request, a server reconstructs the target
URI from the received components in accordance with their local URI from the received components in accordance with their local
configuration and incoming connection context. This reconstruction configuration and incoming connection context. This reconstruction
is specific to each major protocol version. For example, Appendix of is specific to each major protocol version. For example, Appendix of
[Messaging] defines how a server determines the target URI of an [Messaging] defines how a server determines the target URI of an
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distinguish among resources while servicing requests for multiple distinguish among resources while servicing requests for multiple
host names. host names.
In HTTP/2 [RFC7540] and HTTP/3 [HTTP3], the Host header field is, in In HTTP/2 [RFC7540] and HTTP/3 [HTTP3], the Host header field is, in
some cases, supplanted by the ":authority" pseudo-header field of a some cases, supplanted by the ":authority" pseudo-header field of a
request's control data. request's control data.
Host = uri-host [ ":" port ] ; Section 4 Host = uri-host [ ":" port ] ; Section 4
The target URI's authority information is critical for handling a The target URI's authority information is critical for handling a
request. A user agent SHOULD generate Host as the first field in the request. A user agent MUST generate a Host header field in a request
header section of a request unless it has already generated that unless it sends that information as an ":authority" pseudo-header
information as an ":authority" pseudo-header field. field. A user agent that sends Host SHOULD send it as the first
field in the header section of a request.
For example, a GET request to the origin server for For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with: <http://www.example.org/pub/WWW/> would begin with:
GET /pub/WWW/ HTTP/1.1 GET /pub/WWW/ HTTP/1.1
Host: www.example.org Host: www.example.org
Since the host and port information acts as an application-level Since the host and port information acts as an application-level
routing mechanism, it is a frequent target for malware seeking to routing mechanism, it is a frequent target for malware seeking to
poison a shared cache or redirect a request to an unintended server. poison a shared cache or redirect a request to an unintended server.
An interception proxy is particularly vulnerable if it relies on the An interception proxy is particularly vulnerable if it relies on the
host and port information for redirecting requests to internal host and port information for redirecting requests to internal
servers, or for use as a cache key in a shared cache, without first servers, or for use as a cache key in a shared cache, without first
verifying that the intercepted connection is targeting a valid IP verifying that the intercepted connection is targeting a valid IP
address for that host. address for that host.
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that the origin server has rejected the request because it appears to that the origin server has rejected the request because it appears to
have been misdirected (Section 15.5.20). have been misdirected (Section 15.5.20).
7.5. Response Correlation 7.5. Response Correlation
A connection might be used for multiple request/response exchanges. A connection might be used for multiple request/response exchanges.
The mechanism used to correlate between request and response messages The mechanism used to correlate between request and response messages
is version dependent; some versions of HTTP use implicit ordering of is version dependent; some versions of HTTP use implicit ordering of
messages, while others use an explicit identifier. messages, while others use an explicit identifier.
Responses (both final and interim) can be sent at any time after a All responses, regardless of the status code (including interim
request is received, even if it is not yet complete. However, responses) can be sent at any time after a request is received, even
clients (including intermediaries) might abandon a request if the if the request is not yet complete. A response can complete before
response is not forthcoming within a reasonable period of time. its corresponding request is complete. Likewise, clients are not
expected to wait any specific amount of time for a response. Clients
(including intermediaries) might abandon a request if the response is
not forthcoming within a reasonable period of time.
A client that receives a response while it is still sending the
associated request SHOULD continue sending that request, unless it
receives an explicit indication to the contrary (see, e.g.,
Section 9.5 of [Messaging] and Section 6.4 of [RFC7540]).
7.6. Message Forwarding 7.6. Message Forwarding
As described in Section 3.7, intermediaries can serve a variety of As described in Section 3.7, intermediaries can serve a variety of
roles in the processing of HTTP requests and responses. Some roles in the processing of HTTP requests and responses. Some
intermediaries are used to improve performance or availability. intermediaries are used to improve performance or availability.
Others are used for access control or to filter content. Since an Others are used for access control or to filter content. Since an
HTTP stream has characteristics similar to a pipe-and-filter HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an architecture, there are no inherent limits to the extent an
intermediary can enhance (or interfere) with either direction of the intermediary can enhance (or interfere) with either direction of the
stream. stream.
Intermediaries are expected to forward messages even when protocol
elements are not recognized (e.g., new methods, status codes, or
field names), since that preserves extensibility for downstream
recipients.
An intermediary not acting as a tunnel MUST implement the Connection An intermediary not acting as a tunnel MUST implement the Connection
header field, as specified in Section 7.6.1, and exclude fields from header field, as specified in Section 7.6.1, and exclude fields from
being forwarded that are only intended for the incoming connection. being forwarded that are only intended for the incoming connection.
An intermediary MUST NOT forward a message to itself unless it is An intermediary MUST NOT forward a message to itself unless it is
protected from an infinite request loop. In general, an intermediary protected from an infinite request loop. In general, an intermediary
ought to recognize its own server names, including any aliases, local ought to recognize its own server names, including any aliases, local
variations, or literal IP addresses, and respond to such requests variations, or literal IP addresses, and respond to such requests
directly. directly.
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recipients on the chain ("end-to-end"), enabling the message to be recipients on the chain ("end-to-end"), enabling the message to be
self-descriptive and allowing future connection-specific extensions self-descriptive and allowing future connection-specific extensions
to be deployed without fear that they will be blindly forwarded by to be deployed without fear that they will be blindly forwarded by
older intermediaries. older intermediaries.
Furthermore, intermediaries SHOULD remove or replace field(s) whose Furthermore, intermediaries SHOULD remove or replace field(s) whose
semantics are known to require removal before forwarding, whether or semantics are known to require removal before forwarding, whether or
not they appear as a Connection option, after applying those fields' not they appear as a Connection option, after applying those fields'
semantics. This includes but is not limited to: semantics. This includes but is not limited to:
o Proxy-Connection (Appendix C.2.2 of [Messaging]) * Proxy-Connection (Appendix C.2.2 of [Messaging])
o Keep-Alive (Section 19.7.1 of [RFC2068])
o TE (Section 10.1.4) * Keep-Alive (Section 19.7.1 of [RFC2068])
o Trailer (Section 10.1.5) * TE (Section 10.1.4)
o Transfer-Encoding (Section 6.1 of [Messaging]) * Transfer-Encoding (Section 6.1 of [Messaging])
o Upgrade (Section 7.8) * Upgrade (Section 7.8)
The Connection header field's value has the following grammar: The Connection header field's value has the following grammar:
Connection = #connection-option Connection = #connection-option
connection-option = token connection-option = token
Connection options are case-insensitive. Connection options are case-insensitive.
A sender MUST NOT send a connection option corresponding to a field A sender MUST NOT send a connection option corresponding to a field
that is intended for all recipients of the content. For example, that is intended for all recipients of the content. For example,
Cache-Control is never appropriate as a connection option Cache-Control is never appropriate as a connection option
(Section 5.2 of [Caching]). (Section 5.2 of [Caching]).
The connection options do not always correspond to a field present in Connection options do not always correspond to a field present in the
the message, since a connection-specific field might not be needed if message, since a connection-specific field might not be needed if
there are no parameters associated with a connection option. In there are no parameters associated with a connection option. In
contrast, a connection-specific field that is received without a contrast, a connection-specific field received without a
corresponding connection option usually indicates that the field has corresponding connection option usually indicates that the field has
been improperly forwarded by an intermediary and ought to be ignored been improperly forwarded by an intermediary and ought to be ignored
by the recipient. by the recipient.
When defining new connection options, specification authors ought to When defining a new connection option that does not correspond to a
document it as reserved field name and register that definition in field, specification authors ought to reserve the corresponding field
the Hypertext Transfer Protocol (HTTP) Field Name Registry name anyway in order to avoid later collisions. Such reserved field
(Section 16.3.1), to avoid collisions. names are registered in the Hypertext Transfer Protocol (HTTP) Field
Name Registry (Section 16.3.1).
7.6.2. Max-Forwards 7.6.2. Max-Forwards
The "Max-Forwards" header field provides a mechanism with the TRACE The "Max-Forwards" header field provides a mechanism with the TRACE
(Section 9.3.8) and OPTIONS (Section 9.3.7) request methods to limit (Section 9.3.8) and OPTIONS (Section 9.3.7) request methods to limit
the number of times that the request is forwarded by proxies. This the number of times that the request is forwarded by proxies. This
can be useful when the client is attempting to trace a request that can be useful when the client is attempting to trace a request that
appears to be failing or looping mid-chain. appears to be failing or looping mid-chain.
Max-Forwards = 1*DIGIT Max-Forwards = 1*DIGIT
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However, comments in Via are optional, and a recipient MAY remove However, comments in Via are optional, and a recipient MAY remove
them prior to forwarding the message. them prior to forwarding the message.
For example, a request message could be sent from an HTTP/1.0 user 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 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 forward the request to a public proxy at p.example.net, which
completes the request by forwarding it to the origin server at completes the request by forwarding it to the origin server at
www.example.com. The request received by www.example.com would then www.example.com. The request received by www.example.com would then
have the following Via header field: have the following Via header field:
Via: 1.0 fred, 1.1 p.example.net Via: 1.0 fred, 1.1 p.example.net
An intermediary used as a portal through a network firewall SHOULD An intermediary used as a portal through a network firewall SHOULD
NOT forward the names and ports of hosts within the firewall region 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 unless it is explicitly enabled to do so. If not enabled, such an
intermediary SHOULD replace each received-by host of any host behind intermediary SHOULD replace each received-by host of any host behind
the firewall by an appropriate pseudonym for that host. the firewall by an appropriate pseudonym for that host.
An intermediary MAY combine an ordered subsequence of Via header An intermediary MAY combine an ordered subsequence of Via header
field list members into a single member if the entries have identical field list members into a single member if the entries have identical
received-protocol values. For example, received-protocol values. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
A sender SHOULD NOT combine multiple list members unless they are all A sender SHOULD NOT combine multiple list members unless they are all
under the same organizational control and the hosts have already been under the same organizational control and the hosts have already been
replaced by pseudonyms. A sender MUST NOT combine members that have replaced by pseudonyms. A sender MUST NOT combine members that have
different received-protocol values. different received-protocol values.
7.7. Message Transformations 7.7. Message Transformations
Some intermediaries include features for transforming messages and Some intermediaries include features for transforming messages and
their content. A proxy might, for example, convert between image their content. A proxy might, for example, convert between image
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Upgrade header field to indicate the acceptable protocols, in order Upgrade header field to indicate the acceptable protocols, in order
of descending preference. of descending preference.
A server MAY send an Upgrade header field in any other response to A server MAY send an Upgrade header field in any other response to
advertise that it implements support for upgrading to the listed advertise that it implements support for upgrading to the listed
protocols, in order of descending preference, when appropriate for a protocols, in order of descending preference, when appropriate for a
future request. future request.
The following is a hypothetical example sent by a client: The following is a hypothetical example sent by a client:
GET /hello HTTP/1.1 GET /hello HTTP/1.1
Host: www.example.com Host: www.example.com
Connection: upgrade Connection: upgrade
Upgrade: websocket, IRC/6.9, RTA/x11 Upgrade: websocket, IRC/6.9, RTA/x11
The capabilities and nature of the application-level communication The capabilities and nature of the application-level communication
after the protocol change is entirely dependent upon the new after the protocol change is entirely dependent upon the new
protocol(s) chosen. However, immediately after sending the 101 protocol(s) chosen. However, immediately after sending the 101
(Switching Protocols) response, the server is expected to continue (Switching Protocols) response, the server is expected to continue
responding to the original request as if it had received its responding to the original request as if it had received its
equivalent within the new protocol (i.e., the server still has an equivalent within the new protocol (i.e., the server still has an
outstanding request to satisfy after the protocol has been changed, outstanding request to satisfy after the protocol has been changed,
and is expected to do so without requiring the request to be and is expected to do so without requiring the request to be
repeated). repeated).
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follows that with the new protocol's equivalent of a response to a follows that with the new protocol's equivalent of a response to a
GET on the target resource. This allows a connection to be upgraded GET on the target resource. This allows a connection to be upgraded
to protocols with the same semantics as HTTP without the latency cost to protocols with the same semantics as HTTP without the latency cost
of an additional round trip. A server MUST NOT switch protocols of an additional round trip. A server MUST NOT switch protocols
unless the received message semantics can be honored by the new unless the received message semantics can be honored by the new
protocol; an OPTIONS request can be honored by any protocol. protocol; an OPTIONS request can be honored by any protocol.
The following is an example response to the above hypothetical The following is an example response to the above hypothetical
request: request:
HTTP/1.1 101 Switching Protocols HTTP/1.1 101 Switching Protocols
Connection: upgrade Connection: upgrade
Upgrade: websocket Upgrade: websocket
[... data stream switches to websocket with an appropriate response [... data stream switches to websocket with an appropriate response
(as defined by new protocol) to the "GET /hello" request ...] (as defined by new protocol) to the "GET /hello" request ...]
When Upgrade is sent, the sender MUST also send a Connection header A sender of Upgrade MUST also send an "Upgrade" connection option in
field (Section 7.6.1) that contains an "upgrade" connection option, the Connection header field (Section 7.6.1) to inform intermediaries
in order to prevent Upgrade from being accidentally forwarded by not to forward this field. A server that receives an Upgrade header
intermediaries that might not implement the listed protocols. A field in an HTTP/1.0 request MUST ignore that Upgrade field.
server MUST ignore an Upgrade header field that is received in an
HTTP/1.0 request.
A client cannot begin using an upgraded protocol on the connection A client cannot begin using an upgraded protocol on the connection
until it has completely sent the request message (i.e., the client until it has completely sent the request message (i.e., the client
can't change the protocol it is sending in the middle of a message). can't change the protocol it is sending in the middle of a message).
If a server receives both an Upgrade and an Expect header field with If a server receives both an Upgrade and an Expect header field with
the "100-continue" expectation (Section 10.1.1), the server MUST send the "100-continue" expectation (Section 10.1.1), the server MUST send
a 100 (Continue) response before sending a 101 (Switching Protocols) a 100 (Continue) response before sending a 101 (Switching Protocols)
response. response.
The Upgrade header field only applies to switching protocols on top The Upgrade header field only applies to switching protocols on top
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The representation data associated with an HTTP message is either The representation data associated with an HTTP message is either
provided as the content of the message or referred to by the message provided as the content of the message or referred to by the message
semantics and the target URI. The representation data is in a format semantics and the target URI. The representation data is in a format
and encoding defined by the representation metadata header fields. and encoding defined by the representation metadata header fields.
The data type of the representation data is determined via the header The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer, fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model: ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) ) representation-data := Content-Encoding( Content-Type( data ) )
8.2. Representation Metadata 8.2. Representation Metadata
Representation header fields provide metadata about the Representation header fields provide metadata about the
representation. When a message includes content, the representation representation. When a message includes content, the representation
header fields describe how to interpret that data. In a response to header fields describe how to interpret that data. In a response to
a HEAD request, the representation header fields describe the a HEAD request, the representation header fields describe the
representation data that would have been enclosed in the content if representation data that would have been enclosed in the content if
the same request had been a GET. the same request had been a GET.
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message content or the selected representation, as determined by the message content or the selected representation, as determined by the
message semantics. The indicated media type defines both the data message semantics. The indicated media type defines both the data
format and how that data is intended to be processed by a recipient, format and how that data is intended to be processed by a recipient,
within the scope of the received message semantics, after any content within the scope of the received message semantics, after any content
codings indicated by Content-Encoding are decoded. codings indicated by Content-Encoding are decoded.
Content-Type = media-type Content-Type = media-type
Media types are defined in Section 8.3.1. An example of the field is Media types are defined in Section 8.3.1. An example of the field is
Content-Type: text/html; charset=ISO-8859-4 Content-Type: text/html; charset=ISO-8859-4
A sender that generates a message containing content SHOULD generate A sender that generates a message containing content SHOULD generate
a Content-Type header field in that message unless the intended media a Content-Type header field in that message unless the intended media
type of the enclosed representation is unknown to the sender. If a type of the enclosed representation is unknown to the sender. If a
Content-Type header field is not present, the recipient MAY either Content-Type header field is not present, the recipient MAY either
assume a media type of "application/octet-stream" ([RFC2046], assume a media type of "application/octet-stream" ([RFC2046],
Section 4.5.1) or examine the data to determine its type. Section 4.5.1) or examine the data to determine its type.
In practice, resource owners do not always properly configure their In practice, resource owners do not always properly configure their
origin server to provide the correct Content-Type for a given origin server to provide the correct Content-Type for a given
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member of the list, but note that some implementations might have member of the list, but note that some implementations might have
different error handling behaviors, leading to interoperability and/ different error handling behaviors, leading to interoperability and/
or security issues. or security issues.
8.3.1. Media Type 8.3.1. Media Type
HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and
Accept (Section 12.5.1) header fields in order to provide open and Accept (Section 12.5.1) header fields in order to provide open and
extensible data typing and type negotiation. Media types define both extensible data typing and type negotiation. Media types define both
a data format and various processing models: how to process that data a data format and various processing models: how to process that data
in accordance with each context in which it is received. in accordance with the message context.
media-type = type "/" subtype parameters media-type = type "/" subtype parameters
type = token type = token
subtype = token subtype = token
The type and subtype tokens are case-insensitive. The type and subtype tokens are case-insensitive.
The type/subtype MAY be followed by semicolon-delimited parameters The type/subtype MAY be followed by semicolon-delimited parameters
(Section 5.6.6) in the form of name=value pairs. The presence or (Section 5.6.6) in the form of name=value pairs. The presence or
absence of a parameter might be significant to the processing of a absence of a parameter might be significant to the processing of a
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Text/HTML;Charset="utf-8" Text/HTML;Charset="utf-8"
text/html; charset="utf-8" text/html; charset="utf-8"
text/html;charset=UTF-8 text/html;charset=UTF-8
Media types ought to be registered with IANA according to the Media types ought to be registered with IANA according to the
procedures defined in [BCP13]. procedures defined in [BCP13].
8.3.2. Charset 8.3.2. Charset
HTTP uses _charset_ names to indicate or negotiate the character HTTP uses _charset_ names to indicate or negotiate the character
encoding scheme of a textual representation [RFC6365]. A charset is encoding scheme ([RFC6365], Section 1.3) of a textual representation.
identified by a case-insensitive token. In the fields defined by this document, charset names appear either
in parameters (Content-Type), or, for Accept-Encoding, in the form of
charset = token a plain token. In both cases, charset names are matched case-
insensitively.
Charset names ought to be registered in the IANA "Character Sets" Charset names ought to be registered in the IANA "Character Sets"
registry (<https://www.iana.org/assignments/character-sets>) registry (<https://www.iana.org/assignments/character-sets>)
according to the procedures defined in Section 2 of [RFC2978]. according to the procedures defined in Section 2 of [RFC2978].
| *Note:* In theory, charset names are defined by the "mime- | *Note:* In theory, charset names are defined by the "mime-
| charset" ABNF rule defined in Section 2.3 of [RFC2978] (as | charset" ABNF rule defined in Section 2.3 of [RFC2978] (as
| corrected in [Err1912]). That rule allows two characters that | corrected in [Err1912]). That rule allows two characters that
| are not included in "token" ("{" and "}"), but no charset name | are not included in "token" ("{" and "}"), but no charset name
| registered at the time of this writing includes braces (see | registered at the time of this writing includes braces (see
| [Err5433]). | [Err5433]).
8.3.3. Canonicalization and Text Defaults 8.3.3. Multipart Types
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 content (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.
8.3.4. Multipart Types
MIME provides for a number of "multipart" types - encapsulations of MIME provides for a number of "multipart" types - encapsulations of
one or more representations within a single message body. All one or more representations within a single message body. All
multipart types share a common syntax, as defined in Section 5.1.1 of multipart types share a common syntax, as defined in Section 5.1.1 of
[RFC2046], and include a boundary parameter as part of the media type [RFC2046], and include a boundary parameter as part of the media type
value. The message body is itself a protocol element; a sender MUST value. The message body is itself a protocol element; a sender MUST
generate only CRLF to represent line breaks between body parts. generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an HTTP message framing does not use the multipart boundary as an
indicator of message body length, though it might be used by indicator of message body length, though it might be used by
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media type, and thus what decoding mechanisms have to be applied in media type, and thus what decoding mechanisms have to be applied in
order to obtain data in the media type referenced by the Content-Type order to obtain data in the media type referenced by the Content-Type
header field. Content-Encoding is primarily used to allow a header field. Content-Encoding is primarily used to allow a
representation's data to be compressed without losing the identity of representation's data to be compressed without losing the identity of
its underlying media type. its underlying media type.
Content-Encoding = #content-coding Content-Encoding = #content-coding
An example of its use is An example of its use is
Content-Encoding: gzip Content-Encoding: gzip
If one or more encodings have been applied to a representation, the If one or more encodings have been applied to a representation, the
sender that applied the encodings MUST generate a Content-Encoding sender that applied the encodings MUST generate a Content-Encoding
header field that lists the content codings in the order in which header field that lists the content codings in the order in which
they were applied. Note that the coding named "identity" is reserved they were applied. Note that the coding named "identity" is reserved
for its special role in Accept-Encoding, and thus SHOULD NOT be for its special role in Accept-Encoding, and thus SHOULD NOT be
included. included.
Additional information about the encoding parameters can be provided Additional information about the encoding parameters can be provided
by other header fields not defined by this specification. by other header fields not defined by this specification.
