draft-ietf-httpbis-semantics-19.txt   draft-ietf-httpbis-semantics-latest.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: March 14, 2022 September 10, 2021 Expires: March 27, 2023 September 23, 2022
HTTP Semantics HTTP Semantics
draft-ietf-httpbis-semantics-19 draft-ietf-httpbis-semantics-latest
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.
This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232,
7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions 7233, 7235, 7538, 7615, 7694, and portions of 7230.
of RFC 7230.
Editorial Note Editorial Note
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.20. The changes in this draft are summarized in Appendix C.1.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 14, 2022. This Internet-Draft will expire on March 27, 2023.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . 10
1.3. Core Semantics . . . . . . . . . . . . . . . . . . . . . 11 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 . . . . . . . . . . . . . . . . . . 13 2.2. Requirements Notation . . . . . . . . . . . . . . . . . . 12
2.3. Length Requirements . . . . . . . . . . . . . . . . . . . 14 2.3. Length Requirements . . . . . . . . . . . . . . . . . . . 13
2.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 14 2.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 14
2.5. Protocol Version . . . . . . . . . . . . . . . . . . . . 15 2.5. Protocol Version . . . . . . . . . . . . . . . . . . . . 15
3. Terminology and Core Concepts . . . . . . . . . . . . . . . . 16 3. Terminology and Core Concepts . . . . . . . . . . . . . . . . 15
3.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Representations . . . . . . . . . . . . . . . . . . . . . 16 3.2. Representations . . . . . . . . . . . . . . . . . . . . . 16
3.3. Connections, Clients and Servers . . . . . . . . . . . . 17 3.3. Connections, Clients, and Servers . . . . . . . . . . . . 17
3.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . 18 3.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . 17
3.5. User Agents . . . . . . . . . . . . . . . . . . . . . . . 18 3.5. User Agents . . . . . . . . . . . . . . . . . . . . . . . 18
3.6. Origin Server . . . . . . . . . . . . . . . . . . . . . . 19 3.6. Origin Server . . . . . . . . . . . . . . . . . . . . . . 19
3.7. Intermediaries . . . . . . . . . . . . . . . . . . . . . 19 3.7. Intermediaries . . . . . . . . . . . . . . . . . . . . . 19
3.8. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.8. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.9. Example Message Exchange . . . . . . . . . . . . . . . . 22 3.9. Example Message Exchange . . . . . . . . . . . . . . . . 22
4. Identifiers in HTTP . . . . . . . . . . . . . . . . . . . . . 22 4. Identifiers in HTTP . . . . . . . . . . . . . . . . . . . . . 22
4.1. URI References . . . . . . . . . . . . . . . . . . . . . 23 4.1. URI References . . . . . . . . . . . . . . . . . . . . . 23
4.2. HTTP-Related URI Schemes . . . . . . . . . . . . . . . . 23 4.2. HTTP-Related URI Schemes . . . . . . . . . . . . . . . . 23
4.2.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 24 4.2.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 24
4.2.2. https URI Scheme . . . . . . . . . . . . . . . . . . 24 4.2.2. https URI Scheme . . . . . . . . . . . . . . . . . . 24
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 . . . . . . . 26 4.2.4. Deprecation of userinfo in http(s) URIs . . . . . . . 26
4.2.5. http(s) References with Fragment Identifiers . . . . 27 4.2.5. http(s) References with Fragment Identifiers . . . . 27
4.3. Authoritative Access . . . . . . . . . . . . . . . . . . 27 4.3. Authoritative Access . . . . . . . . . . . . . . . . . . 27
4.3.1. URI Origin . . . . . . . . . . . . . . . . . . . . . 27 4.3.1. URI Origin . . . . . . . . . . . . . . . . . . . . . 27
4.3.2. http origins . . . . . . . . . . . . . . . . . . . . 28 4.3.2. http Origins . . . . . . . . . . . . . . . . . . . . 28
4.3.3. https origins . . . . . . . . . . . . . . . . . . . . 29 4.3.3. https Origins . . . . . . . . . . . . . . . . . . . . 29
4.3.4. https certificate verification . . . . . . . . . . . 30 4.3.4. https Certificate Verification . . . . . . . . . . . 30
4.3.5. IP-ID reference identity . . . . . . . . . . . . . . 31 4.3.5. IP-ID Reference Identity . . . . . . . . . . . . . . 31
5. Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5. Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 32 5.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 32
5.2. Field Lines and Combined Field Value . . . . . . . . . . 33 5.2. Field Lines and Combined Field Value . . . . . . . . . . 33
5.3. Field Order . . . . . . . . . . . . . . . . . . . . . . . 33 5.3. Field Order . . . . . . . . . . . . . . . . . . . . . . . 33
5.4. Field Limits . . . . . . . . . . . . . . . . . . . . . . 34 5.4. Field Limits . . . . . . . . . . . . . . . . . . . . . . 34
5.5. Field Values . . . . . . . . . . . . . . . . . . . . . . 34 5.5. Field Values . . . . . . . . . . . . . . . . . . . . . . 34
5.6. Common Rules for Defining Field Values . . . . . . . . . 36 5.6. Common Rules for Defining Field Values . . . . . . . . . 36
5.6.1. Lists (#rule ABNF Extension) . . . . . . . . . . . . 36 5.6.1. Lists (#rule ABNF Extension) . . . . . . . . . . . . 36
5.6.1.1. Sender Requirements . . . . . . . . . . . . . . . 37 5.6.1.1. Sender Requirements . . . . . . . . . . . . . . . 37
5.6.1.2. Recipient Requirements . . . . . . . . . . . . . 37 5.6.1.2. Recipient Requirements . . . . . . . . . . . . . 37
5.6.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 38 5.6.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 38
5.6.3. Whitespace . . . . . . . . . . . . . . . . . . . . . 38 5.6.3. Whitespace . . . . . . . . . . . . . . . . . . . . . 38
5.6.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 39 5.6.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 39
5.6.5. Comments . . . . . . . . . . . . . . . . . . . . . . 39 5.6.5. Comments . . . . . . . . . . . . . . . . . . . . . . 39
5.6.6. Parameters . . . . . . . . . . . . . . . . . . . . . 40 5.6.6. Parameters . . . . . . . . . . . . . . . . . . . . . 40
5.6.7. Date/Time Formats . . . . . . . . . . . . . . . . . . 40 5.6.7. Date/Time Formats . . . . . . . . . . . . . . . . . . 40
6. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 42 6. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 42
6.1. Framing and Completeness . . . . . . . . . . . . . . . . 43 6.1. Framing and Completeness . . . . . . . . . . . . . . . . 44
6.2. Control Data . . . . . . . . . . . . . . . . . . . . . . 44 6.2. Control Data . . . . . . . . . . . . . . . . . . . . . . 44
6.3. Header Fields . . . . . . . . . . . . . . . . . . . . . . 45 6.3. Header Fields . . . . . . . . . . . . . . . . . . . . . . 45
6.4. Content . . . . . . . . . . . . . . . . . . . . . . . . . 45 6.4. Content . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.4.1. Content Semantics . . . . . . . . . . . . . . . . . . 46 6.4.1. Content Semantics . . . . . . . . . . . . . . . . . . 46
6.4.2. Identifying Content . . . . . . . . . . . . . . . . . 47 6.4.2. Identifying Content . . . . . . . . . . . . . . . . . 47
6.5. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 48 6.5. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 48
6.5.1. Limitations on use of Trailers . . . . . . . . . . . 48 6.5.1. Limitations on Use of Trailers . . . . . . . . . . . 49
6.5.2. Processing Trailer Fields . . . . . . . . . . . . . . 49 6.5.2. Processing Trailer Fields . . . . . . . . . . . . . . 49
6.6. Message Metadata . . . . . . . . . . . . . . . . . . . . 50 6.6. Message Metadata . . . . . . . . . . . . . . . . . . . . 50
6.6.1. Date . . . . . . . . . . . . . . . . . . . . . . . . 50 6.6.1. Date . . . . . . . . . . . . . . . . . . . . . . . . 50
6.6.2. Trailer . . . . . . . . . . . . . . . . . . . . . . . 51 6.6.2. Trailer . . . . . . . . . . . . . . . . . . . . . . . 51
7. Routing HTTP Messages . . . . . . . . . . . . . . . . . . . . 51 7. Routing HTTP Messages . . . . . . . . . . . . . . . . . . . . 51
7.1. Determining the Target Resource . . . . . . . . . . . . . 51 7.1. Determining the Target Resource . . . . . . . . . . . . . 52
7.2. Host and :authority . . . . . . . . . . . . . . . . . . . 52 7.2. Host and :authority . . . . . . . . . . . . . . . . . . . 52
7.3. Routing Inbound Requests . . . . . . . . . . . . . . . . 53 7.3. Routing Inbound Requests . . . . . . . . . . . . . . . . 53
7.3.1. To a Cache . . . . . . . . . . . . . . . . . . . . . 53 7.3.1. To a Cache . . . . . . . . . . . . . . . . . . . . . 53
7.3.2. To a Proxy . . . . . . . . . . . . . . . . . . . . . 53 7.3.2. To a Proxy . . . . . . . . . . . . . . . . . . . . . 53
7.3.3. To the Origin . . . . . . . . . . . . . . . . . . . . 54 7.3.3. To the Origin . . . . . . . . . . . . . . . . . . . . 54
7.4. Rejecting Misdirected Requests . . . . . . . . . . . . . 54 7.4. Rejecting Misdirected Requests . . . . . . . . . . . . . 54
7.5. Response Correlation . . . . . . . . . . . . . . . . . . 55 7.5. Response Correlation . . . . . . . . . . . . . . . . . . 55
7.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 55 7.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 55
7.6.1. Connection . . . . . . . . . . . . . . . . . . . . . 56 7.6.1. Connection . . . . . . . . . . . . . . . . . . . . . 56
7.6.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 57 7.6.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 57
7.6.3. Via . . . . . . . . . . . . . . . . . . . . . . . . . 58 7.6.3. Via . . . . . . . . . . . . . . . . . . . . . . . . . 58
7.7. Message Transformations . . . . . . . . . . . . . . . . . 60 7.7. Message Transformations . . . . . . . . . . . . . . . . . 60
7.8. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 61 7.8. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 61
8. Representation Data and Metadata . . . . . . . . . . . . . . 63 8. Representation Data and Metadata . . . . . . . . . . . . . . 63
8.1. Representation Data . . . . . . . . . . . . . . . . . . . 63 8.1. Representation Data . . . . . . . . . . . . . . . . . . . 63
8.2. Representation Metadata . . . . . . . . . . . . . . . . . 63 8.2. Representation Metadata . . . . . . . . . . . . . . . . . 64
8.3. Content-Type . . . . . . . . . . . . . . . . . . . . . . 64 8.3. Content-Type . . . . . . . . . . . . . . . . . . . . . . 64
8.3.1. Media Type . . . . . . . . . . . . . . . . . . . . . 65 8.3.1. Media Type . . . . . . . . . . . . . . . . . . . . . 65
8.3.2. Charset . . . . . . . . . . . . . . . . . . . . . . . 65 8.3.2. Charset . . . . . . . . . . . . . . . . . . . . . . . 65
8.3.3. Multipart Types . . . . . . . . . . . . . . . . . . . 66 8.3.3. Multipart Types . . . . . . . . . . . . . . . . . . . 66
8.4. Content-Encoding . . . . . . . . . . . . . . . . . . . . 66 8.4. Content-Encoding . . . . . . . . . . . . . . . . . . . . 66
8.4.1. Content Codings . . . . . . . . . . . . . . . . . . . 67 8.4.1. Content Codings . . . . . . . . . . . . . . . . . . . 67
8.4.1.1. Compress Coding . . . . . . . . . . . . . . . . . 68 8.4.1.1. Compress Coding . . . . . . . . . . . . . . . . . 68
8.4.1.2. Deflate Coding . . . . . . . . . . . . . . . . . 68 8.4.1.2. Deflate Coding . . . . . . . . . . . . . . . . . 68
8.4.1.3. Gzip Coding . . . . . . . . . . . . . . . . . . . 68 8.4.1.3. Gzip Coding . . . . . . . . . . . . . . . . . . . 68
8.5. Content-Language . . . . . . . . . . . . . . . . . . . . 68 8.5. Content-Language . . . . . . . . . . . . . . . . . . . . 68
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8.6. Content-Length . . . . . . . . . . . . . . . . . . . . . 70 8.6. Content-Length . . . . . . . . . . . . . . . . . . . . . 70
8.7. Content-Location . . . . . . . . . . . . . . . . . . . . 71 8.7. Content-Location . . . . . . . . . . . . . . . . . . . . 71