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representation. representation.
Content-Language = #language-tag Content-Language = #language-tag
Language tags are defined in Section 8.5.1. The primary purpose of Language tags are defined in Section 8.5.1. The primary purpose of
Content-Language is to allow a user to identify and differentiate Content-Language is to allow a user to identify and differentiate
representations according to the users' own preferred language. representations according to the users' own preferred language.
Thus, if the content is intended only for a Danish-literate audience, Thus, if the content is intended only for a Danish-literate audience,
the appropriate field is the appropriate field is
Content-Language: da Content-Language: da
If no Content-Language is specified, the default is that the content If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language, sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended. or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English Waitangi", presented simultaneously in the original Maori and English
versions, would call for versions, would call for
Content-Language: mi, en Content-Language: mi, en
However, just because multiple languages are present within a However, just because multiple languages are present within a
representation does not mean that it is intended for multiple representation does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin", which is clearly intended primer, such as "A First Lesson in Latin", which is clearly intended
to be used by an English-literate audience. In this case, the to be used by an English-literate audience. In this case, the
Content-Language would properly only include "en". Content-Language would properly only include "en".
Content-Language MAY be applied to any media type - it is not limited Content-Language MAY be applied to any media type - it is not limited
to textual documents. to textual documents.
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can be used to delimit framing (e.g., Section 6.2 of [Messaging]). can be used to delimit framing (e.g., Section 6.2 of [Messaging]).
In other cases, Content-Length indicates the selected In other cases, Content-Length indicates the selected
representation's current length, which can be used by recipients to representation's current length, which can be used by recipients to
estimate transfer time or compare to previously stored estimate transfer time or compare to previously stored
representations. representations.
Content-Length = 1*DIGIT Content-Length = 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
A sender MUST NOT send a Content-Length header field in any message
that contains a Transfer-Encoding header field.
A user agent SHOULD send a Content-Length in a request message when A user agent SHOULD send Content-Length in a request when the method
no Transfer-Encoding is sent and the request method defines a meaning defines a meaning for enclosed content and it is not sending
for enclosed content. For example, a Content-Length header field is Transfer-Encoding. For example, a user agent normally sends Content-
normally sent in a POST request even when the value is 0 (indicating Length in a POST request even when the value is 0 (indicating empty
empty content). A user agent SHOULD NOT send a Content-Length header content). A user agent SHOULD NOT send a Content-Length header field
field when the request message does not contain content and the when the request message does not contain content and the method
method semantics do not anticipate such data. semantics do not anticipate such data.
A server MAY send a Content-Length header field in a response to a A server MAY send a Content-Length header field in a response to a
HEAD request (Section 9.3.2); a server MUST NOT send Content-Length HEAD request (Section 9.3.2); a server MUST NOT send Content-Length
in such a response unless its field value equals the decimal number in such a response unless its field value equals the decimal number
of octets that would have been sent in the content of a response if of octets that would have been sent in the content of a response if
the same request had used the GET method. the same request had used the GET method.
A server MAY send a Content-Length header field in a 304 (Not A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 15.4.5); a Modified) response to a conditional GET request (Section 15.4.5); a
server MUST NOT send Content-Length in such a response unless its server MUST NOT send Content-Length in such a response unless its
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Aside from the cases defined above, in the absence of Transfer- Aside from the cases defined above, in the absence of Transfer-
Encoding, an origin server SHOULD send a Content-Length header field Encoding, an origin server SHOULD send a Content-Length header field
when the content size is known prior to sending the complete header when the content size is known prior to sending the complete header
section. This will allow downstream recipients to measure transfer section. This will allow downstream recipients to measure transfer
progress, know when a received message is complete, and potentially progress, know when a received message is complete, and potentially
reuse the connection for additional requests. reuse the connection for additional requests.
Any Content-Length field value greater than or equal to zero is Any Content-Length field value greater than or equal to zero is
valid. Since there is no predefined limit to the length of content, valid. Since there is no predefined limit to the length of content,
a recipient MUST anticipate potentially large decimal numerals and a recipient MUST anticipate potentially large decimal numerals and
prevent parsing errors due to integer conversion overflows prevent parsing errors due to integer conversion overflows or
(Section 17.5). precision loss due to integer conversion (Section 17.5).
If a message is received that has a Content-Length header field value Because Content-Length is used for message delimitation in HTTP/1.1,
consisting of the same decimal value as a comma-separated list its field value can impact how the message is parsed by downstream
(Section 5.6.1) - for example, "Content-Length: 42, 42" - indicating recipients even when the immediate connection is not using HTTP/1.1.
that duplicate Content-Length header fields have been generated or If the message is forwarded by a downstream intermediary, a Content-
combined by an upstream message processor, then the recipient MUST Length field value that is inconsistent with the received message
either reject the message as invalid or replace the duplicated field framing might cause a security failure due to request smuggling or
values with a single valid Content-Length field containing that response splitting.
decimal value.
As a result, a sender MUST NOT forward a message with a Content-
Length header field value that is known to be incorrect.
Likewise, a sender MUST NOT forward a message with a Content-Length
header field value that does not match the ABNF above, with one
exception: A recipient of a Content-Length header field value
consisting of the same decimal value repeated as a comma-separated
list (e.g, "Content-Length: 42, 42"), MAY either reject the message
as invalid or replace that invalid field value with a single instance
of the decimal value, since this likely indicates that a duplicate
was generated or combined by an upstream message processor.
8.7. Content-Location 8.7. Content-Location
The "Content-Location" header field references a URI that can be used The "Content-Location" header field references a URI that can be used
as an identifier for a specific resource corresponding to the as an identifier for a specific resource corresponding to the
representation in this message's content. In other words, if one representation in this message's content. In other words, if one
were to perform a GET request on this URI at the time of this were to perform a GET request on this URI at the time of this
message's generation, then a 200 (OK) response would contain the same message's generation, then a 200 (OK) response would contain the same
representation that is enclosed as content in this message. representation that is enclosed as content in this message.
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local copies without the need for a subsequent GET request. local copies without the need for a subsequent GET request.
If Content-Location is included in a 2xx (Successful) response If Content-Location is included in a 2xx (Successful) response
message and its field value refers to a URI that differs from the message and its field value refers to a URI that differs from the
target URI, then the origin server claims that the URI is an target URI, then the origin server claims that the URI is an
identifier for a different resource corresponding to the enclosed identifier for a different resource corresponding to the enclosed
representation. Such a claim can only be trusted if both identifiers representation. Such a claim can only be trusted if both identifiers
share the same resource owner, which cannot be programmatically share the same resource owner, which cannot be programmatically
determined via HTTP. determined via HTTP.
o For a response to a GET or HEAD request, this is an indication * For a response to a GET or HEAD request, this is an indication
that the target URI refers to a resource that is subject to that the target URI refers to a resource that is subject to
content negotiation and the Content-Location field value is a more content negotiation and the Content-Location field value is a more
specific identifier for the selected representation. specific identifier for the selected representation.
o For a 201 (Created) response to a state-changing method, a * For a 201 (Created) response to a state-changing method, a
Content-Location field value that is identical to the Location Content-Location field value that is identical to the Location
field value indicates that this content is a current field value indicates that this content is a current
representation of the newly created resource. representation of the newly created resource.
o Otherwise, such a Content-Location indicates that this content is * Otherwise, such a Content-Location indicates that this content is
a representation reporting on the requested action's status and a representation reporting on the requested action's status and
that the same report is available (for future access with GET) at that the same report is available (for future access with GET) at
the given URI. For example, a purchase transaction made via a the given URI. For example, a purchase transaction made via a
POST request might include a receipt document as the content of POST request might include a receipt document as the content of
the 200 (OK) response; the Content-Location field value provides the 200 (OK) response; the Content-Location field value provides
an identifier for retrieving a copy of that same receipt in the an identifier for retrieving a copy of that same receipt in the
future. future.
A user agent that sends Content-Location in a request message is A user agent that sends Content-Location in a request message is
stating that its value refers to where the user agent originally stating that its value refers to where the user agent originally
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necessarily the same as the representation enclosed as response necessarily the same as the representation enclosed as response
content. content.
In a successful response to a state-changing request, validator In a successful response to a state-changing request, validator
fields describe the new representation that has replaced the prior fields describe the new representation that has replaced the prior
selected representation as a result of processing the request. selected representation as a result of processing the request.
For example, an ETag field in a 201 (Created) response communicates For example, an ETag field in a 201 (Created) response communicates
the entity-tag of the newly created resource's representation, so the entity-tag of the newly created resource's representation, so
that it can be used in later conditional requests to prevent the that it can be used in later conditional requests to prevent the
"lost update" problem Section 13.1. "lost update" problem (Section 13.1).
This specification defines two forms of metadata that are commonly This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions: used to observe resource state and test for preconditions:
modification dates (Section 8.8.2) and opaque entity tags modification dates (Section 8.8.2) and opaque entity tags
(Section 8.8.3). Additional metadata that reflects resource state (Section 8.8.3). Additional metadata that reflects resource state
has been defined by various extensions of HTTP, such as Web has been defined by various extensions of HTTP, such as Web
Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are
beyond the scope of this specification. A resource metadata value is beyond the scope of this specification. A resource metadata value is
referred to as a _validator_ when it is used within a precondition. referred to as a _validator_ when it is used within a precondition.
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8.8.2. Last-Modified 8.8.2. Last-Modified
The "Last-Modified" header field in a response provides a timestamp The "Last-Modified" header field in a response provides a timestamp
indicating the date and time at which the origin server believes the indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the selected representation was last modified, as determined at the
conclusion of handling the request. conclusion of handling the request.
Last-Modified = HTTP-date Last-Modified = HTTP-date
An example of its use is An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
8.8.2.1. Generation 8.8.2.1. Generation
An origin server SHOULD send Last-Modified for any selected An origin server SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests and consistently determined, since its use in conditional requests
and evaluating cache freshness ([Caching]) results in a substantial and evaluating cache freshness ([Caching]) can substantially reduce
reduction of HTTP traffic on the Internet and can be a significant unnecessary transfers and significantly improve service availability
factor in improving service scalability and reliability. and scalability.
A representation is typically the sum of many parts behind the A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most resource interface. The last-modified time would usually be the most
recent time that any of those parts were changed. How that value is recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how the scope of this specification.
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 An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates 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 Date field value for its response. This allows a recipient to make
an accurate assessment of the representation's modification time, an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the especially if the representation changes near the time that the
response is generated. response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that 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 is later than the server's time of message origination (Date). If
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An origin server without a clock MUST NOT assign Last-Modified values An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource to a response unless these values were associated with the resource
by some other system or user with a reliable clock. by some other system or user with a reliable clock.
8.8.2.2. Comparison 8.8.2.2. Comparison
A Last-Modified time, when used as a validator in a request, is A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong, implicitly weak unless it is possible to deduce that it is strong,
using the following rules: using the following rules:
o The validator is being compared by an origin server to the actual * The validator is being compared by an origin server to the actual
current validator for the representation and, current validator for the representation and,
o That origin server reliably knows that the associated * That origin server reliably knows that the associated
representation did not change twice during the second covered by representation did not change twice during the second covered by
the presented validator; the presented validator;
or or
o The validator is about to be used by a client in an * The validator is about to be used by a client in an
If-Modified-Since, If-Unmodified-Since, or If-Range header field, If-Modified-Since, If-Unmodified-Since, or If-Range header field,
because the client has a cache entry for the associated because the client has a cache entry for the associated
representation, and representation, and
o That cache entry includes a Date value which is at least one * That cache entry includes a Date value which is at least one
second after the Last-Modified value and the client has reason to second after the Last-Modified value and the client has reason to
believe that they were generated by the same clock or that there believe that they were generated by the same clock or that there
is enough difference between the Last-Modified and Date values to is enough difference between the Last-Modified and Date values to
make clock synchronization issues unlikely; make clock synchronization issues unlikely;
or or
o The validator is being compared by an intermediate cache to the * The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, and validator stored in its cache entry for the representation, and
o That cache entry includes a Date value which is at least one * That cache entry includes a Date value which is at least one
second after the Last-Modified value and the cache has reason to second after the Last-Modified value and the cache has reason to
believe that they were generated by the same clock or that there believe that they were generated by the same clock or that there
is enough difference between the Last-Modified and Date values to is enough difference between the Last-Modified and Date values to
make clock synchronization issues unlikely. make clock synchronization issues unlikely.
This method relies on the fact that if two different responses were This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. have a Date value equal to its Last-Modified time.
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| backslash characters in entity tags. | backslash characters in entity tags.
An entity-tag can be more reliable for validation than a modification An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently sufficient, or where modification dates are not consistently
maintained. maintained.
Examples: Examples:
ETag: "xyzzy" ETag: "xyzzy"
ETag: W/"xyzzy" ETag: W/"xyzzy"
ETag: "" ETag: ""
An entity-tag can be either a weak or strong validator, with strong 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 being the default. If an origin server provides an entity-tag for a
representation and the generation of that entity-tag does not satisfy representation and the generation of that entity-tag does not satisfy
all of the characteristics of a strong validator (Section 8.8.1), all of the characteristics of a strong validator (Section 8.8.1),
then the origin server MUST mark the entity-tag as weak by prefixing then the origin server MUST mark the entity-tag as weak by prefixing
its opaque value with "W/" (case-sensitive). its opaque value with "W/" (case-sensitive).
A sender MAY send the Etag field in a trailer section (see A sender MAY send the Etag field in a trailer section (see
Section 6.5). However, since trailers are often ignored, it is Section 6.5). However, since trailers are often ignored, it is
preferable to send Etag as a header field unless the entity-tag is preferable to send Etag as a header field unless the entity-tag is
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applied to all changes might use an internal revision number, perhaps applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other accurately differentiate between representations. Other
implementations might use a collision-resistant hash of implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution. a modification timestamp that has sub-second resolution.
An origin server SHOULD send an ETag for any selected representation An origin server SHOULD send an ETag for any selected representation
for which detection of changes can be reasonably and consistently for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([Caching]) can result in a substantial evaluating cache freshness ([Caching]) can substantially reduce
reduction of HTTP network traffic and can be a significant factor in unnecessary transfers and significantly improve service availability,
improving service scalability and reliability. scalability, and reliability.
8.8.3.2. Comparison 8.8.3.2. Comparison
There are two entity-tag comparison functions, depending on whether There are two entity-tag comparison functions, depending on whether
or not the comparison context allows the use of weak validators: or not the comparison context allows the use of weak validators:
_Strong comparison_: two entity-tags are equivalent if both are not _Strong comparison_: two entity-tags are equivalent if both are not
weak and their opaque-tags match character-by-character. weak and their opaque-tags match character-by-character.
_Weak comparison_: two entity-tags are equivalent if their opaque- _Weak comparison_: two entity-tags are equivalent if their opaque-
tags match character-by-character, regardless of either or both tags match character-by-character, regardless of either or both
being tagged as "weak". being tagged as "weak".
The example below shows the results for a set of entity-tag pairs and The example below shows the results for a set of entity-tag pairs and
both the weak and strong comparison function results: both the weak and strong comparison function results:
-------- -------- ------------------- ----------------- +========+========+===================+=================+
ETag 1 ETag 2 Strong Comparison Weak Comparison | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
-------- -------- ------------------- ----------------- +========+========+===================+=================+
W/"1" W/"1" no match match | W/"1" | W/"1" | no match | match |
W/"1" W/"2" no match no match +--------+--------+-------------------+-----------------+
W/"1" "1" no match match | W/"1" | W/"2" | no match | no match |
"1" "1" match match +--------+--------+-------------------+-----------------+
-------- -------- ------------------- ----------------- | W/"1" | "1" | no match | match |
+--------+--------+-------------------+-----------------+
| "1" | "1" | match | match |
+--------+--------+-------------------+-----------------+
Table 3 Table 3
8.8.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources 8.8.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation Consider a resource that is subject to content negotiation
(Section 12), and where the representations sent in response to a GET (Section 12), and where the representations sent in response to a GET
request vary based on the Accept-Encoding request header field request vary based on the Accept-Encoding request header field
(Section 12.5.3): (Section 12.5.3):
>> Request: >> Request:
GET /index HTTP/1.1 GET /index HTTP/1.1
Host: www.example.com Host: www.example.com
Accept-Encoding: gzip Accept-Encoding: gzip
In this case, the response might or might not use the gzip content In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like: coding. If it does not, the response might look like:
>> Response: >> Response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a" ETag: "123-a"
Content-Length: 70 Content-Length: 70
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would An alternative representation that does use gzip content coding would
be: be:
>> Response: >> Response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b" ETag: "123-b"
Content-Length: 43 Content-Length: 43
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Content-Encoding: gzip Content-Encoding: gzip
...binary data... ...binary data...
| *Note:* Content codings are a property of the representation | *Note:* Content codings are a property of the representation
| data, so a strong entity-tag for a content-encoded | data, so a strong entity-tag for a content-encoded
| representation has to be distinct from the entity tag of an | representation has to be distinct from the entity tag of an
| unencoded representation to prevent potential conflicts during | unencoded representation to prevent potential conflicts during
| cache updates and range requests. In contrast, transfer | cache updates and range requests. In contrast, transfer
| codings (Section 7 of [Messaging]) apply only during message | codings (Section 7 of [Messaging]) apply only during message
| transfer and do not result in distinct entity-tags. | transfer and do not result in distinct entity-tags.
8.8.4. When to Use Entity-Tags and Last-Modified Dates 8.8.4. When to Use Entity-Tags and Last-Modified Dates
In 200 (OK) responses to GET or HEAD, an origin server: In 200 (OK) responses to GET or HEAD, an origin server:
o SHOULD send an entity-tag validator unless it is not feasible to * SHOULD send an entity-tag validator unless it is not feasible to
generate one. generate one.
o MAY send a weak entity-tag instead of a strong entity-tag, if * MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag. if it is unfeasible to send a strong entity-tag.
o SHOULD send a Last-Modified value if it is feasible to send one. * 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 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 send both a strong entity-tag and a Last-Modified value in successful
responses to a retrieval request. responses to a retrieval request.
A client: A client:
o MUST send that entity-tag in any cache validation request (using * 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 If-Match or If-None-Match) if an entity-tag has been provided by
the origin server. the origin server.
o SHOULD send the Last-Modified value in non-subrange cache * SHOULD send the Last-Modified value in non-subrange cache
validation requests (using If-Modified-Since) if only a Last- validation requests (using If-Modified-Since) if only a Last-
Modified value has been provided by the origin server. Modified value has been provided by the origin server.
o MAY send the Last-Modified value in subrange cache validation * MAY send the Last-Modified value in subrange cache validation
requests (using If-Unmodified-Since) if only a Last-Modified value requests (using If-Unmodified-Since) if only a Last-Modified value
has been provided by an HTTP/1.0 origin server. The user agent has been provided by an HTTP/1.0 origin server. The user agent
SHOULD provide a way to disable this, in case of difficulty. SHOULD provide a way to disable this, in case of difficulty.
o SHOULD send both validators in cache validation requests if both * SHOULD send both validators in cache validation requests if both
an entity-tag and a Last-Modified value have been provided by the 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 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately. respond appropriately.
9. Methods 9. Methods
9.1. Overview 9.1. Overview
The request method token is the primary source of request semantics; The request method token is the primary source of request semantics;
it indicates the purpose for which the client has made this request it indicates the purpose for which the client has made this request
and what is expected by the client as a successful result. and what is expected by the client as a successful result.
The request method's semantics might be further specialized by the The request method's semantics might be further specialized by the
semantics of some header fields when present in a request if those semantics of some header fields when present in a request if those
additional semantics do not conflict with the method. For example, a additional semantics do not conflict with the method. For example, a
client can send conditional request header fields (Section 13.1) to client can send conditional request header fields (Section 13.1) to
make the requested action conditional on the current state of the make the requested action conditional on the current state of the
target resource. target resource.
HTTP was originally designed to be usable as an interface to HTTP is designed to be usable as an interface to distributed object
distributed object systems. The request method was envisioned as systems. The request method invokes an action to be applied to a
applying semantics to a target resource in much the same way as target resource in much the same way that a remote method invocation
invoking a defined method on an identified object would apply can be sent to an identified object.
semantics.
method = token method = token
The method token is case-sensitive because it might be used as a The method token is case-sensitive because it might be used as a
gateway to object-based systems with case-sensitive method names. By gateway to object-based systems with case-sensitive method names. By
convention, standardized methods are defined in all-uppercase US- convention, standardized methods are defined in all-uppercase US-
ASCII letters. ASCII letters.
Unlike distributed objects, the standardized request methods in HTTP Unlike distributed objects, the standardized request methods in HTTP
are not resource-specific, since uniform interfaces provide for are not resource-specific, since uniform interfaces provide for
better visibility and reuse in network-based systems [REST]. Once better visibility and reuse in network-based systems [REST]. Once
defined, a standardized method ought to have the same semantics when defined, a standardized method ought to have the same semantics when
applied to any resource, though each resource determines for itself applied to any resource, though each resource determines for itself
whether those semantics are implemented or allowed. whether those semantics are implemented or allowed.
This specification defines a number of standardized methods that are This specification defines a number of standardized methods that are
commonly used in HTTP, as outlined by the following table. commonly used in HTTP, as outlined by the following table.
--------- -------------------------------------------- ------- +=========+============================================+=======+
Method Description Ref. | Method | Description | Ref. |
--------- -------------------------------------------- ------- +=========+============================================+=======+
GET Transfer a current representation of the 9.3.1 | GET | Transfer a current representation of the | 9.3.1 |
target resource. | | target resource. | |
HEAD Same as GET, but do not transfer the 9.3.2 +---------+--------------------------------------------+-------+
response content. | HEAD | Same as GET, but do not transfer the | 9.3.2 |
POST Perform resource-specific processing on 9.3.3 | | response content. | |
the request content. +---------+--------------------------------------------+-------+
PUT Replace all current representations of the 9.3.4 | POST | Perform resource-specific processing on | 9.3.3 |
target resource with the request content. | | the request content. | |
DELETE Remove all current representations of the 9.3.5 +---------+--------------------------------------------+-------+
target resource. | PUT | Replace all current representations of the | 9.3.4 |
CONNECT Establish a tunnel to the server 9.3.6 | | target resource with the request content. | |
identified by the target resource. +---------+--------------------------------------------+-------+
OPTIONS Describe the communication options for the 9.3.7 | DELETE | Remove all current representations of the | 9.3.5 |
target resource. | | target resource. | |
TRACE Perform a message loop-back test along the 9.3.8 +---------+--------------------------------------------+-------+
path to the target resource. | CONNECT | Establish a tunnel to the server | 9.3.6 |
--------- -------------------------------------------- ------- | | identified by the target resource. | |
+---------+--------------------------------------------+-------+
| OPTIONS | Describe the communication options for the | 9.3.7 |
| | target resource. | |
+---------+--------------------------------------------+-------+
| TRACE | Perform a message loop-back test along the | 9.3.8 |
| | path to the target resource. | |
+---------+--------------------------------------------+-------+
Table 4 Table 4
All general-purpose servers MUST support the methods GET and HEAD. All general-purpose servers MUST support the methods GET and HEAD.
All other methods are OPTIONAL. All other methods are OPTIONAL.
The set of methods allowed by a target resource can be listed in an The set of methods allowed by a target resource can be listed in an
Allow header field (Section 10.2.1). However, the set of allowed Allow header field (Section 10.2.1). However, the set of allowed
methods can change dynamically. When a request method is received methods can change dynamically. An origin server that receives a
that is unrecognized or not implemented by an origin server, the request method that is unrecognized or not implemented SHOULD respond
origin server SHOULD respond with the 501 (Not Implemented) status with the 501 (Not Implemented) status code. An origin server that
code. When a request method is received that is known by an origin receives a request method that is recognized and implemented, but not
server but not allowed for the target resource, the origin server allowed for the target resource, SHOULD respond with the 405 (Method
SHOULD respond with the 405 (Method Not Allowed) status code. Not Allowed) status code.
Additional methods, outside the scope of this specification, have Additional methods, outside the scope of this specification, have
been specified for use in HTTP. All such methods ought to be been specified for use in HTTP. All such methods ought to be
registered within the "Hypertext Transfer Protocol (HTTP) Method registered within the "Hypertext Transfer Protocol (HTTP) Method
Registry", as described in Section 16.1. Registry", as described in Section 16.1.
9.2. Common Method Properties 9.2. Common Method Properties
9.2.1. Safe Methods 9.2.1. Safe Methods
Request methods are considered _safe_ if their defined semantics are Request methods are considered _safe_ if their defined semantics are
essentially read-only; i.e., the client does not request, and does essentially read-only; i.e., the client does not request, and does
not expect, any state change on the origin server as a result of not expect, any state change on the origin server as a result of
applying a safe method to a target resource. Likewise, reasonable applying a safe method to a target resource. Likewise, reasonable
use of a safe method is not expected to cause any harm, loss of use of a safe method is not expected to cause any harm, loss of
property, or unusual burden on the origin server. property, or unusual burden on the origin server.
This definition of safe methods does not prevent an implementation This definition of safe methods does not prevent an implementation
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before any response is received, then the client can establish a new before any response is received, then the client can establish a new
connection and retry the idempotent request. It knows that repeating connection and retry the idempotent request. It knows that repeating
the request will have the same intended effect, even if the original the request will have the same intended effect, even if the original
request succeeded, though the response might differ. request succeeded, though the response might differ.
A client SHOULD NOT automatically retry a request with a non- A client SHOULD NOT automatically retry a request with a non-
idempotent method unless it has some means to know that the request idempotent method unless it has some means to know that the request
semantics are actually idempotent, regardless of the method, or some semantics are actually idempotent, regardless of the method, or some
means to detect that the original request was never applied. means to detect that the original request was never applied.
For example, a user agent that knows (through design or For example, a user agent can repeat a POST request automatically if
configuration) that a POST request to a given resource is safe can it knows (through design or configuration) that the request is safe
repeat that request automatically. Likewise, a user agent designed for that resource. Likewise, a user agent designed specifically to
specifically to operate on a version control repository might be able operate on a version control repository might be able to recover from
to recover from partial failure conditions by checking the target partial failure conditions by checking the target resource
resource revision(s) after a failed connection, reverting or fixing revision(s) after a failed connection, reverting or fixing any
any changes that were partially applied, and then automatically changes that were partially applied, and then automatically retrying
retrying the requests that failed. the requests that failed.
Some clients use weaker signals to initiate automatic retries. For Some clients take a riskier approach and attempt to guess when an
example, when a POST request is sent, but the underlying transport automatic retry is possible. For example, a client might
connection is closed before any part of the response is received. automatically retry a POST request if the underlying transport
Although this is commonly implemented, it is not recommended. connection closed before any part of a response is received,
particularly if an idle persistent connection was used.
A proxy MUST NOT automatically retry non-idempotent requests. A A proxy MUST NOT automatically retry non-idempotent requests. A
client SHOULD NOT automatically retry a failed automatic retry. client SHOULD NOT automatically retry a failed automatic retry.
9.2.3. Methods and Caching 9.2.3. Methods and Caching
For a cache to store and use a response, the associated method needs For a cache to store and use a response, the associated method needs
to explicitly allow caching, and detail under what conditions a to explicitly allow caching, and detail under what conditions a
response can be used to satisfy subsequent requests; a method response can be used to satisfy subsequent requests; a method
definition which does not do so cannot be cached. For additional definition which does not do so cannot be cached. For additional
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(see Section 17.9). In some cases, the data can be filtered or (see Section 17.9). In some cases, the data can be filtered or
transformed such that it would not reveal such information. In transformed such that it would not reveal such information. In
others, particularly when there is no benefit from caching a others, particularly when there is no benefit from caching a
response, using the POST method (Section 9.3.3) instead of GET can response, using the POST method (Section 9.3.3) instead of GET can
transmit such information in the request content rather than within transmit such information in the request content rather than within
the target URI. the target URI.
9.3.2. HEAD 9.3.2. HEAD
The HEAD method is identical to GET except that the server MUST NOT The HEAD method is identical to GET except that the server MUST NOT
send content in the response and the response always terminates at send content in the response. HEAD is used to obtain metadata about
the end of the header section. HEAD is used to obtain metadata about
the selected representation without transferring its representation the selected representation without transferring its representation
data, often for the sake of testing hypertext links or finding recent data, often for the sake of testing hypertext links or finding recent
modifications. modifications.
The server SHOULD send the same header fields in response to a HEAD The server SHOULD send the same header fields in response to a HEAD
request as it would have sent if the request method had been GET. request as it would have sent if the request method had been GET.
However, a server MAY omit header fields for which a value is However, a server MAY omit header fields for which a value is
determined only while generating the content. For example, some determined only while generating the content. For example, some
servers buffer a dynamic response to GET until a minimum amount of servers buffer a dynamic response to GET until a minimum amount of
data is generated so that they can more efficiently delimit small data is generated so that they can more efficiently delimit small
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response might also affect previously cached responses to GET; see response might also affect previously cached responses to GET; see
Section 4.3.5 of [Caching]. Section 4.3.5 of [Caching].
9.3.3. POST 9.3.3. POST
The POST method requests that the target resource process the The POST method requests that the target resource process the
representation enclosed in the request according to the resource's representation enclosed in the request according to the resource's
own specific semantics. For example, POST is used for the following own specific semantics. For example, POST is used for the following
functions (among others): functions (among others):
o Providing a block of data, such as the fields entered into an HTML * Providing a block of data, such as the fields entered into an HTML
form, to a data-handling process; form, to a data-handling process;
o Posting a message to a bulletin board, newsgroup, mailing list, * Posting a message to a bulletin board, newsgroup, mailing list,
blog, or similar group of articles; blog, or similar group of articles;
o Creating a new resource that has yet to be identified by the * Creating a new resource that has yet to be identified by the
origin server; and origin server; and
o Appending data to a resource's existing representation(s). * Appending data to a resource's existing representation(s).
An origin server indicates response semantics by choosing an An origin server indicates response semantics by choosing an
appropriate status code depending on the result of processing the appropriate status code depending on the result of processing the
POST request; almost all of the status codes defined by this POST request; almost all of the status codes defined by this
specification could be received in a response to POST (the exceptions specification could be received in a response to POST (the exceptions
being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
Satisfiable)). Satisfiable)).
If one or more resources has been created on the origin server as a If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server result of successfully processing a POST request, the origin server
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trailer fields received in a PUT request (i.e., do not save them as trailer fields received in a PUT request (i.e., do not save them as
part of the resource state). part of the resource state).
An origin server MUST NOT send a validator field (Section 8.8), such An origin server MUST NOT send a validator field (Section 8.8), such
as an ETag or Last-Modified field, in a successful response to PUT as an ETag or Last-Modified field, in a successful response to PUT
unless the request's representation data was saved without any unless the request's representation data was saved without any
transformation applied to the content (i.e., the resource's new transformation applied to the content (i.e., the resource's new
representation data is identical to the content received in the PUT representation data is identical to the content received in the PUT
request) and the validator field value reflects the new request) and the validator field value reflects the new
representation. This requirement allows a user agent to know when representation. This requirement allows a user agent to know when
the representation it has in memory remains current as a result of the representation it sent (and retains in memory) is the result of
the PUT, thus not in need of being retrieved again from the origin the PUT, and thus doesn't need to be retrieved again from the origin
server, and that the new validator(s) received in the response can be server. The new validator(s) received in the response can be used
used for future conditional requests in order to prevent accidental for future conditional requests in order to prevent accidental
overwrites (Section 13.1). overwrites (Section 13.1).
The fundamental difference between the POST and PUT methods is The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation. highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics, enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT replacing the state of the target resource. Hence, the intent of PUT
is idempotent and visible to intermediaries, even though the exact is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server. effect is only known by the origin server.
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field after a 201 (Created) response to a POST request, might allow a field after a 201 (Created) response to a POST request, might allow a
corresponding DELETE request to undo those actions. Similarly, corresponding DELETE request to undo those actions. Similarly,
custom user agent implementations that implement an authoring custom user agent implementations that implement an authoring
function, such as revision control clients using HTTP for remote function, such as revision control clients using HTTP for remote
operations, might use DELETE based on an assumption that the server's operations, might use DELETE based on an assumption that the server's
URI space has been crafted to correspond to a version repository. URI space has been crafted to correspond to a version repository.
If a DELETE method is successfully applied, the origin server SHOULD If a DELETE method is successfully applied, the origin server SHOULD
send send
o a 202 (Accepted) status code if the action will likely succeed but * a 202 (Accepted) status code if the action will likely succeed but
has not yet been enacted, has not yet been enacted,
o a 204 (No Content) status code if the action has been enacted and * a 204 (No Content) status code if the action has been enacted and
no further information is to be supplied, or no further information is to be supplied, or
o a 200 (OK) status code if the action has been enacted and the * a 200 (OK) status code if the action has been enacted and the
response message includes a representation describing the status. response message includes a representation describing the status.
A client SHOULD NOT generate content in a DELETE request. Content A client SHOULD NOT generate content in a DELETE request. Content
received in a DELETE request has no defined semantics, cannot alter received in a DELETE request has no defined semantics, cannot alter
the meaning or target of the request, and might lead some the meaning or target of the request, and might lead some
implementations to reject the request. implementations to reject the request.
Responses to the DELETE method are not cacheable. If a successful Responses to the DELETE method are not cacheable. If a successful
DELETE request passes through a cache that has one or more stored DELETE request passes through a cache that has one or more stored
responses for the target URI, those stored responses will be responses for the target URI, those stored responses will be
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9.3.6. CONNECT 9.3.6. CONNECT
The CONNECT method requests that the recipient establish a tunnel to The CONNECT method requests that the recipient establish a tunnel to
the destination origin server identified by the request target and, the destination origin server identified by the request target and,
if successful, thereafter restrict its behavior to blind forwarding if successful, thereafter restrict its behavior to blind forwarding
of data, in both directions, until the tunnel is closed. Tunnels are of data, in both directions, until the tunnel is closed. Tunnels are
commonly used to create an end-to-end virtual connection, through one commonly used to create an end-to-end virtual connection, through one
or more proxies, which can then be secured using TLS (Transport Layer or more proxies, which can then be secured using TLS (Transport Layer
Security, [RFC8446]). Security, [RFC8446]).
Because CONNECT changes the request/response nature of an HTTP CONNECT uses a special form of request target, unique to this method,
connection, specific HTTP versions might have different ways of consisting of only the host and port number of the tunnel
mapping its semantics into the protocol's wire format. destination, separated by a colon. There is no default port; a
client MUST send the port number even if the CONNECT request is based
on a URI reference that contains an authority component with an
elided port (Section 4.1). For example,
CONNECT is intended only for use in requests to a proxy. An origin CONNECT server.example.com:80 HTTP/1.1
server that receives a CONNECT request for itself MAY respond with a Host: server.example.com
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 component A server MUST reject a CONNECT request that targets an empty or
(described in Section 3.2 of [RFC3986]) as the request target; i.e., invalid port number, typically by responding with a 400 (Bad Request)
the request target consists of only the host name and port number of status code.
the tunnel destination, separated by a colon. For example,
CONNECT server.example.com:80 HTTP/1.1 Because CONNECT changes the request/response nature of an HTTP
Host: server.example.com:80 connection, specific HTTP versions might have different ways of
mapping its semantics into the protocol's wire format.
The recipient proxy can establish a tunnel either by directly CONNECT is intended for use in requests to a proxy. The recipient
connecting to the request target or, if configured to use another can establish a tunnel either by directly connecting to the server
identified by the request target or, if configured to use another
proxy, by forwarding the CONNECT request to the next inbound proxy. proxy, by forwarding the CONNECT request to the next inbound proxy.
An origin server MAY accept a CONNECT request, but most origin
servers do not implement CONNECT.
Any 2xx (Successful) response indicates that the sender (and all Any 2xx (Successful) response indicates that the sender (and all
inbound proxies) will switch to tunnel mode immediately after the inbound proxies) will switch to tunnel mode immediately after the
blank line that concludes the successful response's header section; response header section; data received after that header section is
data received after that blank line is from the server identified by from the server identified by the request target. Any response other
the request target. Any response other than a successful response than a successful response indicates that the tunnel has not yet been
indicates that the tunnel has not yet been formed and that the formed.
connection remains governed by HTTP.
A tunnel is closed when a tunnel intermediary detects that either A tunnel is closed when a tunnel intermediary detects that either
side has closed its connection: the intermediary MUST attempt to send side has closed its connection: the intermediary MUST attempt to send
any outstanding data that came from the closed side to the other any outstanding data that came from the closed side to the other
side, close both connections, and then discard any remaining data side, close both connections, and then discard any remaining data
left undelivered. left undelivered.
Proxy authentication might be used to establish the authority to Proxy authentication might be used to establish the authority to
create a tunnel. For example, create a tunnel. For example,
CONNECT server.example.com:80 HTTP/1.1 CONNECT server.example.com:443 HTTP/1.1
Host: server.example.com:80 Host: server.example.com:443
Proxy-Authorization: basic aGVsbG86d29ybGQ= Proxy-Authorization: basic aGVsbG86d29ybGQ=
There are significant risks in establishing a tunnel to arbitrary There are significant risks in establishing a tunnel to arbitrary
servers, particularly when the destination is a well-known or servers, particularly when the destination is a well-known or
reserved TCP port that is not intended for Web traffic. For example, reserved TCP port that is not intended for Web traffic. For example,
a CONNECT to "example.com:25" would suggest that the proxy connect to a CONNECT to "example.com:25" would suggest that the proxy connect to
the reserved port for SMTP traffic; if allowed, that could trick the the reserved port for SMTP traffic; if allowed, that could trick the
proxy into relaying spam email. Proxies that support CONNECT SHOULD proxy into relaying spam email. Proxies that support CONNECT SHOULD
restrict its use to a limited set of known ports or a configurable restrict its use to a limited set of known ports or a configurable
whitelist of safe request targets. list of safe request targets.
A server MUST NOT send any Transfer-Encoding or Content-Length header A server MUST NOT send any Transfer-Encoding or Content-Length header
fields in a 2xx (Successful) response to CONNECT. A client MUST fields in a 2xx (Successful) response to CONNECT. A client MUST
ignore any Content-Length or Transfer-Encoding header fields received ignore any Content-Length or Transfer-Encoding header fields received
in a successful response to CONNECT. in a successful response to CONNECT.
Content within a CONNECT request message has no defined semantics; A CONNECT request message does not have content. The interpretation
sending content in a CONNECT request might cause some existing of and allowability of data sent after the header section of the
implementations to reject the request. CONNECT request message is specific to the version of HTTP in use.
Responses to the CONNECT method are not cacheable. Responses to the CONNECT method are not cacheable.
9.3.7. OPTIONS 9.3.7. OPTIONS
The OPTIONS method requests information about the communication The OPTIONS method requests information about the communication
options available for the target resource, at either the origin options available for the target resource, at either the origin
server or an intervening intermediary. This method allows a client server or an intervening intermediary. This method allows a client
to determine the options and/or requirements associated with a to determine the options and/or requirements associated with a
resource, or the capabilities of a server, without implying a resource, or the capabilities of a server, without implying a
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media type. Note that this specification does not define any use for media type. Note that this specification does not define any use for
such content. such content.
Responses to the OPTIONS method are not cacheable. Responses to the OPTIONS method are not cacheable.
9.3.8. TRACE 9.3.8. TRACE
The TRACE method requests a remote, application-level loop-back of The TRACE method requests a remote, application-level loop-back of
the request message. The final recipient of the request SHOULD the request message. The final recipient of the request SHOULD
reflect the message received, excluding some fields described below, reflect the message received, excluding some fields described below,
back to the client as the content of a 200 (OK) response with a back to the client as the content of a 200 (OK) response. The
Content-Type of "message/http" (Section 10.1 of [Messaging]). The "message/http" (Section 10.1 of [Messaging]) format is one way to do
final recipient is either the origin server or the first server to so. The final recipient is either the origin server or the first
receive a Max-Forwards value of zero (0) in the request server to receive a Max-Forwards value of zero (0) in the request
(Section 7.6.2). (Section 7.6.2).
A client MUST NOT generate fields in a TRACE request containing A client MUST NOT generate fields in a TRACE request containing
sensitive data that might be disclosed by the response. For example, sensitive data that might be disclosed by the response. For example,
it would be foolish for a user agent to send stored user credentials it would be foolish for a user agent to send stored user credentials
(Section 11) or cookies [RFC6265] in a TRACE request. The final (Section 11) or cookies [RFC6265] in a TRACE request. The final
recipient of the request SHOULD exclude any request fields that are recipient of the request SHOULD exclude any request fields that are
likely to contain sensitive data when that recipient generates the likely to contain sensitive data when that recipient generates the
response content. response content.
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URI, and header fields are not sufficient to cause an immediate URI, and header fields are not sufficient to cause an immediate
success, redirect, or error response. This allows the client to wait success, redirect, or error response. This allows the client to wait
for an indication that it is worthwhile to send the content before for an indication that it is worthwhile to send the content before
actually doing so, which can improve efficiency when the data is huge actually doing so, which can improve efficiency when the data is huge
or when the client anticipates that an error is likely (e.g., when or when the client anticipates that an error is likely (e.g., when
sending a state-changing method, for the first time, without sending a state-changing method, for the first time, without
previously verified authentication credentials). previously verified authentication credentials).
For example, a request that begins with For example, a request that begins with
PUT /somewhere/fun HTTP/1.1 PUT /somewhere/fun HTTP/1.1
Host: origin.example.com Host: origin.example.com
Content-Type: video/h264 Content-Type: video/h264
Content-Length: 1234567890987 Content-Length: 1234567890987
Expect: 100-continue Expect: 100-continue
allows the origin server to immediately respond with an error allows the origin server to immediately respond with an error
message, such as 401 (Unauthorized) or 405 (Method Not Allowed), message, such as 401 (Unauthorized) or 405 (Method Not Allowed),
before the client starts filling the pipes with an unnecessary data before the client starts filling the pipes with an unnecessary data
transfer. transfer.
Requirements for clients: Requirements for clients:
o A client MUST NOT generate a 100-continue expectation in a request * A client MUST NOT generate a 100-continue expectation in a request
that does not include content. that does not include content.
o A client that will wait for a 100 (Continue) response before * A client that will wait for a 100 (Continue) response before
sending the request content MUST send an Expect header field sending the request content MUST send an Expect header field
containing a 100-continue expectation. containing a 100-continue expectation.
o A client that sends a 100-continue expectation is not required to * 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 wait for any specific length of time; such a client MAY proceed to
send the content even if it has not yet received a response. send the content even if it has not yet received a response.