8.8. Validator Fields . . . . . . . . . . . . . . . . . . . . 73 8.8. Validator Fields . . . . . . . . . . . . . . . . . . . . 73
8.8.1. Weak versus Strong . . . . . . . . . . . . . . . . . 74 8.8.1. Weak versus Strong . . . . . . . . . . . . . . . . . 74
8.8.2. Last-Modified . . . . . . . . . . . . . . . . . . . . 75 8.8.2. Last-Modified . . . . . . . . . . . . . . . . . . . . 75
8.8.2.1. Generation . . . . . . . . . . . . . . . . . . . 76 8.8.2.1. Generation . . . . . . . . . . . . . . . . . . . 76
8.8.2.2. Comparison . . . . . . . . . . . . . . . . . . . 76 8.8.2.2. Comparison . . . . . . . . . . . . . . . . . . . 76
8.8.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 77 8.8.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 77
8.8.3.1. Generation . . . . . . . . . . . . . . . . . . . 78 8.8.3.1. Generation . . . . . . . . . . . . . . . . . . . 78
8.8.3.2. Comparison . . . . . . . . . . . . . . . . . . . 79 8.8.3.2. Comparison . . . . . . . . . . . . . . . . . . . 79
8.8.3.3. Example: Entity-Tags Varying on Content-Negotiated 8.8.3.3. Example: Entity Tags Varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . 79 Resources . . . . . . . . . . . . . . . . . . . . . 79
9. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 9. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 81 9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 81
9.2. Common Method Properties . . . . . . . . . . . . . . . . 82 9.2. Common Method Properties . . . . . . . . . . . . . . . . 82
9.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 83 9.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 83
9.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 84 9.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 84
9.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 85 9.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 85
9.3. Method Definitions . . . . . . . . . . . . . . . . . . . 85 9.3. Method Definitions . . . . . . . . . . . . . . . . . . . 85
9.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 85 9.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 85
9.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 86 9.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 86
skipping to change at page 6, line 36 skipping to change at page 6, line 30
14.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . 138 14.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . 138
14.2. Range . . . . . . . . . . . . . . . . . . . . . . . . . 139 14.2. Range . . . . . . . . . . . . . . . . . . . . . . . . . 139
14.3. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 141 14.3. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 141
14.4. Content-Range . . . . . . . . . . . . . . . . . . . . . 142 14.4. Content-Range . . . . . . . . . . . . . . . . . . . . . 142
14.5. Partial PUT . . . . . . . . . . . . . . . . . . . . . . 144 14.5. Partial PUT . . . . . . . . . . . . . . . . . . . . . . 144
14.6. Media Type multipart/byteranges . . . . . . . . . . . . 144 14.6. Media Type multipart/byteranges . . . . . . . . . . . . 144
15. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 146 15. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 146
15.1. Overview of Status Codes . . . . . . . . . . . . . . . . 147 15.1. Overview of Status Codes . . . . . . . . . . . . . . . . 147
15.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 148 15.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 148
15.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 148 15.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 148
15.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 148 15.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 149
15.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 149 15.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 149
15.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 149 15.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 149
15.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 150 15.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 150
15.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 150 15.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 150
15.3.4. 203 Non-Authoritative Information . . . . . . . . . 151 15.3.4. 203 Non-Authoritative Information . . . . . . . . . 151
15.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 151 15.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 151
15.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 152 15.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 152
15.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 152 15.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 152
15.3.7.1. Single Part . . . . . . . . . . . . . . . . . . 153 15.3.7.1. Single Part . . . . . . . . . . . . . . . . . . 153
15.3.7.2. Multiple Parts . . . . . . . . . . . . . . . . . 153 15.3.7.2. Multiple Parts . . . . . . . . . . . . . . . . . 153
15.3.7.3. Combining Parts . . . . . . . . . . . . . . . . 155 15.3.7.3. Combining Parts . . . . . . . . . . . . . . . . 155
15.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 155 15.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 156
15.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 157 15.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 158
15.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 158 15.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 159
15.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 159 15.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 159
15.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 159 15.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 160
15.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 160 15.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 160
15.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 161 15.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 161
15.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 161 15.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 161
15.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 161 15.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 161
15.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 161 15.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 162
15.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 162 15.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 162
15.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 162 15.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 162
15.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 162 15.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 163
15.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 163 15.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 163
15.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 163 15.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 163
15.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 163 15.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 163
15.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 163 15.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 164
15.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 164 15.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 164
15.5.8. 407 Proxy Authentication Required . . . . . . . . . 164 15.5.8. 407 Proxy Authentication Required . . . . . . . . . 164
15.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 164 15.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 164
15.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 164 15.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 165
15.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 165 15.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 165
15.5.12. 411 Length Required . . . . . . . . . . . . . . . . 165 15.5.12. 411 Length Required . . . . . . . . . . . . . . . . 166
15.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 165 15.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 166
15.5.14. 413 Content Too Large . . . . . . . . . . . . . . . 166 15.5.14. 413 Content Too Large . . . . . . . . . . . . . . . 166
15.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 166 15.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 166
15.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 166 15.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 167
15.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 167 15.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 167
15.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 167 15.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 168
15.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 167 15.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 168
15.5.20. 421 Misdirected Request . . . . . . . . . . . . . . 168 15.5.20. 421 Misdirected Request . . . . . . . . . . . . . . 168
15.5.21. 422 Unprocessable Content . . . . . . . . . . . . . 168 15.5.21. 422 Unprocessable Content . . . . . . . . . . . . . 169
15.5.22. 426 Upgrade Required . . . . . . . . . . . . . . . . 168 15.5.22. 426 Upgrade Required . . . . . . . . . . . . . . . . 169
15.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 169 15.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 169
15.6.1. 500 Internal Server Error . . . . . . . . . . . . . 169 15.6.1. 500 Internal Server Error . . . . . . . . . . . . . 169
15.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 169 15.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 170
15.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 169 15.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 170
15.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 170 15.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 170
15.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 170 15.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 170
15.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 170 15.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 170
16. Extending HTTP . . . . . . . . . . . . . . . . . . . . . . . 170 16. Extending HTTP . . . . . . . . . . . . . . . . . . . . . . . 171
16.1. Method Extensibility . . . . . . . . . . . . . . . . . . 171 16.1. Method Extensibility . . . . . . . . . . . . . . . . . . 171
16.1.1. Method Registry . . . . . . . . . . . . . . . . . . 171 16.1.1. Method Registry . . . . . . . . . . . . . . . . . . 171
16.1.2. Considerations for New Methods . . . . . . . . . . . 171 16.1.2. Considerations for New Methods . . . . . . . . . . . 172
16.2. Status Code Extensibility . . . . . . . . . . . . . . . 172 16.2. Status Code Extensibility . . . . . . . . . . . . . . . 173
16.2.1. Status Code Registry . . . . . . . . . . . . . . . . 172 16.2.1. Status Code Registry . . . . . . . . . . . . . . . . 173
16.2.2. Considerations for New Status Codes . . . . . . . . 173 16.2.2. Considerations for New Status Codes . . . . . . . . 173
16.3. Field Extensibility . . . . . . . . . . . . . . . . . . 174 16.3. Field Extensibility . . . . . . . . . . . . . . . . . . 174
16.3.1. Field Name Registry . . . . . . . . . . . . . . . . 174 16.3.1. Field Name Registry . . . . . . . . . . . . . . . . 174
16.3.2. Considerations for New Fields . . . . . . . . . . . 175 16.3.2. Considerations for New Fields . . . . . . . . . . . 176
16.3.2.1. Considerations for New Field Names . . . . . . . 176 16.3.2.1. Considerations for New Field Names . . . . . . . 177
16.3.2.2. Considerations for New Field Values . . . . . . 177 16.3.2.2. Considerations for New Field Values . . . . . . 177
16.4. Authentication Scheme Extensibility . . . . . . . . . . 178 16.4. Authentication Scheme Extensibility . . . . . . . . . . 178
16.4.1. Authentication Scheme Registry . . . . . . . . . . . 178 16.4.1. Authentication Scheme Registry . . . . . . . . . . . 178
16.4.2. Considerations for New Authentication Schemes . . . 178 16.4.2. Considerations for New Authentication Schemes . . . 179