Furthermore, since 100 (Continue) responses cannot be sent through Furthermore, since 100 (Continue) responses cannot be sent through
an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
indefinite period before sending the content. indefinite period before sending the content.
o A client that receives a 417 (Expectation Failed) status code in * A client that receives a 417 (Expectation Failed) status code in
response to a request containing a 100-continue expectation SHOULD response to a request containing a 100-continue expectation SHOULD
repeat that request without a 100-continue expectation, since the repeat that request without a 100-continue expectation, since the
417 response merely indicates that the response chain does not 417 response merely indicates that the response chain does not
support expectations (e.g., it passes through an HTTP/1.0 server). support expectations (e.g., it passes through an HTTP/1.0 server).
Requirements for servers: Requirements for servers:
o A server that receives a 100-continue expectation in an HTTP/1.0 * A server that receives a 100-continue expectation in an HTTP/1.0
request MUST ignore that expectation. request MUST ignore that expectation.
o A server MAY omit sending a 100 (Continue) response if it has * A server MAY omit sending a 100 (Continue) response if it has
already received some or all of the content for the corresponding already received some or all of the content for the corresponding
request, or if the framing indicates that there is no content. request, or if the framing indicates that there is no content.
o A server that sends a 100 (Continue) response MUST ultimately send * A server that sends a 100 (Continue) response MUST ultimately send
a final status code, once the request content is received and a final status code, once it receives and processes the request
processed, unless the connection is closed prematurely. content, unless the connection is closed prematurely.
o A server that responds with a final status code before reading the * A server that responds with a final status code before reading the
entire request content SHOULD indicate whether it intends to close entire request content SHOULD indicate whether it intends to close
the connection (e.g., see Section 9.6 of [Messaging]) or continue the connection (e.g., see Section 9.6 of [Messaging]) or continue
reading the request content. reading the request content.
An origin server MUST, upon receiving an HTTP/1.1 (or later) request An origin server MUST, upon receiving an HTTP/1.1 (or later) request
that has a method, target URI, and complete header section that that has a method, target URI, and complete header section that
contains a 100-continue expectation and an indication that request contains a 100-continue expectation and an indication that request
content will follow, either send an immediate response with a final content will follow, either send an immediate response with a final
status code, if that status can be determined by examining just the status code, if that status can be determined by examining just the
method, target URI, and header fields, or send an immediate 100 method, target URI, and header fields, or send an immediate 100
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human user who controls the requesting user agent. The address ought human user who controls the requesting user agent. The address ought
to be machine-usable, as defined by "mailbox" in Section 3.4 of to be machine-usable, as defined by "mailbox" in Section 3.4 of
[RFC5322]: [RFC5322]:
From = mailbox From = mailbox
mailbox = <mailbox, see [RFC5322], Section 3.4> mailbox = <mailbox, see [RFC5322], Section 3.4>
An example is: An example is:
From: webmaster@example.org From: webmaster@example.org
The From header field is rarely sent by non-robotic user agents. A The From header field is rarely sent by non-robotic user agents. A
user agent SHOULD NOT send a From header field without explicit user agent SHOULD NOT send a From header field without explicit
configuration by the user, since that might conflict with the user's configuration by the user, since that might conflict with the user's
privacy interests or their site's security policy. privacy interests or their site's security policy.
A robotic user agent SHOULD send a valid From header field so that A robotic user agent SHOULD send a valid From header field so that
the person responsible for running the robot can be contacted if the person responsible for running the robot can be contacted if
problems occur on servers, such as if the robot is sending excessive, problems occur on servers, such as if the robot is sending excessive,
unwanted, or invalid requests. unwanted, or invalid requests.
A server SHOULD NOT use the From header field for access control or A server SHOULD NOT use the From header field for access control or
authentication, since most recipients will assume that the field authentication.
value is public information.
10.1.3. Referer 10.1.3. Referer
The "Referer" [sic] header field allows the user agent to specify a The "Referer" [sic] header field allows the user agent to specify a
URI reference for the resource from which the target URI was obtained URI reference for the resource from which the target URI was obtained
(i.e., the "referrer", though the field name is misspelled). A user (i.e., the "referrer", though the field name is misspelled). A user
agent MUST NOT include the fragment and userinfo components of the agent MUST NOT include the fragment and userinfo components of the
URI reference [RFC3986], if any, when generating the Referer field URI reference [RFC3986], if any, when generating the Referer field
value. value.
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The Referer header field allows servers to generate back-links to The Referer header field allows servers to generate back-links to
other resources for simple analytics, logging, optimized caching, other resources for simple analytics, logging, optimized caching,
etc. It also allows obsolete or mistyped links to be found for etc. It also allows obsolete or mistyped links to be found for
maintenance. Some servers use the Referer header field as a means of maintenance. Some servers use the Referer header field as a means of
denying links from other sites (so-called "deep linking") or denying links from other sites (so-called "deep linking") or
restricting cross-site request forgery (CSRF), but not all requests restricting cross-site request forgery (CSRF), but not all requests
contain it. contain it.
Example: Example:
Referer: http://www.example.org/hypertext/Overview.html Referer: http://www.example.org/hypertext/Overview.html
If the target URI was obtained from a source that does not have its 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 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 user's bookmarks/favorites), the user agent MUST either exclude the
Referer header field or send it with a value of "about:blank". Referer header field or send it with a value of "about:blank".
The Referer header field value need not convey the full URI of the
referring resource; a user agent MAY truncate parts other than the
referring origin.
The Referer header field has the potential to reveal information The Referer header field has the potential to reveal information
about the request context or browsing history of the user, which is a about the request context or browsing history of the user, which is a
privacy concern if the referring resource's identifier reveals privacy concern if the referring resource's identifier reveals
personal information (such as an account name) or a resource that is personal information (such as an account name) or a resource that is
supposed to be confidential (such as behind a firewall or internal to supposed to be confidential (such as behind a firewall or internal to
a secured service). Most general-purpose user agents do not send the a secured service). Most general-purpose user agents do not send the
Referer header field when the referring resource is a local "file" or Referer header field when the referring resource is a local "file" or
"data" URI. A user agent MUST NOT send a Referer header field in an "data" URI. A user agent SHOULD NOT send a Referer header field if
unsecured HTTP request if the referring page was received with a the referring resource was accessed with a secure protocol and the
request target has an origin differing from that of the referring
resource, unless the referring resource explicitly allows Referer to
be sent. A user agent MUST NOT send a Referer header field in an
unsecured HTTP request if the referring resource was accessed with a
secure protocol. See Section 17.9 for additional security secure protocol. See Section 17.9 for additional security
considerations. considerations.
Some intermediaries have been known to indiscriminately remove Some intermediaries have been known to indiscriminately remove
Referer header fields from outgoing requests. This has the Referer header fields from outgoing requests. This has the
unfortunate side effect of interfering with protection against CSRF unfortunate side effect of interfering with protection against CSRF
attacks, which can be far more harmful to their users. attacks, which can be far more harmful to their users.
Intermediaries and user agent extensions that wish to limit Intermediaries and user agent extensions that wish to limit
information disclosure in Referer ought to restrict their changes to information disclosure in Referer ought to restrict their changes to
specific edits, such as replacing internal domain names with specific edits, such as replacing internal domain names with
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[Messaging]). [Messaging]).
The TE field value consists of a list of tokens, each allowing for The TE field value consists of a list of tokens, each allowing for
optional parameters (except for the special case "trailers"). optional parameters (except for the special case "trailers").
TE = #t-codings TE = #t-codings
t-codings = "trailers" / ( transfer-coding [ weight ] ) t-codings = "trailers" / ( transfer-coding [ weight ] )
transfer-coding = token *( OWS ";" OWS transfer-parameter ) transfer-coding = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = token BWS "=" BWS ( token / quoted-string ) transfer-parameter = token BWS "=" BWS ( token / quoted-string )
A sender of TE MUST also send a "TE" connection option within the
Connection header field (Section 7.6.1) to inform intermediaries not
to forward this field.
10.1.5. Trailer 10.1.5. Trailer
The "Trailer" header field provides a list of field names that the The "Trailer" header field provides a list of field names that the
sender anticipates sending as trailer fields within that message. sender anticipates sending as trailer fields within that message.
This allows a recipient to prepare for receipt of the indicated This allows a recipient to prepare for receipt of the indicated
metadata before it starts processing the content. metadata before it starts processing the content.
Trailer = #field-name Trailer = #field-name
For example, a sender might indicate that a message integrity check For example, a sender might indicate that a message integrity check
will be computed as the content is being streamed and provide the will be computed as the content is being streamed and provide the
final signature as a trailer field. This allows a recipient to final signature as a trailer field. This allows a recipient to
perform the same check on the fly as the content is received. perform the same check on the fly as it receives the content.
Because the Trailer field is not removed by intermediaries, it can
also be used by downstream recipients to discover when a trailer
field has been removed from a message.
A sender that intends to generate one or more trailer fields in a A sender that intends to generate one or more trailer fields in a
message SHOULD generate a Trailer header field in the header section message SHOULD generate a Trailer header field in the header section
of that message to indicate which fields might be present in the of that message to indicate which fields might be present in the
trailers. trailers.
10.1.6. User-Agent 10.1.6. User-Agent
The "User-Agent" header field contains information about the user The "User-Agent" header field contains information about the user
agent originating the request, which is often used by servers to help agent originating the request, which is often used by servers to help
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A sender SHOULD limit generated product identifiers to what is A sender SHOULD limit generated product identifiers to what is
necessary to identify the product; a sender MUST NOT generate necessary to identify the product; a sender MUST NOT generate
advertising or other nonessential information within the product advertising or other nonessential information within the product
identifier. A sender SHOULD NOT generate information in identifier. A sender SHOULD NOT generate information in
product-version that is not a version identifier (i.e., successive product-version that is not a version identifier (i.e., successive
versions of the same product name ought to differ only in the versions of the same product name ought to differ only in the
product-version portion of the product identifier). product-version portion of the product identifier).
Example: Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3 User-Agent: CERN-LineMode/2.15 libwww/2.17b3
A user agent SHOULD NOT generate a User-Agent header field containing A user agent SHOULD NOT generate a User-Agent header field containing
needlessly fine-grained detail and SHOULD limit the addition of needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly long and detailed User-Agent subproducts by third parties. Overly long and detailed User-Agent
field values increase request latency and the risk of a user being field values increase request latency and the risk of a user being
identified against their wishes ("fingerprinting"). identified against their wishes ("fingerprinting").
Likewise, implementations are encouraged not to use the product Likewise, implementations are encouraged not to use the product
tokens of other implementations in order to declare compatibility tokens of other implementations in order to declare compatibility
with them, as this circumvents the purpose of the field. If a user with them, as this circumvents the purpose of the field. If a user
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The "Allow" header field lists the set of methods advertised as The "Allow" header field lists the set of methods advertised as
supported by the target resource. The purpose of this field is supported by the target resource. The purpose of this field is
strictly to inform the recipient of valid request methods associated strictly to inform the recipient of valid request methods associated
with the resource. with the resource.
Allow = #method Allow = #method
Example of use: Example of use:
Allow: GET, HEAD, PUT Allow: GET, HEAD, PUT
The actual set of allowed methods is defined by the origin server at 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 the time of each request. An origin server MUST generate an Allow
header field in a 405 (Method Not Allowed) response and MAY do so in header field in a 405 (Method Not Allowed) response and MAY do so in
any other response. An empty Allow field value indicates that the any other response. An empty Allow field value indicates that the
resource allows no methods, which might occur in a 405 response if resource allows no methods, which might occur in a 405 response if
the resource has been temporarily disabled by configuration. the resource has been temporarily disabled by configuration.
A proxy MUST NOT modify the Allow header field - it does not need to 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 understand all of the indicated methods in order to handle them
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The "Date" header field represents the date and time at which the The "Date" header field represents the date and time at which the
message was originated, having the same semantics as the Origination message was originated, having the same semantics as the Origination
Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The
field value is an HTTP-date, as defined in Section 5.6.7. field value is an HTTP-date, as defined in Section 5.6.7.
Date = HTTP-date Date = HTTP-date
An example is An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT Date: Tue, 15 Nov 1994 08:12:31 GMT
When a Date header field is generated, the sender SHOULD generate its A sender that generates a Date header field SHOULD generate its field
field value as the best available approximation of the date and time value as the best available approximation of the date and time of
of message generation. In theory, the date ought to represent the message generation. In theory, the date ought to represent the
moment just before the content is generated. In practice, the date moment just before generating the message content. In practice, a
can be generated at any time during message origination. sender can generate the date value at any time during message
origination.
An origin server MUST NOT send a Date header field if it does not 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 have a clock capable of providing a reasonable approximation of the
current instance in Coordinated Universal Time. An origin server MAY current instance in Coordinated Universal Time. An origin server MAY
send a Date header field if the response is in the 1xx send a Date header field if the response is in the 1xx
(Informational) or 5xx (Server Error) class of status codes. An (Informational) or 5xx (Server Error) class of status codes. An
origin server MUST send a Date header field in all other cases. origin server MUST send a Date header field in all other cases.
A recipient with a clock that receives a response message without a 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 Date header field MUST record the time it was received and append a
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If the Location value provided in a 3xx (Redirection) response does If the Location value provided in a 3xx (Redirection) response does
not have a fragment component, a user agent MUST process the not have a fragment component, a user agent MUST process the
redirection as if the value inherits the fragment component of the redirection as if the value inherits the fragment component of the
URI reference used to generate the target URI (i.e., the redirection URI reference used to generate the target URI (i.e., the redirection
inherits the original reference's fragment, if any). inherits the original reference's fragment, if any).
For example, a GET request generated for the URI reference For example, a GET request generated for the URI reference
"http://www.example.org/~tim" might result in a 303 (See Other) "http://www.example.org/~tim" might result in a 303 (See Other)
response containing the header field: response containing the header field:
Location: /People.html#tim Location: /People.html#tim
which suggests that the user agent redirect to which suggests that the user agent redirect to
"http://www.example.org/People.html#tim" "http://www.example.org/People.html#tim"
Likewise, a GET request generated for the URI reference Likewise, a GET request generated for the URI reference
"http://www.example.org/index.html#larry" might result in a 301 "http://www.example.org/index.html#larry" might result in a 301
(Moved Permanently) response containing the header field: (Moved Permanently) response containing the header field:
Location: http://www.example.net/index.html Location: http://www.example.net/index.html
which suggests that the user agent redirect to which suggests that the user agent redirect to
"http://www.example.net/index.html#larry", preserving the original "http://www.example.net/index.html#larry", preserving the original
fragment identifier. fragment identifier.
There are circumstances in which a fragment identifier in a Location There are circumstances in which a fragment identifier in a Location
value would not be appropriate. For example, the Location header value would not be appropriate. For example, the Location header
field in a 201 (Created) response is supposed to provide a URI that field in a 201 (Created) response is supposed to provide a URI that
is specific to the created resource. is specific to the created resource.
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Servers send the "Retry-After" header field to indicate how long the Servers send the "Retry-After" header field to indicate how long the
user agent ought to wait before making a follow-up request. When user agent ought to wait before making a follow-up request. When
sent with a 503 (Service Unavailable) response, Retry-After indicates sent with a 503 (Service Unavailable) response, Retry-After indicates
how long the service is expected to be unavailable to the client. how long the service is expected to be unavailable to the client.
When sent with any 3xx (Redirection) response, Retry-After indicates When sent with any 3xx (Redirection) response, Retry-After indicates
the minimum time that the user agent is asked to wait before issuing the minimum time that the user agent is asked to wait before issuing
the redirected request. the redirected request.
The Retry-After field value can be either an HTTP-date or a number of The Retry-After field value can be either an HTTP-date or a number of
seconds to delay after the response is received. seconds to delay after receiving the response.
Retry-After = HTTP-date / delay-seconds Retry-After = HTTP-date / delay-seconds
A delay-seconds value is a non-negative decimal integer, representing A delay-seconds value is a non-negative decimal integer, representing
time in seconds. time in seconds.
delay-seconds = 1*DIGIT delay-seconds = 1*DIGIT
Two examples of its use are Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120 Retry-After: 120
In the latter example, the delay is 2 minutes. In the latter example, the delay is 2 minutes.
10.2.5. Server 10.2.5. Server
The "Server" header field contains information about the software The "Server" header field contains information about the software
used by the origin server to handle the request, which is often used used by the origin server to handle the request, which is often used
by clients to help identify the scope of reported interoperability by clients to help identify the scope of reported interoperability
problems, to work around or tailor requests to avoid particular problems, to work around or tailor requests to avoid particular
server limitations, and for analytics regarding server or operating server limitations, and for analytics regarding server or operating
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The Server header field value consists of one or more product The Server header field value consists of one or more product
identifiers, each followed by zero or more comments (Section 5.6.5), identifiers, each followed by zero or more comments (Section 5.6.5),
which together identify the origin server software and its which together identify the origin server software and its
significant subproducts. By convention, the product identifiers are significant subproducts. By convention, the product identifiers are
listed in decreasing order of their significance for identifying the listed in decreasing order of their significance for identifying the
origin server software. Each product identifier consists of a name origin server software. Each product identifier consists of a name
and optional version, as defined in Section 10.1.6. and optional version, as defined in Section 10.1.6.
Example: Example:
Server: CERN/3.0 libwww/2.17 Server: CERN/3.0 libwww/2.17
An origin server SHOULD NOT generate a Server header field containing An origin server SHOULD NOT generate a Server header field containing
needlessly fine-grained detail and SHOULD limit the addition of needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly long and detailed Server field subproducts by third parties. Overly long and detailed Server field
values increase response latency and potentially reveal internal values increase response latency and potentially reveal internal
implementation details that might make it (slightly) easier for implementation details that might make it (slightly) easier for
attackers to find and exploit known security holes. attackers to find and exploit known security holes.
11. HTTP Authentication 11. HTTP Authentication
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is a string, generally assigned by the origin server, that can have is a string, generally assigned by the origin server, that can have
additional semantics specific to the authentication scheme. Note additional semantics specific to the authentication scheme. Note
that a response can have multiple challenges with the same auth- that a response can have multiple challenges with the same auth-
scheme but with different realms. scheme but with different realms.
The protection space determines the domain over which credentials can The protection space determines the domain over which credentials can
be automatically applied. If a prior request has been authorized, be automatically applied. If a prior request has been authorized,
the user agent MAY reuse the same credentials for all other requests the user agent MAY reuse the same credentials for all other requests
within that protection space for a period of time determined by the within that protection space for a period of time determined by the
authentication scheme, parameters, and/or user preferences (such as a authentication scheme, parameters, and/or user preferences (such as a
configurable inactivity timeout). Unless specifically allowed by the configurable inactivity timeout).
authentication scheme, a single protection space cannot extend
outside the scope of its server. The extent of a protection space, and therefore the requests to which
credentials might be automatically applied, is not necessarily known
to clients without additional information. An authentication scheme
might define parameters that describe the extent of a protection
space. 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 For historical reasons, a sender MUST only generate the quoted-string
syntax. Recipients might have to support both token and quoted- syntax. Recipients might have to support both token and quoted-
string syntax for maximum interoperability with existing clients that string syntax for maximum interoperability with existing clients that
have been accepting both notations for a long time. have been accepting both notations for a long time.
11.6. Authenticating Users to Origin Servers 11.6. Authenticating Users to Origin Servers
11.6.1. WWW-Authenticate 11.6.1. WWW-Authenticate
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header fields in that response. header fields in that response.
User agents are advised to take special care in parsing the field User agents are advised to take special care in parsing the field
value, as it might contain more than one challenge, and each value, as it might contain more than one challenge, and each
challenge can contain a comma-separated list of authentication challenge can contain a comma-separated list of authentication
parameters. Furthermore, the header field itself can occur multiple parameters. Furthermore, the header field itself can occur multiple
times. times.
For instance: For instance:
WWW-Authenticate: Newauth realm="apps", type=1, WWW-Authenticate: Basic realm="simple", Newauth realm="apps",
title="Login to \"apps\"", Basic realm="simple" type=1, title="Login to \"apps\""
This header field contains two challenges; one for the "Newauth" This header field contains two challenges; one for the "Basic" scheme
scheme with a realm value of "apps", and two additional parameters with a realm value of "simple", and another for the "Newauth" scheme
"type" and "title", and another one for the "Basic" scheme with a with a realm value of "apps", and two additional parameters "type"
realm value of "simple". and "title".
Some user agents do not recognise this form, however. As a result, Some user agents do not recognise this form, however. As a result,
sending a WWW-Authenticate field value with more than one member on sending a WWW-Authenticate field value with more than one member on
the same field line might not be interoperable. the same field line might not be interoperable.
| *Note:* The challenge grammar production uses the list syntax | *Note:* The challenge grammar production uses the list syntax
| as well. Therefore, a sequence of comma, whitespace, and comma | as well. Therefore, a sequence of comma, whitespace, and comma
| can be considered either as applying to the preceding | can be considered either as applying to the preceding
| challenge, or to be an empty entry in the list of challenges. | challenge, or to be an empty entry in the list of challenges.
| In practice, this ambiguity does not affect the semantics of | In practice, this ambiguity does not affect the semantics of
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time, is determined entirely by whatever entity or algorithm selects time, is determined entirely by whatever entity or algorithm selects
or generates those responses. or generates those responses.