16.5. Range Unit Extensibility . . . . . . . . . . . . . . . . 180 16.5. Range Unit Extensibility . . . . . . . . . . . . . . . . 180
16.5.1. Range Unit Registry . . . . . . . . . . . . . . . . 180 16.5.1. Range Unit Registry . . . . . . . . . . . . . . . . 180
16.5.2. Considerations for New Range Units . . . . . . . . . 180 16.5.2. Considerations for New Range Units . . . . . . . . . 180
16.6. Content Coding Extensibility . . . . . . . . . . . . . . 180 16.6. Content Coding Extensibility . . . . . . . . . . . . . . 180
16.6.1. Content Coding Registry . . . . . . . . . . . . . . 180 16.6.1. Content Coding Registry . . . . . . . . . . . . . . 181
16.6.2. Considerations for New Content Codings . . . . . . . 181 16.6.2. Considerations for New Content Codings . . . . . . . 181
16.7. Upgrade Token Registry . . . . . . . . . . . . . . . . . 181 16.7. Upgrade Token Registry . . . . . . . . . . . . . . . . . 181
17. Security Considerations . . . . . . . . . . . . . . . . . . . 182 17. Security Considerations . . . . . . . . . . . . . . . . . . . 182
17.1. Establishing Authority . . . . . . . . . . . . . . . . . 182 17.1. Establishing Authority . . . . . . . . . . . . . . . . . 182
17.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 183 17.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 184
17.3. Attacks Based on File and Path Names . . . . . . . . . . 184 17.3. Attacks Based on File and Path Names . . . . . . . . . . 184
17.4. Attacks Based on Command, Code, or Query Injection . . . 184 17.4. Attacks Based on Command, Code, or Query Injection . . . 185
17.5. Attacks via Protocol Element Length . . . . . . . . . . 185 17.5. Attacks via Protocol Element Length . . . . . . . . . . 185
17.6. Attacks using Shared-dictionary Compression . . . . . . 186 17.6. Attacks Using Shared-Dictionary Compression . . . . . . 186
17.7. Disclosure of Personal Information . . . . . . . . . . . 186 17.7. Disclosure of Personal Information . . . . . . . . . . . 186
17.8. Privacy of Server Log Information . . . . . . . . . . . 186 17.8. Privacy of Server Log Information . . . . . . . . . . . 186
17.9. Disclosure of Sensitive Information in URIs . . . . . . 187 17.9. Disclosure of Sensitive Information in URIs . . . . . . 187
17.10. Application Handling of Field Names . . . . . . . . . . 187 17.10. Application Handling of Field Names . . . . . . . . . . 188
17.11. Disclosure of Fragment after Redirects . . . . . . . . . 188 17.11. Disclosure of Fragment after Redirects . . . . . . . . . 189
17.12. Disclosure of Product Information . . . . . . . . . . . 189 17.12. Disclosure of Product Information . . . . . . . . . . . 189
17.13. Browser Fingerprinting . . . . . . . . . . . . . . . . . 189 17.13. Browser Fingerprinting . . . . . . . . . . . . . . . . . 189
17.14. Validator Retention . . . . . . . . . . . . . . . . . . 190 17.14. Validator Retention . . . . . . . . . . . . . . . . . . 190
17.15. Denial-of-Service Attacks Using Range . . . . . . . . . 190 17.15. Denial-of-Service Attacks Using Range . . . . . . . . . 191
17.16. Authentication Considerations . . . . . . . . . . . . . 191 17.16. Authentication Considerations . . . . . . . . . . . . . 191
17.16.1. Confidentiality of Credentials . . . . . . . . . . 191 17.16.1. Confidentiality of Credentials . . . . . . . . . . 191
17.16.2. Credentials and Idle Clients . . . . . . . . . . . 191 17.16.2. Credentials and Idle Clients . . . . . . . . . . . 192
17.16.3. Protection Spaces . . . . . . . . . . . . . . . . . 192 17.16.3. Protection Spaces . . . . . . . . . . . . . . . . . 192
17.16.4. Additional Response Fields . . . . . . . . . . . . 192 17.16.4. Additional Response Fields . . . . . . . . . . . . 193
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 192 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 193
18.1. URI Scheme Registration . . . . . . . . . . . . . . . . 193 18.1. URI Scheme Registration . . . . . . . . . . . . . . . . 193
18.2. Method Registration . . . . . . . . . . . . . . . . . . 193 18.2. Method Registration . . . . . . . . . . . . . . . . . . 193
18.3. Status Code Registration . . . . . . . . . . . . . . . . 193 18.3. Status Code Registration . . . . . . . . . . . . . . . . 194
18.4. Field Name Registration . . . . . . . . . . . . . . . . 195 18.4. Field Name Registration . . . . . . . . . . . . . . . . 195
18.5. Authentication Scheme Registration . . . . . . . . . . . 197 18.5. Authentication Scheme Registration . . . . . . . . . . . 197
18.6. Content Coding Registration . . . . . . . . . . . . . . 197 18.6. Content Coding Registration . . . . . . . . . . . . . . 198
18.7. Range Unit Registration . . . . . . . . . . . . . . . . 197 18.7. Range Unit Registration . . . . . . . . . . . . . . . . 198
18.8. Media Type Registration . . . . . . . . . . . . . . . . 198 18.8. Media Type Registration . . . . . . . . . . . . . . . . 198
18.9. Port Registration . . . . . . . . . . . . . . . . . . . 198 18.9. Port Registration . . . . . . . . . . . . . . . . . . . 199
18.10. Upgrade Token Registration . . . . . . . . . . . . . . . 198 18.10. Upgrade Token Registration . . . . . . . . . . . . . . . 199
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 198 19. References . . . . . . . . . . . . . . . . . . . . . . . . . 199
19.1. Normative References . . . . . . . . . . . . . . . . . . 198 19.1. Normative References . . . . . . . . . . . . . . . . . . 199
19.2. Informative References . . . . . . . . . . . . . . . . . 200 19.2. Informative References . . . . . . . . . . . . . . . . . 201
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 207 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 208
Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 211 Appendix B. Changes from Previous RFCs . . . . . . . . . . . . . 212
B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 211 B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 212