12.1. Proactive Negotiation 12.1. Proactive Negotiation
When content negotiation preferences are sent by the user agent in a When content negotiation preferences are sent by the user agent in a
request to encourage an algorithm located at the server to select the request to encourage an algorithm located at the server to select the
preferred representation, it is called _proactive negotiation_ preferred representation, it is called _proactive negotiation_
(a.k.a., _server-driven negotiation_). Selection is based on the (a.k.a., _server-driven negotiation_). Selection is based on the
available representations for a response (the dimensions over which available representations for a response (the dimensions over which
it might vary, such as language, content-coding, etc.) compared to it might vary, such as language, content coding, etc.) compared to
various information supplied in the request, including both the various information supplied in the request, including both the
explicit negotiation header fields below and implicit explicit negotiation header fields below and implicit
characteristics, such as the client's network address or parts of the characteristics, such as the client's network address or parts of the
User-Agent field. User-Agent field.
Proactive negotiation is advantageous when the algorithm for Proactive negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to selecting from among the available representations is difficult to
describe to a user agent, or when the server desires to send its describe to a user agent, or when the server desires to send its
"best guess" to the user agent along with the first response (hoping "best guess" to the user agent along with the first response (hoping
to avoid the round trip delay of a subsequent request if the "best to avoid the round trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the guess" is good enough for the user). In order to improve the
server's guess, a user agent MAY send request header fields that server's guess, a user agent MAY send request header fields that
describe its preferences. describe its preferences.
Proactive negotiation has serious disadvantages: Proactive negotiation has serious disadvantages:
o It is impossible for the server to accurately determine what might * It is impossible for the server to accurately determine what might
be "best" for any given user, since that would require complete be "best" for any given user, since that would require complete
knowledge of both the capabilities of the user agent and the 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 intended use for the response (e.g., does the user want to view it
on screen or print it on paper?); on screen or print it on paper?);
o Having the user agent describe its capabilities in every request * Having the user agent describe its capabilities in every request
can be both very inefficient (given that only a small percentage can be both very inefficient (given that only a small percentage
of responses have multiple representations) and a potential risk of responses have multiple representations) and a potential risk
to the user's privacy; to the user's privacy;
o It complicates the implementation of an origin server and the * It complicates the implementation of an origin server and the
algorithms for generating responses to a request; and, algorithms for generating responses to a request; and,
o It limits the reusability of responses for shared caching. * It limits the reusability of responses for shared caching.
A user agent cannot rely on proactive negotiation preferences being A user agent cannot rely on proactive negotiation preferences being
consistently honored, since the origin server might not implement consistently honored, since the origin server might not implement
proactive negotiation for the requested resource or might decide that proactive negotiation for the requested resource or might decide that
sending a response that doesn't conform to the user agent's sending a response that doesn't conform to the user agent's
preferences is better than sending a 406 (Not Acceptable) response. preferences is better than sending a 406 (Not Acceptable) response.
A Vary header field (Section 12.5.5) is often sent in a response A Vary header field (Section 12.5.5) is often sent in a response
subject to proactive negotiation to indicate what parts of the subject to proactive negotiation to indicate what parts of the
request information were used in the selection algorithm. request information were used in the selection algorithm.
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and Accept-Language are defined below for a user agent to engage in and Accept-Language are defined below for a user agent to engage in
proactive negotiation of the response content. The preferences sent proactive negotiation of the response content. The preferences sent
in these fields apply to any content in the response, including in these fields apply to any content in the response, including
representations of the target resource, representations of error or representations of the target resource, representations of error or
processing status, and potentially even the miscellaneous text processing status, and potentially even the miscellaneous text
strings that might appear within the protocol. strings that might appear within the protocol.
12.2. Reactive Negotiation 12.2. Reactive Negotiation
With _reactive negotiation_ (a.k.a., _agent-driven negotiation_), With _reactive negotiation_ (a.k.a., _agent-driven negotiation_),
selection of the best response representation (regardless of the selection of content (regardless of the status code) is performed by
status code) is performed by the user agent after receiving an the user agent after receiving an initial response. The mechanism
initial response from the origin server that contains a list of for reactive negotiation might be as simple as a list of references
resources for alternative representations. to 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.
Note that the above refers to representations of the response, in If the user agent is not satisfied by the initial response content,
general, not representations of the resource. The alternative it can perform a GET request on one or more of the alternative
representations are only considered representations of the target resources to obtain a different representation. Selection of such
resource if the response in which those alternatives are provided has alternatives might be performed automatically (by the user agent) or
the semantics of being a representation of the target resource (e.g., manually (e.g., by the user selecting from a hypertext menu).
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 A server might choose not to send an initial representation, other
than the list of alternatives, and thereby indicate that reactive than the list of alternatives, and thereby indicate that reactive
negotiation by the user agent is preferred. For example, the negotiation by the user agent is preferred. For example, the
alternatives listed in responses with the 300 (Multiple Choices) and alternatives listed in responses with the 300 (Multiple Choices) and
406 (Not Acceptable) status codes include information about the 406 (Not Acceptable) status codes include information about available
available representations so that the user or user agent can react by representations so that the user or user agent can react by making a
making a selection. selection.
Reactive negotiation is advantageous when the response would vary Reactive negotiation is advantageous when the response would vary
over commonly used dimensions (such as type, language, or encoding), over commonly used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage. caches are used to distribute server load and reduce network usage.
Reactive negotiation suffers from the disadvantages of transmitting a Reactive negotiation suffers from the disadvantages of transmitting a
list of alternatives to the user agent, which degrades user-perceived list of alternatives to the user agent, which degrades user-perceived
latency if transmitted in the header section, and needing a second latency if transmitted in the header section, and needing a second
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| *Note:* Use of the "q" parameter name to control content | *Note:* Use of the "q" parameter name to control content
| negotiation is due to historical practice. Although this | negotiation is due to historical practice. Although this
| prevents any media type parameter named "q" from being used | prevents any media type parameter named "q" from being used
| with a media range, such an event is believed to be unlikely | with a media range, such an event is believed to be unlikely
| given the lack of any "q" parameters in the IANA media type | given the lack of any "q" parameters in the IANA media type
| registry and the rare usage of any media type parameters in | registry and the rare usage of any media type parameters in
| Accept. Future media types are discouraged from registering | Accept. Future media types are discouraged from registering
| any parameter named "q". | any parameter named "q".
The example The example
Accept: audio/*; q=0.2, audio/basic
Accept: audio/*; q=0.2, audio/basic
is interpreted as "I prefer audio/basic, but send me any audio type 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". if it is the best available after an 80% markdown in quality".
A more elaborate example is A more elaborate example is
Accept: text/plain; q=0.5, text/html, Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are Verbally, this would be interpreted as "text/html and text/x-c are
the equally preferred media types, but if they do not exist, then 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 send the text/x-dvi representation, and if that does not exist, send
the text/plain representation". the text/plain representation".
Media ranges can be overridden by more specific media ranges or Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a specific media types. If more than one media range applies to a
given type, the most specific reference has precedence. For example, given type, the most specific reference has precedence. For example,
Accept: text/*, text/plain, text/plain;format=flowed, */* Accept: text/*, text/plain, text/plain;format=flowed, */*
have the following precedence: have the following precedence:
1. text/plain;format=flowed 1. text/plain;format=flowed
2. text/plain 2. text/plain
3. text/* 3. text/*
4. */* 4. */*
The media type quality factor associated with a given type is The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence determined by finding the media range with the highest precedence
that matches the type. For example, that matches the type. For example,
Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed, Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
text/plain;format=fixed;q=0.4, */*;q=0.5 text/plain;format=fixed;q=0.4, */*;q=0.5
would cause the following values to be associated: would cause the following values to be associated:
-------------------------- --------------- +==========================+===============+
Media Type Quality Value | Media Type | Quality Value |
-------------------------- --------------- +==========================+===============+
text/plain;format=flowed 1 | text/plain;format=flowed | 1 |
text/plain 0.7 +--------------------------+---------------+
text/html 0.3 | text/plain | 0.7 |
image/jpeg 0.5 +--------------------------+---------------+
text/plain;format=fixed 0.4 | text/html | 0.3 |
text/html;level=3 0.7 +--------------------------+---------------+
-------------------------- --------------- | image/jpeg | 0.5 |
+--------------------------+---------------+
| text/plain;format=fixed | 0.4 |
+--------------------------+---------------+
| text/html;level=3 | 0.7 |
+--------------------------+---------------+
Table 5 Table 5
| *Note:* A user agent might be provided with a default set of | *Note:* A user agent might be provided with a default set of
| quality values for certain media ranges. However, unless the | quality values for certain media ranges. However, unless the
| user agent is a closed system that cannot interact with other | user agent is a closed system that cannot interact with other
| rendering agents, this default set ought to be configurable by | rendering agents, this default set ought to be configurable by
| the user. | the user.
12.5.2. Accept-Charset 12.5.2. Accept-Charset
The "Accept-Charset" header field can be sent by a user agent to The "Accept-Charset" header field can be sent by a user agent to
indicate its preferences for charsets in textual response content. indicate its preferences for charsets in textual response content.
For example, this field allows user agents capable of understanding For example, this field allows user agents capable of understanding
more comprehensive or special-purpose charsets to signal that more comprehensive or special-purpose charsets to signal that
capability to an origin server that is capable of representing capability to an origin server that is capable of representing
information in those charsets. information in those charsets.
Accept-Charset = #( ( charset / "*" ) [ weight ] ) Accept-Charset = #( ( token / "*" ) [ weight ] )
Charset names are defined in Section 8.3.2. A user agent MAY Charset names are defined in Section 8.3.2. A user agent MAY
associate a quality value with each charset to indicate the user's associate a quality value with each charset to indicate the user's
relative preference for that charset, as defined in Section 12.4.2. relative preference for that charset, as defined in Section 12.4.2.
An example is An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset header field, The special value "*", if present in the Accept-Charset header field,
matches every charset that is not mentioned elsewhere in the field. matches every charset that is not mentioned elsewhere in the field.
| *Note:* Accept-Charset is deprecated because UTF-8 has become | *Note:* Accept-Charset is deprecated because UTF-8 has become
| nearly ubiquitous and sending a detailed list of user-preferred | nearly ubiquitous and sending a detailed list of user-preferred
| charsets wastes bandwidth, increases latency, and makes passive | charsets wastes bandwidth, increases latency, and makes passive
| fingerprinting far too easy (Section 17.12). Most general- | fingerprinting far too easy (Section 17.12). Most general-
| purpose user agents do not send Accept-Charset, unless | purpose user agents do not send Accept-Charset, unless
| specifically configured to do so. | specifically configured to do so.
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An "identity" token is used as a synonym for "no encoding" in order An "identity" token is used as a synonym for "no encoding" in order
to communicate when no encoding is preferred. to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ weight ] ) Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*" codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value Each codings value MAY be given an associated quality value
representing the preference for that encoding, as defined in representing the preference for that encoding, as defined in
Section 12.4.2. The asterisk "*" symbol in an Accept-Encoding field Section 12.4.2. The asterisk "*" symbol in an Accept-Encoding field
matches any available content-coding not explicitly listed in the matches any available content coding not explicitly listed in the
field. field.
For example, For example,
Accept-Encoding: compress, gzip Accept-Encoding: compress, gzip
Accept-Encoding: Accept-Encoding:
Accept-Encoding: * Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0 Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0 Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding for a given representation is A server tests whether a content coding for a given representation is
acceptable using these rules: acceptable using these rules:
1. If no Accept-Encoding header field is in the request, any 1. If no Accept-Encoding header field is in the request, any content
content-coding is considered acceptable by the user agent. coding is considered acceptable by the user agent.
2. If the representation has no content-coding, then it is 2. If the representation has no content coding, then it is
acceptable by default unless specifically excluded by the Accept- acceptable by default unless specifically excluded by the Accept-
Encoding header field stating either "identity;q=0" or "*;q=0" Encoding header field stating either "identity;q=0" or "*;q=0"
without a more specific entry for "identity". without a more specific entry for "identity".
3. If the representation's content-coding is one of the content- 3. If the representation's content coding is one of the content
codings listed in the Accept-Encoding field value, then it is codings listed in the Accept-Encoding field value, then it is
acceptable unless it is accompanied by a qvalue of 0. (As acceptable unless it is accompanied by a qvalue of 0. (As
defined in Section 12.4.2, a qvalue of 0 means "not acceptable".) defined in Section 12.4.2, a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable A representation could be encoded with multiple content codings.
content-coding with the highest non-zero qvalue is preferred. However, most content codings are alternative ways to accomplish the
same purpose (e.g., data compression). When selecting between
multiple content codings that have the same purpose, the acceptable
content coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a field value that is empty An Accept-Encoding header field with a field value that is empty
implies that the user agent does not want any content-coding in implies that the user agent does not want any content coding in
response. If an Accept-Encoding header field is present in a request response. If an Accept-Encoding header field is present in a request
and none of the available representations for the response have a and none of the available representations for the response have a
content-coding that is listed as acceptable, the origin server SHOULD content coding that is listed as acceptable, the origin server SHOULD
send a response without any content-coding. send a response without any content coding.
When the Accept-Encoding header field is present in a response, it When the Accept-Encoding header field is present in a response, it
indicates what content codings the resource was willing to accept in indicates what content codings the resource was willing to accept in
the associated request. The field value is evaluated the same way as the associated request. The field value is evaluated the same way as
in a request. in a request.
Note that this information is specific to the associated request; the Note that this information is specific to the associated request; the
set of supported encodings might be different for other resources on set of supported encodings might be different for other resources on
the same server and could change over time or depend on other aspects the same server and could change over time or depend on other aspects
of the request (such as the request method). of the request (such as the request method).
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response. Language tags are defined in Section 8.5.1. response. Language tags are defined in Section 8.5.1.
Accept-Language = #( language-range [ weight ] ) Accept-Language = #( language-range [ weight ] )
language-range = language-range =
<language-range, see [RFC4647], Section 2.1> <language-range, see [RFC4647], Section 2.1>
Each language-range can be given an associated quality value Each language-range can be given an associated quality value
representing an estimate of the user's preference for the languages representing an estimate of the user's preference for the languages
specified by that range, as defined in Section 12.4.2. For example, specified by that range, as defined in Section 12.4.2. For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7 Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and would mean: "I prefer Danish, but will accept British English and
other types of English". other types of English".
Note that some recipients treat the order in which language tags are Note that some recipients treat the order in which language tags are
listed as an indication of descending priority, particularly for tags listed as an indication of descending priority, particularly for tags
that are assigned equal quality values (no value is the same as q=1). that are assigned equal quality values (no value is the same as q=1).
However, this behavior cannot be relied upon. For consistency and to However, this behavior cannot be relied upon. For consistency and to
maximize interoperability, many user agents assign each language tag maximize interoperability, many user agents assign each language tag
a unique quality value while also listing them in order of decreasing a unique quality value while also listing them in order of decreasing
quality. Additional discussion of language priority lists can be quality. Additional discussion of language priority lists can be
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If the list contains "*", it signals that other aspects of the If the list contains "*", it signals that other aspects of the
request might play a role in selecting the response representation, request might play a role in selecting the response representation,
possibly including elements outside the message syntax (e.g., the possibly including elements outside the message syntax (e.g., the
client's network address). A recipient will not be able to determine client's network address). A recipient will not be able to determine
whether this response is appropriate for a later request without whether this response is appropriate for a later request without
forwarding the request to the origin server. A proxy MUST NOT forwarding the request to the origin server. A proxy MUST NOT
generate "*" in a Vary field value. generate "*" in a Vary field value.
For example, a response that contains For example, a response that contains
Vary: accept-encoding, accept-language Vary: accept-encoding, accept-language
indicates that the origin server might have used the request's indicates that the origin server might have used the request's
Accept-Encoding and Accept-Language header fields (or lack thereof) Accept-Encoding and Accept-Language header fields (or lack thereof)
as determining factors while choosing the content for this response. as determining factors while choosing the content for this response.
An origin server might send Vary with a list of header fields for two An origin server might send Vary with a list of header fields for two
purposes: purposes:
1. To inform cache recipients that they MUST NOT use this response 1. To inform cache recipients that they MUST NOT use this response
to satisfy a later request unless the later request has the same to satisfy a later request unless the later request has the same
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applying the request method to the target resource. Section 13.2 applying the request method to the target resource. Section 13.2
defines when to evaluate preconditions and their order of precedence defines when to evaluate preconditions and their order of precedence
when more than one precondition is present. when more than one precondition is present.
Conditional GET requests are the most efficient mechanism for HTTP Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Caching]. Conditionals can also be applied to state- cache updates [Caching]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel. another client that has been acting in parallel.
Conditional request preconditions are based on the state of the 13.1. Preconditions
Preconditions are usually defined with respect to a state of the
target resource as a whole (its current value set) or the state as target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each set). If a resource has multiple current representations, each with
with its own observable state. The conditional request mechanisms its own observable state, a precondition will assume that the mapping
assume that the mapping of requests to a selected representation of each request to a selected representation (Section 3.2) is
(Section 3.2) will be consistent over time if the server intends to consistent over time. Regardless, if the mapping is inconsistent or
take advantage of conditionals. Regardless, if the mapping is the server is unable to select an appropriate representation, then no
inconsistent and the server is unable to select the appropriate harm will result when the precondition evaluates to false.
representation, then no harm will result when the precondition
evaluates to false.
13.1. Preconditions Each precondition defined below 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 8.8). 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 13.2.
The request header fields below allow a client to place a Other preconditions, defined by other specifications as extension
precondition on the state of the target resource, so that the action fields, might place conditions on all recipients, on the state of the
corresponding to the method semantics will not be applied if the target resource in general, or on a group of resources. For
precondition evaluates to false. Each precondition defined by this instance, the "If" header field in WebDAV can make a request
specification consists of a comparison between a set of validators conditional on various aspects of multiple resources, such as locks,
obtained from prior representations of the target resource to the if the recipient understands and implements that field ([RFC4918],
current state of validators for the selected representation Section 10.4).
(Section 8.8). Hence, these preconditions evaluate whether the state
of the target resource has changed since a given state known by the Extensibility of preconditions is only possible when the precondition
client. The effect of such an evaluation depends on the method can be safely ignored if unknown (like If-Modified-Since), when
semantics and choice of conditional, as defined in Section 13.2. deployment can be assumed for a given use case, or when
implementation is signaled by some other property of the target
resource. This encourages a focus on mutually agreed deployment of
common standards.
13.1.1. If-Match 13.1.1. If-Match
The "If-Match" header field makes the request method conditional on The "If-Match" header field makes the request method conditional on
the recipient origin server either having at least one current the recipient origin server either having at least one current
representation of the target resource, when the field value is "*", representation of the target resource, when the field value is "*",
or having a current representation of the target resource that has an or having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in entity-tag matching a member of the list of entity-tags provided in
the field value. the field value.
An origin server MUST use the strong comparison function when An origin server MUST use the strong comparison function when
comparing entity-tags for If-Match (Section 8.8.3.2), since the comparing entity-tags for If-Match (Section 8.8.3.2), since the
client intends this precondition to prevent the method from being client intends this precondition to prevent the method from being
applied if there have been any changes to the representation data. applied if there have been any changes to the representation data.
If-Match = "*" / #entity-tag If-Match = "*" / #entity-tag
Examples: Examples:
If-Match: "xyzzy" If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: * If-Match: *
If-Match is most often used with state-changing methods (e.g., POST, If-Match is most often used with state-changing methods (e.g., POST,
PUT, DELETE) to prevent accidental overwrites when multiple user PUT, DELETE) to prevent accidental overwrites when multiple user
agents might be acting in parallel on the same resource (i.e., to agents might be acting in parallel on the same resource (i.e., to
prevent the "lost update" problem). It can also be used with any prevent the "lost update" problem). It can also be used with any
method to abort a request if the selected representation does not method to abort a request if the selected representation does not
match one that the client has already stored (or partially stored) match one that the client has already stored (or partially stored)
from a prior request. from a prior request.
An origin server that receives an If-Match header field MUST evaluate An origin server that receives an If-Match header field MUST evaluate
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A recipient MUST use the weak comparison function when comparing A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (Section 8.8.3.2), since weak entity- entity-tags for If-None-Match (Section 8.8.3.2), since weak entity-
tags can be used for cache validation even if there have been changes tags can be used for cache validation even if there have been changes
to the representation data. to the representation data.
If-None-Match = "*" / #entity-tag If-None-Match = "*" / #entity-tag
Examples: Examples:
If-None-Match: "xyzzy" If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy" If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: * If-None-Match: *
If-None-Match is primarily used in conditional GET requests to enable If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of efficient updates of cached information with a minimum amount of
transaction overhead. When a client desires to update one or more transaction overhead. When a client desires to update one or more
stored responses that have entity-tags, the client SHOULD generate an stored responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags If-None-Match header field containing a list of those entity-tags
when making a GET request; this allows recipient servers to send a when making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those stored 304 (Not Modified) response to indicate when one of those stored
responses matches the selected representation. responses matches the selected representation.
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The "If-Modified-Since" header field makes a GET or HEAD request The "If-Modified-Since" header field makes a GET or HEAD request
method conditional on the selected representation's modification date method conditional on the selected representation's modification date
being more recent than the date provided in the field value. being more recent than the date provided in the field value.
Transfer of the selected representation's data is avoided if that Transfer of the selected representation's data is avoided if that
data has not changed. data has not changed.
If-Modified-Since = HTTP-date If-Modified-Since = HTTP-date
An example of the field is: An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Modified-Since if the request contains an A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is If-None-Match header field; the condition in If-None-Match is
considered to be a more accurate replacement for the condition in If- considered to be a more accurate replacement for the condition in If-
Modified-Since, and the two are only combined for the sake of Modified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement interoperating with older intermediaries that might not implement
If-None-Match. If-None-Match.
A recipient MUST ignore the If-Modified-Since header field if the A recipient MUST ignore the If-Modified-Since header field if the
received field value is not a valid HTTP-date, the field value has received field value is not a valid HTTP-date, the field value has
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The "If-Unmodified-Since" header field makes the request method The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field value. being earlier than or equal to the date provided in the field value.
This field accomplishes the same purpose as If-Match for cases where This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation. the user agent does not have an entity-tag for the representation.
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
An example of the field is: An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Unmodified-Since if the request contains A recipient MUST ignore If-Unmodified-Since if the request contains
an If-Match header field; the condition in If-Match is considered to an If-Match header field; the condition in If-Match is considered to
be a more accurate replacement for the condition in If-Unmodified- be a more accurate replacement for the condition in If-Unmodified-
Since, and the two are only combined for the sake of interoperating Since, and the two are only combined for the sake of interoperating
with older intermediaries that might not implement If-Match. with older intermediaries that might not implement If-Match.
A recipient MUST ignore the If-Unmodified-Since header field if the A recipient MUST ignore the If-Unmodified-Since header field if the
received field value is not a valid HTTP-date (including when the received field value is not a valid HTTP-date (including when the
field value appears to be a list of dates). field value appears to be a list of dates).
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MUST ignore the conditional request header fields defined by this MUST ignore the conditional request header fields defined by this
specification when received with a request method that does not specification when received with a request method that does not
involve the selection or modification of a selected representation, involve the selection or modification of a selected representation,
such as CONNECT, OPTIONS, or TRACE. such as CONNECT, OPTIONS, or TRACE.
Note that protocol extensions can modify the conditions under which Note that protocol extensions can modify the conditions under which
revalidation is triggered. For example, the "immutable" cache revalidation is triggered. For example, the "immutable" cache
directive (defined by [RFC8246]) instructs caches to forgo directive (defined by [RFC8246]) instructs caches to forgo
revalidation of fresh responses even when requested by the client. revalidation of fresh responses even when requested by the client.
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 Although conditional request header fields are defined as being
usable with the HEAD method (to keep HEAD's semantics consistent with usable with the HEAD method (to keep HEAD's semantics consistent with
those of GET), there is no point in sending a conditional HEAD those of GET), there is no point in sending a conditional HEAD
because a successful response is around the same size as a 304 (Not because a successful response is around the same size as a 304 (Not
Modified) response and more useful than a 412 (Precondition Failed) Modified) response and more useful than a 412 (Precondition Failed)
response. response.
13.2.2. Precedence of Preconditions 13.2.2. Precedence of Preconditions
When more than one conditional request header field is present in a When more than one conditional request header field is present in a
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validation, a validated cache is more efficient than a partial validation, a validated cache is more efficient than a partial
response, and entity tags are presumed to be more accurate than date response, and entity tags are presumed to be more accurate than date
validators. validators.
A recipient cache or origin server MUST evaluate the request A recipient cache or origin server MUST evaluate the request
preconditions defined by this specification in the following order: preconditions defined by this specification in the following order:
1. When recipient is the origin server and If-Match is present, 1. When recipient is the origin server and If-Match is present,
evaluate the If-Match precondition: evaluate the If-Match precondition:
o if true, continue to step 3 * if true, continue to step 3
o if false, respond 412 (Precondition Failed) unless it can be * if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already determined that the state-changing request has already
succeeded (see Section 13.1.1) succeeded (see Section 13.1.1)
2. When recipient is the origin server, If-Match is not present, and 2. When recipient is the origin server, If-Match is not present, and
If-Unmodified-Since is present, evaluate the If-Unmodified-Since If-Unmodified-Since is present, evaluate the If-Unmodified-Since
precondition: precondition:
o if true, continue to step 3 * if true, continue to step 3
o if false, respond 412 (Precondition Failed) unless it can be * if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already determined that the state-changing request has already
succeeded (see Section 13.1.4) succeeded (see Section 13.1.4)
3. When If-None-Match is present, evaluate the If-None-Match 3. When If-None-Match is present, evaluate the If-None-Match
precondition: precondition:
o if true, continue to step 5 * if true, continue to step 5
o if false for GET/HEAD, respond 304 (Not Modified) * if false for GET/HEAD, respond 304 (Not Modified)
o if false for other methods, respond 412 (Precondition Failed) * if false for other methods, respond 412 (Precondition Failed)
4. When the method is GET or HEAD, If-None-Match is not present, and 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 If-Modified-Since is present, evaluate the If-Modified-Since
precondition: precondition:
o if true, continue to step 5 * if true, continue to step 5
o if false, respond 304 (Not Modified)
* if false, respond 304 (Not Modified)
5. When the method is GET and both Range and If-Range are present, 5. When the method is GET and both Range and If-Range are present,
evaluate the If-Range precondition: evaluate the If-Range precondition:
o if the validator matches and the Range specification is * if the validator matches and the Range specification is
applicable to the selected representation, respond 206 applicable to the selected representation, respond 206
(Partial Content) (Partial Content)
6. Otherwise, 6. Otherwise,
* all conditions are met, so perform the requested action and
o all conditions are met, so perform the requested action and
respond according to its success or failure. respond according to its success or failure.
Any extension to HTTP that defines additional conditional request Any extension to HTTP that defines additional conditional request
header fields ought to define its own expectations regarding the header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice. document and other conditionals that might be found in practice.
14. Range Requests 14. Range Requests
Clients often encounter interrupted data transfers as a result of Clients often encounter interrupted data transfers as a result of
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last byte in the range; that is, the byte positions specified are last byte in the range; that is, the byte positions specified are
inclusive. Byte offsets start at zero. inclusive. Byte offsets start at zero.
If the representation data has a content coding applied, each byte If the representation data has a content coding applied, each byte
range is calculated with respect to the encoded sequence of bytes, range is calculated with respect to the encoded sequence of bytes,
not the sequence of underlying bytes that would be obtained after not the sequence of underlying bytes that would be obtained after
decoding. decoding.
Examples of bytes range specifiers: Examples of bytes range specifiers:
o The first 500 bytes (byte offsets 0-499, inclusive): * The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499 bytes=0-499
o The second 500 bytes (byte offsets 500-999, inclusive): * The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999 bytes=500-999
A client can limit the number of bytes requested without knowing the A client can limit the number of bytes requested without knowing the
size of the selected representation. If the last-pos value is size of the selected representation. If the last-pos value is
absent, or if the value is greater than or equal to the current absent, or if the value is greater than or equal to the current
length of the representation data, the byte range is interpreted as length of the representation data, the byte range is interpreted as
the remainder of the representation (i.e., the server replaces the the remainder of the representation (i.e., the server replaces the
value of last-pos with a value that is one less than the current value of last-pos with a value that is one less than the current
length of the selected representation). length of the selected representation).
A client can request the last N bytes (N > 0) of the selected A client can request the last N bytes (N > 0) of the selected
representation using a suffix-range. If the selected representation representation using a suffix-range. If the selected representation
is shorter than the specified suffix-length, the entire is shorter than the specified suffix-length, the entire
representation is used. representation is used.
Additional examples, assuming a representation of length 10000: Additional examples, assuming a representation of length 10000:
o The final 500 bytes (byte offsets 9500-9999, inclusive): * The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500 bytes=-500
Or: Or:
bytes=9500- bytes=9500-
o The first and last bytes only (bytes 0 and 9999): * The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1 bytes=0-0,-1
o The first, middle, and last 1000 bytes: * The first, middle, and last 1000 bytes:
bytes= 0-999, 4500-5499, -1000 bytes= 0-999, 4500-5499, -1000
o Other valid (but not canonical) specifications of the second 500 * Other valid (but not canonical) specifications of the second 500
bytes (byte offsets 500-999, inclusive): bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999 bytes=500-600,601-999
bytes=500-700,601-999 bytes=500-700,601-999
If a valid bytes range-set includes at least one range-spec with a If a valid bytes range-set includes at least one range-spec with a
first-pos that is less than the current length of the representation, first-pos that is less than the current length of the representation,
or at least one suffix-range with a non-zero suffix-length, then the or at least one suffix-range with a non-zero suffix-length, then the
bytes range-set is satisfiable. Otherwise, the bytes range-set is bytes range-set is satisfiable. Otherwise, the bytes range-set is
unsatisfiable. unsatisfiable.
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The "Accept-Ranges" header field allows a server to indicate that it The "Accept-Ranges" header field allows a server to indicate that it
supports range requests for the target resource. supports range requests for the target resource.
Accept-Ranges = acceptable-ranges Accept-Ranges = acceptable-ranges
acceptable-ranges = 1#range-unit / "none" acceptable-ranges = 1#range-unit / "none"
An origin server that supports byte-range requests for a given target An origin server that supports byte-range requests for a given target
resource MAY send resource MAY send
Accept-Ranges: bytes Accept-Ranges: bytes
to indicate what range units are supported. A client MAY generate to indicate what range units are supported. A client MAY generate
range requests without having received this header field for the range requests without having received this header field for the
resource involved. Range units are defined in Section 14.1. resource involved. Range units are defined in Section 14.1.
A server that does not support any kind of range request for the A server that does not support any kind of range request for the
target resource MAY send target resource MAY send
Accept-Ranges: none Accept-Ranges: none
to advise the client not to attempt a range request. to advise the client not to attempt a range request.
14.4. Content-Range 14.4. Content-Range
The "Content-Range" header field is sent in a single part 206 The "Content-Range" header field is sent in a single part 206
(Partial Content) response to indicate the partial range of the (Partial Content) response to indicate the partial range of the
selected representation enclosed as the message content, sent in each selected representation enclosed as the message content, sent in each
part of a multipart 206 response to indicate the range enclosed part of a multipart 206 response to indicate the range enclosed
within each body part, and sent in 416 (Range Not Satisfiable) within each body part, and sent in 416 (Range Not Satisfiable)
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For byte ranges, a sender SHOULD indicate the complete length of the For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the representation from which the range has been extracted, unless the
complete length is unknown or difficult to determine. An asterisk complete length is unknown or difficult to determine. An asterisk
character ("*") in place of the complete-length indicates that the character ("*") in place of the complete-length indicates that the
representation length was unknown when the header field was representation length was unknown when the header field was
generated. generated.
The following example illustrates when the complete length of the The following example illustrates when the complete length of the
selected representation is known by the sender to be 1234 bytes: selected representation is known by the sender to be 1234 bytes:
Content-Range: bytes 42-1233/1234 Content-Range: bytes 42-1233/1234
and this second example illustrates when the complete length is and this second example illustrates when the complete length is
unknown: unknown:
Content-Range: bytes 42-1233/* Content-Range: bytes 42-1233/*
A Content-Range field value is invalid if it contains a range-resp A Content-Range field value is invalid if it contains a range-resp
that has a last-pos value less than its first-pos value, or a that has a last-pos value less than its first-pos value, or a
complete-length value less than or equal to its last-pos value. The complete-length value less than or equal to its last-pos value. The
recipient of an invalid Content-Range MUST NOT attempt to recombine recipient of an invalid Content-Range MUST NOT attempt to recombine
the received content with a stored representation. the received content with a stored representation.
A server generating a 416 (Range Not Satisfiable) response to a byte- A server generating a 416 (Range Not Satisfiable) response to a byte-
range request SHOULD send a Content-Range header field with an range request SHOULD send a Content-Range header field with an
unsatisfied-range value, as in the following example: unsatisfied-range value, as in the following example:
Content-Range: bytes */1234 Content-Range: bytes */1234
The complete-length in a 416 response indicates the current length of The complete-length in a 416 response indicates the current length of
the selected representation. the selected representation.
The Content-Range header field has no meaning for status codes that The Content-Range header field has no meaning for status codes that
do not explicitly describe its semantic. For this specification, do not explicitly describe its semantic. For this specification,
only the 206 (Partial Content) and 416 (Range Not Satisfiable) status only the 206 (Partial Content) and 416 (Range Not Satisfiable) status
codes describe a meaning for Content-Range. codes describe a meaning for Content-Range.
The following are examples of Content-Range values in which the The following are examples of Content-Range values in which the
selected representation contains a total of 1234 bytes: selected representation contains a total of 1234 bytes:
o The first 500 bytes: * The first 500 bytes:
Content-Range: bytes 0-499/1234 Content-Range: bytes 0-499/1234
o The second 500 bytes: * The second 500 bytes:
Content-Range: bytes 500-999/1234 Content-Range: bytes 500-999/1234
o All except for the first 500 bytes: * All except for the first 500 bytes:
Content-Range: bytes 500-1233/1234 Content-Range: bytes 500-1233/1234
o The last 500 bytes: * The last 500 bytes:
Content-Range: bytes 734-1233/1234 Content-Range: bytes 734-1233/1234
14.5. Partial PUT 14.5. Partial PUT
Some origin servers support PUT of a partial representation when a Some origin servers support PUT of a partial representation when the
Content-Range header field (Section 14.4) is sent in the request, user agent sends a Content-Range header field (Section 14.4) in the
though such support is inconsistent and depends on private agreements request, though such support is inconsistent and depends on private
with user agents. In general, it requests that the state of the agreements with user agents. In general, it requests that the state
target resource be partly replaced with the enclosed content at an of the target resource be partly replaced with the enclosed content
offset and length indicated by the Content-Range value, where the at an offset and length indicated by the Content-Range value, where
offset is relative to the current selected representation. the offset is relative to the current selected representation.
An origin server SHOULD respond with a 400 (Bad Request) status code An origin server SHOULD respond with a 400 (Bad Request) status code
if it receives Content-Range on a PUT for a target resource that does if it receives Content-Range on a PUT for a target resource that does
not support partial PUT requests. not support partial PUT requests.
Partial PUT is not backwards compatible with the original definition Partial PUT is not backwards compatible with the original definition
of PUT. It may result in the content being written as a complete of PUT. It may result in the content being written as a complete
replacement for the current representation. replacement for the current representation.
Partial resource updates are also possible by targeting a separately Partial resource updates are also possible by targeting a separately
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3. A number of clients and servers were coded to an early draft of 3. A number of clients and servers were coded to an early draft of
the byteranges specification that used a media type of multipart/ the byteranges specification that used a media type of multipart/
x-byteranges , which is almost (but not quite) compatible with x-byteranges , which is almost (but not quite) compatible with
this type. this type.
Despite the name, the "multipart/byteranges" media type is not Despite the name, the "multipart/byteranges" media type is not
limited to byte ranges. The following example uses an "exampleunit" limited to byte ranges. The following example uses an "exampleunit"
range unit: range unit:
HTTP/1.1 206 Partial Content HTTP/1.1 206 Partial Content
Date: Tue, 14 Nov 1995 06:25:24 GMT Date: Tue, 14 Nov 1995 06:25:24 GMT
Last-Modified: Tue, 14 July 04:58:08 GMT Last-Modified: Tue, 14 July 04:58:08 GMT
Content-Length: 2331785 Content-Length: 2331785
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES --THIS_STRING_SEPARATES
Content-Type: video/example Content-Type: video/example
Content-Range: exampleunit 1.2-4.3/25 Content-Range: exampleunit 1.2-4.3/25
...the first range... ...the first range...
--THIS_STRING_SEPARATES --THIS_STRING_SEPARATES
Content-Type: video/example Content-Type: video/example
Content-Range: exampleunit 11.2-14.3/25 Content-Range: exampleunit 11.2-14.3/25
...the second range ...the second range
--THIS_STRING_SEPARATES-- --THIS_STRING_SEPARATES--
The following information serves as the registration form for the The following information serves as the registration form for the
multipart/byteranges media type. multipart/byteranges media type.
Type name: multipart Type name: multipart
Subtype name: byteranges Subtype name: byteranges
Required parameters: boundary Required parameters: boundary
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Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: N/A Restrictions on usage: N/A
Author: See Authors' Addresses section. Author: See Authors' Addresses section.
Change controller: IESG Change controller: IESG
15. Status Codes 15. Status Codes
The (response) status code is a three-digit integer code giving the The status code of a response is a three-digit integer code that
result of the attempt to understand and satisfy the request. describes the result of the request and the semantics of the
response, including whether the request was successful and what
HTTP status codes are extensible. HTTP clients are not required to content is enclosed (if any). All valid status codes are within the
understand the meaning of all registered status codes, though such range of 100 to 599, inclusive.
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.
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 first digit of the status code defines the class of response.
The last two digits do not have any categorization role. There are The last two digits do not have any categorization role. There are
five values for the first digit: five values for the first digit:
o 1xx (Informational): The request was received, continuing process * 1xx (Informational): The request was received, continuing process
o 2xx (Successful): The request was successfully received,
* 2xx (Successful): The request was successfully received,
understood, and accepted understood, and accepted
o 3xx (Redirection): Further action needs to be taken in order to * 3xx (Redirection): Further action needs to be taken in order to
complete the request complete the request
o 4xx (Client Error): The request contains bad syntax or cannot be * 4xx (Client Error): The request contains bad syntax or cannot be
fulfilled fulfilled
o 5xx (Server Error): The server failed to fulfill an apparently * 5xx (Server Error): The server failed to fulfill an apparently
valid request valid request
HTTP status codes are extensible. A client is 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.
For example, if a client receives an unrecognized status code of 471,
it can see from the first digit 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.
Values outside the range 100..599 are invalid. Implementations often
use three-digit integer values outside of that range (i.e., 600..999)
for internal communication of non-HTTP status (e.g., library errors).
A client that receives a response with an invalid status code SHOULD
process the response as if it had a 5xx (Server Error) status code.
A single request can have multiple associated responses: zero or more A single request can have multiple associated responses: zero or more
_interim_ (non-final) responses with status codes in the _interim_ (non-final) responses with status codes in the
"informational" (1xx) range, followed by exactly one _final_ response "informational" (1xx) range, followed by exactly one _final_ response
with a status code in one of the other ranges. with a status code in one of the other ranges.
15.1. Overview of Status Codes 15.1. Overview of Status Codes
The status codes listed below are defined in this specification. The The status codes listed below are defined in this specification. The
reason phrases listed here are only recommendations - they can be reason phrases listed here are only recommendations - they can be
replaced by local equivalents or left out altogether without replaced by local equivalents or left out altogether without
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100-continue expectation, the client can simply discard this interim 100-continue expectation, the client can simply discard this interim
response. response.
15.2.2. 101 Switching Protocols 15.2.2. 101 Switching Protocols
The _101 (Switching Protocols)_ status code indicates that the server The _101 (Switching Protocols)_ status code indicates that the server
understands and is willing to comply with the client's request, via understands and is willing to comply with the client's request, via
the Upgrade header field (Section 7.8), for a change in the the Upgrade header field (Section 7.8), for a change in the
application protocol being used on this connection. The server MUST application protocol being used on this connection. The server MUST
generate an Upgrade header field in the response that indicates which generate an Upgrade header field in the response that indicates which
protocol(s) will be switched to immediately after the empty line that protocol(s) will be in effect after this response.
terminates the 101 response.
It is assumed that the server will only agree to switch protocols It is assumed that the server will only agree to switch protocols
when it is advantageous to do so. For example, switching to a newer when it is advantageous to do so. For example, switching to a newer
version of HTTP might be advantageous over older versions, and version of HTTP might be advantageous over older versions, and
switching to a real-time, synchronous protocol might be advantageous switching to a real-time, synchronous protocol might be advantageous
when delivering resources that use such features. when delivering resources that use such features.
15.3. Successful 2xx 15.3. Successful 2xx
The _2xx (Successful)_ class of status code indicates that the The _2xx (Successful)_ class of status code indicates that the
client's request was successfully received, understood, and accepted. client's request was successfully received, understood, and accepted.
15.3.1. 200 OK 15.3.1. 200 OK
The _200 (OK)_ status code indicates that the request has succeeded. The _200 (OK)_ status code indicates that the request has succeeded.
The content sent in a 200 response depends on the request method. The content sent in a 200 response depends on the request method.
For the methods defined by this specification, the intended meaning For the methods defined by this specification, the intended meaning
of the content can be summarized as: of the content can be summarized as:
---------------- -------------------------------------------- +================+============================================+
request method response content is a representation of | request method | response content is a representation of |
---------------- -------------------------------------------- +================+============================================+
GET the target resource | GET | the target resource |
HEAD the target resource, like GET, but without +----------------+--------------------------------------------+
transferring the representation data | HEAD | the target resource, like GET, but without |
POST the status of, or results obtained from, | | transferring the representation data |
the action +----------------+--------------------------------------------+
PUT, DELETE the status of the action | POST | the status of, or results obtained from, |
OPTIONS communication options for the target | | the action |
resource +----------------+--------------------------------------------+
TRACE the request message as received by the | PUT, DELETE | the status of the action |
server returning the trace +----------------+--------------------------------------------+
---------------- -------------------------------------------- | OPTIONS | communication options for the target |
| | resource |
+----------------+--------------------------------------------+
| TRACE | the request message as received by the |
| | server returning the trace |
+----------------+--------------------------------------------+
Table 6 Table 6
Aside from responses to CONNECT, a 200 response always has content, Aside from responses to CONNECT, a 200 response always has content,
though an origin server MAY generate content of zero length. If no though an origin server MAY generate content of zero length. If no
content is desired, an origin server ought to send _204 (No Content)_ content is desired, an origin server ought to send _204 (No Content)_
instead. For CONNECT, no content is allowed because the successful instead. For CONNECT, no content is allowed because the successful
result is a tunnel, which begins immediately after the 200 response result is a tunnel, which begins immediately after the 200 response
header section. header section.
skipping to change at page 149, line 13 skipping to change at page 150, line 18
However, a server might want to send only a subset of the data However, a server might want to send only a subset of the data
requested for reasons of its own, such as temporary unavailability, requested for reasons of its own, such as temporary unavailability,
cache efficiency, load balancing, etc. Since a 206 response is self- cache efficiency, load balancing, etc. Since a 206 response is self-
descriptive, the client can still understand a response that only descriptive, the client can still understand a response that only
partially satisfies its range request. partially satisfies its range request.