B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 211 B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 212
B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 212 B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 213
B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 214 B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 215
B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 215 B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 216
B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 215 B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 216
B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 215 B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 216
B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 215 B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 216
B.9. Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 215 B.9. Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 216
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 215 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 216
C.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 215 C.1. Since draft-ietf-httpbis-semantics-19 . . . . . . . . . . 217
C.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 216 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 217
C.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 216 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
C.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 218 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 229
C.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 219
C.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 219
C.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 220
C.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 221
C.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 223
C.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 224
C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . . . . . 225
C.12. Since draft-ietf-httpbis-semantics-10 . . . . . . . . . . 225
C.13. Since draft-ietf-httpbis-semantics-11 . . . . . . . . . . 227
C.14. Since draft-ietf-httpbis-semantics-12 . . . . . . . . . . 227
C.15. Since draft-ietf-httpbis-semantics-13 . . . . . . . . . . 229
C.16. Since draft-ietf-httpbis-semantics-14 . . . . . . . . . . 230
C.17. Since draft-ietf-httpbis-semantics-15 . . . . . . . . . . 232
C.18. Since draft-ietf-httpbis-semantics-16 . . . . . . . . . . 233
C.19. Since draft-ietf-httpbis-semantics-17 . . . . . . . . . . 233
C.20. Since draft-ietf-httpbis-semantics-18 . . . . . . . . . . 235
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 236
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 248
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 10, line 43 skipping to change at page 10, line 25
and HTTP/1.0 (see [HTTP/1.0]). and HTTP/1.0 (see [HTTP/1.0]).
HTTP/1.1 was designed to refine the protocol's features while HTTP/1.1 was designed to refine the protocol's features while
retaining compatibility with the existing text-based messaging retaining compatibility with the existing text-based messaging
syntax, improving its interoperability, scalability, and robustness syntax, improving its interoperability, scalability, and robustness
across the Internet. This included length-based data delimiters for across the Internet. This included length-based data delimiters for
both fixed and dynamic (chunked) content, a consistent framework for both fixed and dynamic (chunked) content, a consistent framework for
content negotiation, opaque validators for conditional requests, content negotiation, opaque validators for conditional requests,
cache controls for better cache consistency, range requests for cache controls for better cache consistency, range requests for
partial updates, and default persistent connections. HTTP/1.1 was partial updates, and default persistent connections. HTTP/1.1 was
introduced in 1995 and published on the standards track in 1997 introduced in 1995 and published on the Standards Track in 1997
[RFC2068], revised in 1999 [RFC2616], and revised again in 2014 [RFC2068], revised in 1999 [RFC2616], and revised again in 2014
([RFC7230] - [RFC7235]). ([RFC7230] through [RFC7235]).
HTTP/2 ([HTTP/2]) introduced a multiplexed session layer on top of HTTP/2 ([HTTP/2]) introduced a multiplexed session layer on top of
the existing TLS and TCP protocols for exchanging concurrent HTTP the existing TLS and TCP protocols for exchanging concurrent HTTP
messages with efficient field compression and server push. HTTP/3 messages with efficient field compression and server push. HTTP/3
([HTTP/3]) provides greater independence for concurrent messages by ([HTTP/3]) provides greater independence for concurrent messages by
using QUIC as a secure multiplexed transport over UDP instead of TCP. using QUIC as a secure multiplexed transport over UDP instead of TCP.
All three major versions of HTTP rely on the semantics defined by All three major versions of HTTP rely on the semantics defined by
this document. They have not obsoleted each other because each one this document. They have not obsoleted each other because each one
has specific benefits and limitations depending on the context of has specific benefits and limitations depending on the context of
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HTTP semantics include the intentions defined by each request method HTTP semantics include the intentions defined by each request method
(Section 9), extensions to those semantics that might be described in (Section 9), extensions to those semantics that might be described in
request header fields, status codes that describe the response request header fields, status codes that describe the response
(Section 15), and other control data and resource metadata that might (Section 15), and other control data and resource metadata that might
be given in response fields. be given in response fields.
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
This document obsoletes the following specifications: 1.4. Specifications Obsoleted by This Document
+--------------------------------------------+-----------+---------+ +--------------------------------------------+-----------+-----+
| Title | Reference | Changes | | Title | Reference | See |
+--------------------------------------------+-----------+---------+ +--------------------------------------------+-----------+-----+
| 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 Message Syntax and Routing [*] | [RFC7230] | B.2 |
| HTTP/1.1 Semantics and Content | [RFC7231] | B.3 | | HTTP/1.1 Semantics and Content | [RFC7231] | B.3 |
| HTTP/1.1 Conditional Requests | [RFC7232] | B.4 | | HTTP/1.1 Conditional Requests | [RFC7232] | B.4 |
| HTTP/1.1 Range Requests | [RFC7233] | B.5 | | HTTP/1.1 Range Requests | [RFC7233] | B.5 |
| HTTP/1.1 Authentication | [RFC7235] | B.6 | | HTTP/1.1 Authentication | [RFC7235] | B.6 |
| HTTP Status Code 308 (Permanent Redirect) | [RFC7538] | B.7 | | HTTP Status Code 308 (Permanent Redirect) | [RFC7538] | B.7 |
| HTTP Authentication-Info and Proxy- | [RFC7615] | B.8 | | HTTP Authentication-Info and Proxy- | [RFC7615] | B.8 |
| Authentication-Info Response Header Fields | | | | Authentication-Info Response Header Fields | | |
| HTTP Client-Initiated Content-Encoding | [RFC7694] | B.9 | | 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" [HTTP/1.1]. "HTTP/1.1" [HTTP/1.1].
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
standard ABNF notation. standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote As a convention, ABNF rule names prefixed with "obs-" denote obsolete
"obsolete" grammar rules that appear for historical reasons. grammar rules that appear for historical reasons.
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
(line feed), OCTET (any 8-bit sequence of data), SP (space), and (line feed), OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character). VCHAR (any visible US-ASCII character).
Section 5.6 defines some generic syntactic components for field Section 5.6 defines some generic syntactic components for field
values. values.
skipping to change at page 14, line 27 skipping to change at page 14, line 12
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
implementation. Hence, interoperability between senders and implementation. Hence, interoperability between senders and
recipients depends on shared expectations regarding what is a recipients depends on shared expectations regarding what is a
reasonable length for each protocol element. Furthermore, what is reasonable length for each protocol element. Furthermore, what is
commonly understood to be a reasonable length for some protocol commonly understood to be a reasonable length for some protocol
elements has changed over the course of the past two decades of HTTP elements has changed over the course of the past three decades of
use and is expected to continue changing in the future. HTTP use and is expected to continue changing in the future.
At a minimum, a recipient MUST be able to parse and process protocol At a minimum, a recipient MUST be able to parse and process protocol
element lengths that are at least as long as the values that it element lengths that are at least as long as the values that it
generates for those same protocol elements in other messages. For generates for those same protocol elements in other messages. For
example, an origin server that publishes very long URI references to example, an origin server that publishes very long URI references to
its own resources needs to be able to parse and process those same its own resources needs to be able to parse and process those same
references when received as a target URI. references when received as a target URI.
Many received protocol elements are only parsed to the extent Many received protocol elements are only parsed to the extent
necessary to identify and forward that element downstream. For necessary to identify and forward that element downstream. For
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Location header field doesn't parse according to the ABNF, whereas a Location header field doesn't parse according to the ABNF, whereas a
systems control client might consider any form of error recovery to systems control client might consider any form of error recovery to
be dangerous. be dangerous.
Some requests can be automatically retried by a client in the event Some requests can be automatically retried by a client in the event
of an underlying connection failure, as described in Section 9.2.2. of an underlying connection failure, as described in Section 9.2.2.
2.5. Protocol Version 2.5. Protocol Version
HTTP's version number consists of two decimal digits separated by a HTTP's version number consists of two decimal digits separated by a
"." (period or decimal point). The first digit ("major version") "." (period or decimal point). The first digit (major version)
indicates the messaging syntax, whereas the second digit ("minor indicates the messaging syntax, whereas the second digit (minor
version") indicates the highest minor version within that major version) indicates the highest minor version within that major
version to which the sender is conformant (able to understand for version to which the sender is conformant (able to understand for
future communication). future communication).
While HTTP's core semantics don't change between protocol versions, While HTTP's core semantics don't change between protocol versions,
the expression of them "on the wire" can change, and so the HTTP their expression "on the wire" can change, and so the HTTP version
version number changes when incompatible changes are made to the wire number changes when incompatible changes are made to the wire format.
format. Additionally, HTTP allows incremental, backwards-compatible Additionally, HTTP allows incremental, backwards-compatible changes
changes to be made to the protocol without changing its version to be made to the protocol without changing its version through the
through the use of defined extension points (Section 16). use of defined extension points (Section 16).
The protocol version as a whole indicates the sender's conformance The protocol version as a whole indicates the sender's conformance
with the set of requirements laid out in that version's corresponding with the set of requirements laid out in that version's corresponding
specification of HTTP. For example, the version "HTTP/1.1" is specification(s). For example, the version "HTTP/1.1" is defined by
defined by the combined specifications of this document, "HTTP the combined specifications of this document, "HTTP Caching"
Caching" [CACHING], and "HTTP/1.1" [HTTP/1.1]. [CACHING], and "HTTP/1.1" [HTTP/1.1].
HTTP's major version number is incremented when an incompatible HTTP's major version number is incremented when an incompatible
message syntax is introduced. The minor number is incremented when message syntax is introduced. The minor number is incremented when
changes made to the protocol have the effect of adding to the message changes made to the protocol have the effect of adding to the message
semantics or implying additional capabilities of the sender. semantics or implying additional capabilities of the sender.