A client MUST inspect a 206 response's Content-Type and Content-Range A client MUST inspect a 206 response's Content-Type and Content-Range
field(s) to determine what parts are enclosed and whether additional field(s) to determine what parts are enclosed and whether additional
requests are needed. requests are needed.
When a 206 response is generated, the server MUST generate the A server that generates a 206 response MUST generate the following
following header fields, in addition to those required in the header fields, in addition to those required in the subsections
subsections below, if the field would have been sent in a 200 (OK) below, if the field would have been sent in a 200 (OK) response to
response to the same request: Date, Cache-Control, ETag, Expires, the same request: Date, Cache-Control, ETag, Expires,
Content-Location, and Vary. Content-Location, and Vary.
A Content-Length header field present in a 206 response indicates the A Content-Length header field present in a 206 response indicates the
number of octets in the content of this message, which is usually not number of octets in the content of this message, which is usually not
the complete length of the selected representation. Each the complete length of the selected representation. Each
Content-Range header field includes information about the selected Content-Range header field includes information about the selected
representation's complete length. representation's complete length.
If a 206 is generated in response to a request with an If-Range A sender that generates a 206 response with an If-Range header field
header field, the sender SHOULD NOT generate other representation SHOULD NOT generate other representation header fields beyond those
header fields beyond those required, because the client is understood required, because the client already has a prior response containing
to already have a prior response containing those header fields. those header fields. Otherwise, a sender MUST generate all of the
Otherwise, the sender MUST generate all of the representation header representation header fields that would have been sent in a 200 (OK)
fields that would have been sent in a 200 (OK) response to the same response to the same request.
request.
A 206 response is heuristically cacheable; i.e., unless otherwise A 206 response is heuristically cacheable; i.e., unless otherwise
indicated by explicit cache controls (see Section 4.2.2 of indicated by explicit cache controls (see Section 4.2.2 of
[Caching]). [Caching]).
15.3.7.1. Single Part 15.3.7.1. Single Part
If a single part is being transferred, the server generating the 206 If a single part is being transferred, the server generating the 206
response MUST generate a Content-Range header field, describing what response MUST generate a Content-Range header field, describing what
range of the selected representation is enclosed, and a content range of the selected representation is enclosed, and a content
consisting of the range. For example: consisting of the range. For example:
HTTP/1.1 206 Partial Content HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022 Content-Range: bytes 21010-47021/47022
Content-Length: 26012 Content-Length: 26012
Content-Type: image/gif Content-Type: image/gif
... 26012 bytes of partial image data ... ... 26012 bytes of partial image data ...
15.3.7.2. Multiple Parts 15.3.7.2. Multiple Parts
If multiple parts are being transferred, the server generating the If multiple parts are being transferred, the server generating the
206 response MUST generate "multipart/byteranges" content, as defined 206 response MUST generate "multipart/byteranges" content, as defined
in Section 14.6, and a Content-Type header field containing the in Section 14.6, and a Content-Type header field containing the
multipart/byteranges media type and its required boundary parameter. multipart/byteranges media type and its required boundary parameter.
To avoid confusion with single-part responses, a server MUST NOT To avoid confusion with single-part responses, a server MUST NOT
generate a Content-Range header field in the HTTP header section of a generate a Content-Range header field in the HTTP header section of a
multiple part response (this field will be sent in each part multiple part response (this field will be sent in each part
instead). instead).
Within the header area of each body part in the multipart content, Within the header area of each body part in the multipart content,
the server MUST generate a Content-Range header field corresponding the server MUST generate a Content-Range header field corresponding
to the range being enclosed in that body part. If the selected to the range being enclosed in that body part. If the selected
representation would have had a Content-Type header field in a 200 representation would have had a Content-Type header field in a 200
(OK) response, the server SHOULD generate that same Content-Type (OK) response, the server SHOULD generate that same Content-Type
header field in the header area of each body part. For example: header field in the header area of each body part. For example:
HTTP/1.1 206 Partial Content HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Length: 1741 Content-Length: 1741
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES --THIS_STRING_SEPARATES
Content-Type: application/pdf Content-Type: application/pdf
Content-Range: bytes 500-999/8000 Content-Range: bytes 500-999/8000
...the first range... ...the first range...
--THIS_STRING_SEPARATES --THIS_STRING_SEPARATES
Content-Type: application/pdf Content-Type: application/pdf
Content-Range: bytes 7000-7999/8000 Content-Range: bytes 7000-7999/8000
...the second range ...the second range
--THIS_STRING_SEPARATES-- --THIS_STRING_SEPARATES--
When multiple ranges are requested, a server MAY coalesce any of the 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 ranges that overlap, or that are separated by a gap that is smaller
than the overhead of sending multiple parts, regardless of the order than the overhead of sending multiple parts, regardless of the order
in which the corresponding range-spec appeared in the received Range in which the corresponding range-spec appeared in the received Range
header field. Since the typical overhead between each part of a header field. Since the typical overhead between each part of a
multipart/byteranges is around 80 bytes, depending on the selected multipart/byteranges is around 80 bytes, depending on the selected
representation's media type and the chosen boundary parameter length, representation's media type and the chosen boundary parameter length,
it can be less efficient to transfer many small disjoint parts than it can be less efficient to transfer many small disjoint parts than
it is to transfer the entire selected representation. it is to transfer the entire selected representation.
A server MUST NOT generate a multipart response to a request for a A server MUST NOT generate a multipart response to a request for a
single range, since a client that does not request multiple parts single range, since a client that does not request multiple parts
might not support multipart responses. However, a server MAY might not support multipart responses. However, a server MAY
generate a multipart/byteranges response with only a single body part generate a multipart/byteranges response with only a single body part
if multiple ranges were requested and only one range was found to be if multiple ranges were requested and only one range was found to be
satisfiable or only one range remained after coalescing. A client satisfiable or only one range remained after coalescing. A client
that cannot process a multipart/byteranges response MUST NOT generate that cannot process a multipart/byteranges response MUST NOT generate
a request that asks for multiple ranges. a request that asks for multiple ranges.
When a multipart response is generated, the server SHOULD send the A server that generates a multipart response SHOULD send the parts in
parts in the same order that the corresponding range-spec appeared in the same order that the corresponding range-spec appeared in the
the received Range header field, excluding those ranges that were received Range header field, excluding those ranges that were deemed
deemed unsatisfiable or that were coalesced into other ranges. A unsatisfiable or that were coalesced into other ranges. A client
client that receives a multipart response MUST inspect the that receives a multipart response MUST inspect the Content-Range
Content-Range header field present in each body part in order to header field present in each body part in order to determine which
determine which range is contained in that body part; a client cannot range is contained in that body part; a client cannot rely on
rely on receiving the same ranges that it requested, nor the same receiving the same ranges that it requested, nor the same order that
order that it requested. it requested.
15.3.7.3. Combining Parts 15.3.7.3. Combining Parts
A response might transfer only a subrange of a representation if the A response might transfer only a subrange of a representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifications. After several such transfers, a client might Range specifications. After several such transfers, a client might
have received several ranges of the same representation. These have received several ranges of the same representation. These
ranges can only be safely combined if they all have in common the ranges can only be safely combined if they all have in common the
same strong validator (Section 8.8.1). same strong validator (Section 8.8.1).
skipping to change at page 162, line 17 skipping to change at page 163, line 31
The _411 (Length Required)_ status code indicates that the server The _411 (Length Required)_ status code indicates that the server
refuses to accept the request without a defined Content-Length refuses to accept the request without a defined Content-Length
(Section 8.6). The client MAY repeat the request if it adds a valid (Section 8.6). The client MAY repeat the request if it adds a valid
Content-Length header field containing the length of the request Content-Length header field containing the length of the request
content. content.
15.5.13. 412 Precondition Failed 15.5.13. 412 Precondition Failed
The _412 (Precondition Failed)_ status code indicates that one or The _412 (Precondition Failed)_ status code indicates that one or
more conditions given in the request header fields evaluated to false more conditions given in the request header fields evaluated to false
when tested on the server. This response status code allows the when tested on the server (Section 13). This response status code
client to place preconditions on the current resource state (its allows the client to place preconditions on the current resource
current representations and metadata) and, thus, prevent the request state (its current representations and metadata) and, thus, prevent
method from being applied if the target resource is in an unexpected the request method from being applied if the target resource is in an
state. unexpected state.
15.5.14. 413 Content Too Large 15.5.14. 413 Content Too Large
The _413 (Content Too Large)_ status code indicates that the server The _413 (Content Too Large)_ status code indicates that the server
is refusing to process a request because the request content is is refusing to process a request because the request content is
larger than the server is willing or able to process. The server MAY larger than the server is willing or able to process. The server MAY
terminate the request, if the protocol version in use allows it; terminate the request, if the protocol version in use allows it;
otherwise, the server MAY close the connection. otherwise, the server MAY close the connection.
If the condition is temporary, the server SHOULD generate a If the condition is temporary, the server SHOULD generate a
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The _416 (Range Not Satisfiable)_ status code indicates that the set The _416 (Range Not Satisfiable)_ status code indicates that the set
of ranges in the request's Range header field (Section 14.2) has been of ranges in the request's Range header field (Section 14.2) has been
rejected either because none of the requested ranges are satisfiable rejected either because none of the requested ranges are satisfiable
or because the client has requested an excessive number of small or or because the client has requested an excessive number of small or
overlapping ranges (a potential denial of service attack). overlapping ranges (a potential denial of service attack).
Each range unit defines what is required for its own range sets to be Each range unit defines what is required for its own range sets to be
satisfiable. For example, Section 14.1.2 defines what makes a bytes satisfiable. For example, Section 14.1.2 defines what makes a bytes
range set satisfiable. range set satisfiable.
When this status code is generated in response to a byte-range A server that generates a 416 response to a byte-range request SHOULD
request, the sender SHOULD generate a Content-Range header field generate a Content-Range header field specifying the current length
specifying the current length of the selected representation of the selected representation (Section 14.4).
(Section 14.4).
For example: For example:
HTTP/1.1 416 Range Not Satisfiable HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022 Content-Range: bytes */47022
| *Note:* Because servers are free to ignore Range, many | *Note:* Because servers are free to ignore Range, many
| implementations will respond with the entire selected | implementations will respond with the entire selected
| representation in a 200 (OK) response. That is partly because | representation in a 200 (OK) response. That is partly because
| most clients are prepared to receive a 200 (OK) to complete the | most clients are prepared to receive a 200 (OK) to complete the
| task (albeit less efficiently) and partly because clients might | task (albeit less efficiently) and partly because clients might
| not stop making an invalid range request until they have | not stop making an invalid range request until they have
| received a complete representation. Thus, clients cannot | received a complete representation. Thus, clients cannot
| depend on receiving a 416 (Range Not Satisfiable) response even | depend on receiving a 416 (Range Not Satisfiable) response even
| when it is most appropriate. | when it is most appropriate.
skipping to change at page 164, line 33 skipping to change at page 166, line 9
Therefore, the 418 status code is reserved in the IANA HTTP Status Therefore, the 418 status code is reserved in the IANA HTTP Status
Code Registry. This indicates that the status code cannot be Code Registry. This indicates that the status code cannot be
assigned to other applications currently. If future circumstances assigned to other applications currently. If future circumstances
require its use (e.g., exhaustion of 4NN status codes), it can be re- require its use (e.g., exhaustion of 4NN status codes), it can be re-
assigned to another use. assigned to another use.
15.5.20. 421 Misdirected Request 15.5.20. 421 Misdirected Request
The 421 (Misdirected Request) status code indicates that the request The 421 (Misdirected Request) status code indicates that the request
was directed at a server that is unable or unwilling to produce an was directed at a server that is unable or unwilling to produce an
authoritative response for the target URI. A 421 is sent when an authoritative response for the target URI. An origin server (or
origin server (or gateway acting on behalf of the origin server) gateway acting on behalf of the origin server) sends 421 to reject a
rejects a target URI that does not match an origin for which the target URI that does not match an origin for which the server has
server has been configured (Section 4.3.1) or does not match the been configured (Section 4.3.1) or does not match the connection
connection context over which the request was received (Section 7.4). context over which the request was received (Section 7.4).
A client that receives a 421 (Misdirected Request) response MAY retry A client that receives a 421 (Misdirected Request) response MAY retry
the request, whether or not the request method is idempotent, over a the request, whether or not the request method is idempotent, over a
different connection, such as a fresh connection specific to the different connection, such as a fresh connection specific to the
target resource's origin, or via an alternative service [RFC7838]. target resource's origin, or via an alternative service [RFC7838].
A proxy MUST NOT generate a 421 response. A proxy MUST NOT generate a 421 response.
15.5.21. 422 Unprocessable Content 15.5.21. 422 Unprocessable Content
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15.5.22. 426 Upgrade Required 15.5.22. 426 Upgrade Required
The _426 (Upgrade Required)_ status code indicates that the server The _426 (Upgrade Required)_ status code indicates that the server
refuses to perform the request using the current protocol but might refuses to perform the request using the current protocol but might
be willing to do so after the client upgrades to a different be willing to do so after the client upgrades to a different
protocol. The server MUST send an Upgrade header field in a 426 protocol. The server MUST send an Upgrade header field in a 426
response to indicate the required protocol(s) (Section 7.8). response to indicate the required protocol(s) (Section 7.8).
Example: Example:
HTTP/1.1 426 Upgrade Required HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0 Upgrade: HTTP/3.0
Connection: Upgrade Connection: Upgrade
Content-Length: 53 Content-Length: 53
Content-Type: text/plain Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol. This service requires use of the HTTP/3.0 protocol.
15.6. Server Error 5xx 15.6. Server Error 5xx
The _5xx (Server Error)_ class of status code indicates that the The _5xx (Server Error)_ class of status code indicates that the
server is aware that it has erred or is incapable of performing the server is aware that it has erred or is incapable of performing the
requested method. Except when responding to a HEAD request, the requested method. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent error situation, and whether it is a temporary or permanent
condition. A user agent SHOULD display any included representation condition. A user agent SHOULD display any included representation
to the user. These response codes are applicable to any request to the user. These response codes are applicable to any request
skipping to change at page 167, line 47 skipping to change at page 169, line 19
16.1. Method Extensibility 16.1. Method Extensibility
16.1.1. Method Registry 16.1.1. Method Registry
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained
by IANA at <https://www.iana.org/assignments/http-methods>, registers by IANA at <https://www.iana.org/assignments/http-methods>, registers
method names. method names.
HTTP method registrations MUST include the following fields: HTTP method registrations MUST include the following fields:
o Method Name (see Section 9) * Method Name (see Section 9)
o Safe ("yes" or "no", see Section 9.2.1) * Safe ("yes" or "no", see Section 9.2.1)
o Idempotent ("yes" or "no", see Section 9.2.2) * Idempotent ("yes" or "no", see Section 9.2.2)
o Pointer to specification text
* Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
16.1.2. Considerations for New Methods 16.1.2. Considerations for New Methods
Standardized methods are generic; that is, they are potentially Standardized methods are generic; that is, they are potentially
applicable to any resource, not just one particular media type, kind applicable to any resource, not just one particular media type, kind
of resource, or application. As such, it is preferred that new of resource, or application. As such, it is preferred that new
methods be registered in a document that isn't specific to a single methods be registered in a document that isn't specific to a single
application or data format, since orthogonal technologies deserve application or data format, since orthogonal technologies deserve
orthogonal specification. orthogonal specification.
Since message parsing (Section 6 of [Messaging]) needs to be Since message parsing (Section 6) needs to be independent of method
independent of method semantics (aside from responses to HEAD), semantics (aside from responses to HEAD), definitions of new methods
definitions of new methods cannot change the parsing algorithm or cannot change the parsing algorithm or prohibit the presence of
prohibit the presence of content on either the request or the content on either the request or the response message. Definitions
response message. Definitions of new methods can specify that only a of new methods can specify that only a zero-length content is allowed
zero-length content is allowed by requiring a Content-Length header by requiring a Content-Length header field with a value of "0".
field with a value of "0".
Likewise, new methods cannot use the special host:port and asterisk Likewise, new methods cannot use the special host:port and asterisk
forms of request target that are allowed for CONNECT and OPTIONS, forms of request target that are allowed for CONNECT and OPTIONS,
respectively (Section 7.1). A full URI in absolute form is needed respectively (Section 7.1). A full URI in absolute form is needed
for the target URI, which means either the request target needs to be for the target URI, which means either the request target needs to be
sent in absolute form or the target URI will be reconstructed from sent in absolute form or the target URI will be reconstructed from
the request context in the same way it is for other methods. the request context in the same way it is for other methods.
A new method definition needs to indicate whether it is safe A new method definition needs to indicate whether it is safe
(Section 9.2.1), idempotent (Section 9.2.2), cacheable (Section 9.2.1), idempotent (Section 9.2.2), cacheable
skipping to change at page 169, line 4 skipping to change at page 170, line 23
(Section 13.1) and, if so, how a server responds when the condition (Section 13.1) and, if so, how a server responds when the condition
is false. Likewise, if the new method might have some use for is false. Likewise, if the new method might have some use for
partial response semantics (Section 14.2), it ought to document this, partial response semantics (Section 14.2), it ought to document this,
too. too.
| *Note:* Avoid defining a method name that starts with "M-", | *Note:* Avoid defining a method name that starts with "M-",
| since that prefix might be misinterpreted as having the | since that prefix might be misinterpreted as having the
| semantics assigned to it by [RFC2774]. | semantics assigned to it by [RFC2774].
16.2. Status Code Extensibility 16.2. Status Code Extensibility
16.2.1. Status Code Registry 16.2.1. Status Code Registry
The "Hypertext Transfer Protocol (HTTP) Status Code Registry", The "Hypertext Transfer Protocol (HTTP) Status Code Registry",
maintained by IANA at <https://www.iana.org/assignments/http-status- maintained by IANA at <https://www.iana.org/assignments/http-status-
codes>, registers status code numbers. codes>, registers status code numbers.
A registration MUST include the following fields: A registration MUST include the following fields:
o Status Code (3 digits) * Status Code (3 digits)
o Short Description * Short Description
o Pointer to specification text * Pointer to specification text
Values to be added to the HTTP status code namespace require IETF Values to be added to the HTTP status code namespace require IETF
Review (see [RFC8126], Section 4.8). Review (see [RFC8126], Section 4.8).
16.2.2. Considerations for New Status Codes 16.2.2. Considerations for New Status Codes
When it is necessary to express semantics for a response that are not 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. defined by current status codes, a new status code can be registered.
Status codes are generic; they are potentially applicable to any Status codes are generic; they are potentially applicable to any
resource, not just one particular media type, kind of resource, or resource, not just one particular media type, kind of resource, or
skipping to change at page 171, line 5 skipping to change at page 172, line 21
in prior messages, or its use of a specific media type. Likewise, in prior messages, or its use of a specific media type. Likewise,
direct inspection of support might be possible through an OPTIONS direct inspection of support might be possible through an OPTIONS
request or by interacting with a defined well-known URI [RFC8615] if request or by interacting with a defined well-known URI [RFC8615] if
such inspection is defined along with the field being introduced. such inspection is defined along with the field being introduced.
16.3.1. Field Name Registry 16.3.1. Field Name Registry
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines
the namespace for HTTP field names. the namespace for HTTP field names.
Any party can request registration of a HTTP field. See Any party can request registration of an HTTP field. See
Section 16.3.2 for considerations to take into account when creating Section 16.3.2 for considerations to take into account when creating
a new HTTP field. a new HTTP field.
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is
located at <https://www.iana.org/assignments/http-fields/>. located at <https://www.iana.org/assignments/http-fields/>.
Registration requests can be made by following the instructions Registration requests can be made by following the instructions
located there or by sending an email to the "ietf-http-wg@ietf.org" located there or by sending an email to the "ietf-http-wg@ietf.org"
mailing list. mailing list.
Field names are registered on the advice of a Designated Expert Field names are registered on the advice of a Designated Expert
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HTTP header and trailer fields are a widely-used extension point for HTTP header and trailer fields are a widely-used extension point for
the protocol. While they can be used in an ad hoc fashion, fields the protocol. While they can be used in an ad hoc fashion, fields
that are intended for wider use need to be carefully documented to that are intended for wider use need to be carefully documented to
ensure interoperability. ensure interoperability.
In particular, authors of specifications defining new fields are In particular, authors of specifications defining new fields are
advised to consider and, where appropriate, document the following advised to consider and, where appropriate, document the following
aspects: aspects:
o Under what conditions the field can be used; e.g., only in * Under what conditions the field can be used; e.g., only in
responses or requests, in all messages, only on responses to a responses or requests, in all messages, only on responses to a
particular request method, etc. particular request method, etc.
o Whether the field semantics are further refined by their context, * Whether the field semantics are further refined by their context,
such as their use with certain request methods or status codes. such as their use with certain request methods or status codes.
o The scope of applicability for the information conveyed. By * The scope of applicability for the information conveyed. By
default, fields apply only to the message they are associated default, fields apply only to the message they are associated
with, but some response fields are designed to apply to all with, but some response fields are designed to apply to all
representations of a resource, the resource itself, or an even representations of a resource, the resource itself, or an even
broader scope. Specifications that expand the scope of a response broader scope. Specifications that expand the scope of a response
field will need to carefully consider issues such as content field will need to carefully consider issues such as content
negotiation, the time period of applicability, and (in some cases) negotiation, the time period of applicability, and (in some cases)
multi-tenant server deployments. multi-tenant server deployments.
o Under what conditions intermediaries are allowed to insert, * Under what conditions intermediaries are allowed to insert,
delete, or modify the field's value. delete, or modify the field's value.
o If the field is allowable in trailers; by default, it will not be * If the field is allowable in trailers; by default, it will not be
(see Section 6.5.1). (see Section 6.5.1).
o Whether it is appropriate to list the field name in the Connection * Whether it is appropriate to list the field name in the Connection
header field (i.e., if the field is to be hop-by-hop; see header field (i.e., if the field is to be hop-by-hop; see
Section 7.6.1). Section 7.6.1).
o Whether the field introduces any additional security * Whether the field introduces any additional security
considerations, such as disclosure of privacy-related data. considerations, such as disclosure of privacy-related data.
Request header fields have additional considerations that need to be Request header fields have additional considerations that need to be
documented if the default behaviour is not appropriate: documented if the default behaviour is not appropriate:
o If it is appropriate to list the field name in a Vary response * If 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 header field (e.g., when the request header field is used by an
origin server's content selection algorithm; see Section 12.5.5). origin server's content selection algorithm; see Section 12.5.5).
o If the field is intended to be stored when received in a PUT * If the field is intended to be stored when received in a PUT
request (see Section 9.3.4). request (see Section 9.3.4).
o If the field ought to be removed when automatically redirecting a * If the field ought to be removed when automatically redirecting a
request, due to security concerns (see Section 15.4). request, due to security concerns (see Section 15.4).
16.3.2.1. Considerations for New Field Names 16.3.2.1. Considerations for New Field Names
Authors of specifications defining new fields are advised to choose a Authors of specifications defining new fields are advised to choose a
short but descriptive field name. Short names avoid needless data short but descriptive field name. Short names avoid needless data
transmission; descriptive names avoid confusion and "squatting" on transmission; descriptive names avoid confusion and "squatting" on
names that might have broader uses. names that might have broader uses.
To that end, limited-use fields (such as a header confined to a To that end, limited-use fields (such as a header confined to a
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do not trigger automatic processing. do not trigger automatic processing.
16.3.2.2. Considerations for New Field Values 16.3.2.2. Considerations for New Field Values
A major task in the definition of a new HTTP field is the A major task in the definition of a new HTTP field is the
specification of the field value syntax: what senders should specification of the field value syntax: what senders should
generate, and how recipients should infer semantics from what is generate, and how recipients should infer semantics from what is
received. received.
Authors are encouraged (but not required) to use either the ABNF Authors are encouraged (but not required) to use either the ABNF
rules in this specification or those in [RFCSTRF] to define the rules in this specification or those in [RFC8941] to define the
syntax of new field values. syntax of new field values.
Authors are advised to carefully consider how the combination of Authors are advised to carefully consider how the combination of
multiple field lines will impact them (see Section 5.3). Because multiple field lines will impact them (see Section 5.3). Because
senders might send erroneously send multiple values, and both senders might send erroneously send multiple values, and both
intermediaries and HTTP libraries can perform combination intermediaries and HTTP libraries can perform combination
automatically, this applies to all field values - even when only a automatically, this applies to all field values - even when only a
single value is anticipated. single value is anticipated.
Therefore, authors are advised to delimit or encode values that Therefore, authors are advised to delimit or encode values that
contain commas (e.g., with the quoted-string rule of Section 5.6.4, contain commas (e.g., with the quoted-string rule of Section 5.6.4,
the String data type of [RFCSTRF]), or a field-specific encoding). the String data type of [RFC8941]), or a field-specific encoding).
This ensures that commas within field data are not confused with the This ensures that commas within field data are not confused with the
commas that delimit a list value. commas that delimit a list value.
For example, the Content-Type field value only allows commas inside For example, the Content-Type field value only allows commas inside
quoted strings, which can be reliably parsed even when multiple quoted strings, which can be reliably parsed even when multiple
values are present. The Location field value provides a counter- values are present. The Location field value provides a counter-
example that should not be emulated: because URIs can include commas, example that should not be emulated: because URIs can include commas,
it is not possible to reliably distinguish between a single value it is not possible to reliably distinguish between a single value
that includes a comma from two values. that includes a comma from two values.
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example that should not be emulated: because URIs can include commas, example that should not be emulated: because URIs can include commas,
it is not possible to reliably distinguish between a single value it is not possible to reliably distinguish between a single value
that includes a comma from two values. that includes a comma from two values.
Authors of fields with a singleton value (see Section 5.5) are Authors of fields with a singleton value (see Section 5.5) are
additionally advised to document how to treat messages where the additionally advised to document how to treat messages where the
multiple members are present (a sensible default would be to ignore multiple members are present (a sensible default would be to ignore
the field, but this might not always be the right choice). the field, but this might not always be the right choice).
16.4. Authentication Scheme Extensibility 16.4. Authentication Scheme Extensibility
16.4.1. Authentication Scheme Registry 16.4.1. Authentication Scheme Registry
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" defines the namespace for the authentication schemes in Registry" defines the namespace for the authentication schemes in
challenges and credentials. It is maintained at challenges and credentials. It is maintained at
<https://www.iana.org/assignments/http-authschemes>. <https://www.iana.org/assignments/http-authschemes>.
Registrations MUST include the following fields: Registrations MUST include the following fields:
o Authentication Scheme Name * Authentication Scheme Name
o Pointer to specification text * Pointer to specification text
o Notes (optional) * Notes (optional)
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
16.4.2. Considerations for New Authentication Schemes 16.4.2. Considerations for New Authentication Schemes
There are certain aspects of the HTTP Authentication framework that There are certain aspects of the HTTP Authentication framework that
put constraints on how new authentication schemes can work: put constraints on how new authentication schemes can work:
o HTTP authentication is presumed to be stateless: all of the * HTTP authentication is presumed to be stateless: all of the
information necessary to authenticate a request MUST be provided information necessary to authenticate a request MUST be provided
in the request, rather than be dependent on the server remembering in the request, rather than be dependent on the server remembering
prior requests. Authentication based on, or bound to, the prior requests. Authentication based on, or bound to, the
underlying connection is outside the scope of this specification underlying connection is outside the scope of this specification
and inherently flawed unless steps are taken to ensure that the and inherently flawed unless steps are taken to ensure that the
connection cannot be used by any party other than the connection cannot be used by any party other than the
authenticated user (see Section 3.7). authenticated user (see Section 3.7).
o The authentication parameter "realm" is reserved for defining * The authentication parameter "realm" is reserved for defining
protection spaces as described in Section 11.5. New schemes MUST protection spaces as described in Section 11.5. New schemes MUST
NOT use it in a way incompatible with that definition. NOT use it in a way incompatible with that definition.
o The "token68" notation was introduced for compatibility with * The "token68" notation was introduced for compatibility with
existing authentication schemes and can only be used once per existing authentication schemes and can only be used once per
challenge or credential. Thus, new schemes ought to use the auth- challenge or credential. Thus, new schemes ought to use the auth-
param syntax instead, because otherwise future extensions will be param syntax instead, because otherwise future extensions will be
impossible. impossible.
o The parsing of challenges and credentials is defined by this * The parsing of challenges and credentials is defined by this
specification and cannot be modified by new authentication specification and cannot be modified by new authentication
schemes. When the auth-param syntax is used, all parameters ought schemes. When the auth-param syntax is used, all parameters ought
to support both token and quoted-string syntax, and syntactical to support both token and quoted-string syntax, and syntactical
constraints ought to be defined on the field value after parsing constraints ought to be defined on the field value after parsing
(i.e., quoted-string processing). This is necessary so that (i.e., quoted-string processing). This is necessary so that
recipients can use a generic parser that applies to all recipients can use a generic parser that applies to all
authentication schemes. authentication schemes.
*Note:* The fact that the value syntax for the "realm" parameter *Note:* The fact that the value syntax for the "realm" parameter
is restricted to quoted-string was a bad design choice not to be is restricted to quoted-string was a bad design choice not to be
repeated for new parameters. repeated for new parameters.
o Definitions of new schemes ought to define the treatment of * Definitions of new schemes ought to define the treatment of
unknown extension parameters. In general, a "must-ignore" rule is unknown extension parameters. In general, a "must-ignore" rule is
preferable to a "must-understand" rule, because otherwise it will preferable to a "must-understand" rule, because otherwise it will
be hard to introduce new parameters in the presence of legacy be hard to introduce new parameters in the presence of legacy
recipients. Furthermore, it's good to describe the policy for recipients. Furthermore, it's good to describe the policy for
defining new parameters (such as "update the specification" or defining new parameters (such as "update the specification" or
"use this registry"). "use this registry").
o Authentication schemes need to document whether they are usable in * Authentication schemes need to document whether they are usable in
origin-server authentication (i.e., using WWW-Authenticate), and/ origin-server authentication (i.e., using WWW-Authenticate), and/
or proxy authentication (i.e., using Proxy-Authenticate). or proxy authentication (i.e., using Proxy-Authenticate).
o The credentials carried in an Authorization header field are * The credentials carried in an Authorization header field are
specific to the user agent and, therefore, have the same effect on specific to the user agent and, therefore, have the same effect on
HTTP caches as the "private" Cache-Control response directive HTTP caches as the "private" Cache-Control response directive
(Section 5.2.2.7 of [Caching]), within the scope of the request in (Section 5.2.2.7 of [Caching]), within the scope of the request in
which they appear. which they appear.
Therefore, new authentication schemes that choose not to carry Therefore, new authentication schemes that choose not to carry
credentials in the Authorization header field (e.g., using a newly credentials in the Authorization header field (e.g., using a newly
defined header field) will need to explicitly disallow caching, by defined header field) will need to explicitly disallow caching, by
mandating the use of Cache-Control response directives (e.g., mandating the use of Cache-Control response directives (e.g.,
"private"). "private").
o Schemes using Authentication-Info, Proxy-Authentication-Info, or * Schemes using Authentication-Info, Proxy-Authentication-Info, or
any other authentication related response header field need to any other authentication related response header field need to
consider and document the related security considerations (see consider and document the related security considerations (see
Section 17.15.4). Section 17.15.4).
16.5. Range Unit Extensibility 16.5. Range Unit Extensibility
16.5.1. Range Unit Registry 16.5.1. Range Unit Registry
The "HTTP Range Unit Registry" defines the namespace for the range The "HTTP Range Unit Registry" defines the namespace for the range
unit names and refers to their corresponding specifications. It is unit names and refers to their corresponding specifications. It is
maintained at <https://www.iana.org/assignments/http-parameters>. maintained at <https://www.iana.org/assignments/http-parameters>.
Registration of an HTTP Range Unit MUST include the following fields: Registration of an HTTP Range Unit MUST include the following fields:
o Name * Name
o Description * Description
o Pointer to specification text * Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
16.5.2. Considerations for New Range Units 16.5.2. Considerations for New Range Units
Other range units, such as format-specific boundaries like pages, Other range units, such as format-specific boundaries like pages,
sections, records, rows, or time, are potentially usable in HTTP for sections, records, rows, or time, are potentially usable in HTTP for
application-specific purposes, but are not commonly used in practice. application-specific purposes, but are not commonly used in practice.
Implementors of alternative range units ought to consider how they Implementors of alternative range units ought to consider how they
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16.5.2. Considerations for New Range Units 16.5.2. Considerations for New Range Units
Other range units, such as format-specific boundaries like pages, Other range units, such as format-specific boundaries like pages,
sections, records, rows, or time, are potentially usable in HTTP for sections, records, rows, or time, are potentially usable in HTTP for
application-specific purposes, but are not commonly used in practice. application-specific purposes, but are not commonly used in practice.
Implementors of alternative range units ought to consider how they Implementors of alternative range units ought to consider how they
would work with content codings and general-purpose intermediaries. would work with content codings and general-purpose intermediaries.
16.6. Content Coding Extensibility 16.6. Content Coding Extensibility
16.6.1. Content Coding Registry 16.6.1. Content Coding Registry
The "HTTP Content Coding Registry", maintained by IANA at The "HTTP Content Coding Registry", maintained by IANA at
<https://www.iana.org/assignments/http-parameters/>, registers <https://www.iana.org/assignments/http-parameters/>, registers
content-coding names. content-coding names.
Content coding registrations MUST include the following fields: Content coding registrations MUST include the following fields:
o Name * Name
o Description * Description
o Pointer to specification text * Pointer to specification text
Names of content codings MUST NOT overlap with names of transfer Names of content codings MUST NOT overlap with names of transfer
codings (Section 7 of [Messaging]), unless the encoding codings (as per the "HTTP Transfer Coding registry", located at
transformation is identical (as is the case for the compression <https://www.iana.org/assignments/http-parameters/>), unless the
codings defined in Section 8.4.1). encoding transformation is identical (as is the case for the
compression codings defined in Section 8.4.1).
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
Section 4.8 of [RFC8126]) and MUST conform to the purpose of content Section 4.8 of [RFC8126]) and MUST conform to the purpose of content
coding defined in Section 8.4.1. coding defined in Section 8.4.1.
16.6.2. Considerations for New Content Codings 16.6.2. Considerations for New Content Codings
New content codings ought to be self-descriptive whenever possible, New content codings ought to be self-descriptive whenever possible,
with optional parameters discoverable within the coding format with optional parameters discoverable within the coding format
itself, rather than rely on external metadata that might be lost itself, rather than rely on external metadata that might be lost
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Some attacks on encrypted protocols use the differences in size Some attacks on encrypted protocols use the differences in size
created by dynamic compression to reveal confidential information; created by dynamic compression to reveal confidential information;
for example, [BREACH]. These attacks rely on creating a redundancy for example, [BREACH]. These attacks rely on creating a redundancy
between attacker-controlled content and the confidential information, between attacker-controlled content and the confidential information,
such that a dynamic compression algorithm using the same dictionary such that a dynamic compression algorithm using the same dictionary
for both content will compress more efficiently when the attacker- for both content will compress more efficiently when the attacker-
controlled content matches parts of the confidential content. controlled content matches parts of the confidential content.
HTTP messages can be compressed in a number of ways, including using HTTP messages can be compressed in a number of ways, including using
TLS compression, content-codings, transfer-codings, and other TLS compression, content codings, transfer codings, and other
extension or version-specific mechanisms. extension or version-specific mechanisms.
The most effective mitigation for this risk is to disable compression The most effective mitigation for this risk is to disable compression
on sensitive data, or to strictly separate sensitive data from on sensitive data, or to strictly separate sensitive data from
attacker-controlled data so that they cannot share the same attacker-controlled data so that they cannot share the same
compression dictionary. With careful design, a compression scheme compression dictionary. With careful design, a compression scheme
can be designed in a way that is not considered exploitable in can be designed in a way that is not considered exploitable in
limited use cases, such as HPACK ([RFC7541]). limited use cases, such as HPACK ([RFC7541]).
17.7. Disclosure of Personal Information 17.7. Disclosure of Personal Information
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of unique information that is least expected by users is proactive of unique information that is least expected by users is proactive
negotiation (Section 12.1), including the Accept, Accept-Charset, negotiation (Section 12.1), including the Accept, Accept-Charset,
Accept-Encoding, and Accept-Language header fields. Accept-Encoding, and Accept-Language header fields.
In addition to the fingerprinting concern, detailed use of the In addition to the fingerprinting concern, detailed use of the
Accept-Language header field can reveal information the user might Accept-Language header field can reveal information the user might
consider to be of a private nature. For example, understanding a consider to be of a private nature. For example, understanding a
given language set might be strongly correlated to membership in a given language set might be strongly correlated to membership in a
particular ethnic group. An approach that limits such loss of particular ethnic group. An approach that limits such loss of
privacy would be for a user agent to omit the sending of Accept- privacy would be for a user agent to omit the sending of Accept-
Language except for sites that have been whitelisted, perhaps via Language except for sites that have been explicitly permitted,
interaction after detecting a Vary header field that indicates perhaps via interaction after detecting a Vary header field that
language negotiation might be useful. indicates language negotiation might be useful.
In environments where proxies are used to enhance privacy, user In environments where proxies are used to enhance privacy, user
agents ought to be conservative in sending proactive negotiation agents ought to be conservative in sending proactive negotiation
header fields. General-purpose user agents that provide a high header fields. General-purpose user agents that provide a high
degree of header field configurability ought to inform users about degree of header field configurability ought to inform users about
the loss of privacy that might result if too much detail is provided. the loss of privacy that might result if too much detail is provided.
As an extreme privacy measure, proxies could filter the proactive As an extreme privacy measure, proxies could filter the proactive
negotiation header fields in relayed requests. negotiation header fields in relayed requests.
17.13. Validator Retention 17.13. Validator Retention
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information indefinitely. HTTP does not provide a mechanism for the information indefinitely. HTTP does not provide a mechanism for the
origin server to direct clients to discard these cached credentials, origin server to direct clients to discard these cached credentials,
since the protocol has no awareness of how credentials are obtained since the protocol has no awareness of how credentials are obtained
or managed by the user agent. The mechanisms for expiring or or managed by the user agent. The mechanisms for expiring or
revoking credentials can be specified as part of an authentication revoking credentials can be specified as part of an authentication
scheme definition. scheme definition.
Circumstances under which credential caching can interfere with the Circumstances under which credential caching can interfere with the
application's security model include but are not limited to: application's security model include but are not limited to:
o Clients that have been idle for an extended period, following * Clients that have been idle for an extended period, following
which the server might wish to cause the client to re-prompt the which the server might wish to cause the client to re-prompt the
user for credentials. user for credentials.
o Applications that include a session termination indication (such * Applications that include a session termination indication (such
as a "logout" or "commit" button on a page) after which the server as a "logout" or "commit" button on a page) after which the server
side of the application "knows" that there is no further reason side of the application "knows" that there is no further reason
for the client to retain the credentials. for the client to retain the credentials.
User agents that cache credentials are encouraged to provide a User agents that cache credentials are encouraged to provide a
readily accessible mechanism for discarding cached credentials under readily accessible mechanism for discarding cached credentials under
user control. user control.
17.15.3. Protection Spaces 17.15.3. Protection Spaces
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<https://www.iana.org/assignments/uri-schemes/> with the permanent <https://www.iana.org/assignments/uri-schemes/> with the permanent
schemes listed in the table in Section 4.2. schemes listed in the table in Section 4.2.
18.2. Method Registration 18.2. Method Registration
Please update the "Hypertext Transfer Protocol (HTTP) Method Please update the "Hypertext Transfer Protocol (HTTP) Method
Registry" at <https://www.iana.org/assignments/http-methods> with the Registry" at <https://www.iana.org/assignments/http-methods> with the
registration procedure of Section 16.1.1 and the method names registration procedure of Section 16.1.1 and the method names
summarized in the following table. summarized in the following table.
--------- ------ ------------ ------- +=========+======+============+=======+
Method Safe Idempotent Ref. | Method | Safe | Idempotent | Ref. |
--------- ------ ------------ ------- +=========+======+============+=======+
CONNECT no no 9.3.6 | CONNECT | no | no | 9.3.6 |
DELETE no yes 9.3.5 +---------+------+------------+-------+
GET yes yes 9.3.1 | DELETE | no | yes | 9.3.5 |
HEAD yes yes 9.3.2 +---------+------+------------+-------+
OPTIONS yes yes 9.3.7 | GET | yes | yes | 9.3.1 |
POST no no 9.3.3 +---------+------+------------+-------+
PUT no yes 9.3.4 | HEAD | yes | yes | 9.3.2 |
TRACE yes yes 9.3.8 +---------+------+------------+-------+
* no no 18.2 | OPTIONS | yes | yes | 9.3.7 |
--------- ------ ------------ ------- +---------+------+------------+-------+
| POST | no | no | 9.3.3 |
+---------+------+------------+-------+
| PUT | no | yes | 9.3.4 |
+---------+------+------------+-------+
| TRACE | yes | yes | 9.3.8 |
+---------+------+------------+-------+
| * | no | no | 18.2 |
+---------+------+------------+-------+
Table 7 Table 7
The method name "*" is reserved, since using that name as HTTP method The method name "*" is reserved, since using "*" as a method name
name might conflict with special semantics in fields such as "Access- would conflict with its usage as a wildcard in some fields (e.g.,
Control-Request-Method". "Access-Control-Request-Method").
18.3. Status Code Registration 18.3. Status Code Registration
Please update the "Hypertext Transfer Protocol (HTTP) Status Code Please update the "Hypertext Transfer Protocol (HTTP) Status Code
Registry" at <https://www.iana.org/assignments/http-status-codes> Registry" at <https://www.iana.org/assignments/http-status-codes>
with the registration procedure of Section 16.2.1 and the status code with the registration procedure of Section 16.2.1 and the status code
values summarized in the following table. values summarized in the following table.
------- ------------------------------- --------- +=======+===============================+=========+
Value Description Ref. | Value | Description | Ref. |
------- ------------------------------- --------- +=======+===============================+=========+
100 Continue 15.2.1 | 100 | Continue | 15.2.1