The minor version advertises the sender's communication capabilities The minor version advertises the sender's communication capabilities
even when the sender is only using a backwards-compatible subset of even when the sender is only using a backwards-compatible subset of
the protocol, thereby letting the recipient know that more advanced the protocol, thereby letting the recipient know that more advanced
features can be used in response (by servers) or in future requests features can be used in response (by servers) or in future requests
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3. Terminology and Core Concepts 3. Terminology and Core Concepts
HTTP was created for the World Wide Web (WWW) architecture and has HTTP was created for the World Wide Web (WWW) architecture and has
evolved over time to support the scalability needs of a worldwide evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the hypertext system. Much of that architecture is reflected in the
terminology used to define HTTP. terminology used to define HTTP.
3.1. Resources 3.1. Resources
The target of an HTTP request is called a _resource_. HTTP does not The target of an HTTP request is called a "resource". HTTP does not
limit the nature of a resource; it merely defines an interface that limit the nature of a resource; it merely defines an interface that
might be used to interact with resources. Most resources are might be used to interact with resources. Most resources are
identified by a Uniform Resource Identifier (URI), as described in identified by a Uniform Resource Identifier (URI), as described in
Section 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. A resource cannot treat a request in a modifying header fields. A resource cannot treat a request in a
manner inconsistent with the semantics of the method of the request. manner inconsistent with the semantics of the method of the request.
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are not safe, a client can expect the resource to avoid actions 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 are unsafe when processing a request with a safe method (see
Section 9.2.1). Section 9.2.1).
HTTP relies upon the Uniform Resource Identifier (URI) standard [URI] HTTP relies upon the Uniform Resource Identifier (URI) standard [URI]
to indicate the target resource (Section 7.1) and relationships to indicate the target resource (Section 7.1) and relationships
between resources. 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
of a set of representation metadata and a potentially unbounded of a set of representation metadata and a potentially unbounded
stream of representation data (Section 8). stream of representation data (Section 8).
HTTP allows "information hiding" behind its uniform interface by HTTP allows "information hiding" behind its uniform interface by
defining communication with respect to a transferable representation defining communication with respect to a transferable representation
of the resource state, rather than transferring the resource itself. of the resource state, rather than transferring the resource itself.
This allows the resource identified by a URI to be anything, This allows the resource identified by a URI to be anything,
including temporal functions like "the current weather in Laguna including temporal functions like "the current weather in Laguna
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or desired state of that thing in our communications. When a or desired state of that thing in our communications. When a
representation is hypertext, it can provide both a representation of representation is hypertext, it can provide both a representation of
the resource state and processing instructions that help guide the the resource state and processing instructions that help guide the
recipient's future interactions. recipient's future interactions.
A target resource might be provided with, or be capable of A target resource might be provided with, or be capable of
generating, multiple representations that are each intended to generating, multiple representations that are each intended to
reflect the resource's current state. An algorithm, usually based on reflect the resource's current state. An algorithm, usually based on
content negotiation (Section 12), would be used to select one of content negotiation (Section 12), would be used to select one of
those representations as being most applicable to a given request. those representations as being most applicable to a given request.
This _selected representation_ provides the data and metadata for This "selected representation" provides the data and metadata for
evaluating conditional requests (Section 13) and constructing the evaluating conditional requests (Section 13) and constructing the
content for 200 (OK), 206 (Partial Content), and 304 (Not Modified) content for 200 (OK), 206 (Partial Content), and 304 (Not Modified)
responses to GET (Section 9.3.1). responses to GET (Section 9.3.1).
3.3. Connections, Clients and Servers 3.3. Connections, Clients, and Servers
HTTP is a client/server protocol that operates over a reliable HTTP is a client/server protocol that operates over a reliable
transport- or session-layer _connection_. transport- or session-layer "connection".
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 HTTP is defined as a stateless protocol, meaning that each request
message's semantics can be understood in isolation, and that the message's semantics can be understood in isolation, and that the
relationship between connections and messages on them has no impact relationship between connections and messages on them has no impact
on the interpretation of those messages. For example, a CONNECT on the interpretation of those messages. For example, a CONNECT
request (Section 9.3.6) or a request with the Upgrade header field 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 (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 a connection. Many implementations depend on HTTP's stateless design
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As a result, a server MUST NOT assume that two requests on the same 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 connection are from the same user agent unless the connection is
secured and specific to that agent. Some non-standard HTTP secured and specific to that agent. Some non-standard HTTP
extensions (e.g., [RFC4559]) have been known to violate this extensions (e.g., [RFC4559]) have been known to violate this
requirement, resulting in security and interoperability problems. 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
request modifiers, client information, and representation metadata, request modifiers, client information, and representation metadata,
content (Section 6.4) intended for processing in accordance with the content (Section 6.4) intended for processing in accordance with the
method, and trailer fields (Section 6.5) to communicate information method, and trailer fields (Section 6.5) to communicate information
collected while sending the content. collected while sending the content.
A server responds to a client's request by sending one or more A server responds to a client's request by sending one or more
_response_ messages, each including a status code (Section 15). The "response" messages, each including a status code (Section 15). The
response might also contain header fields for server information, response might also contain header fields for server information,
resource metadata, and representation metadata, content to be resource metadata, and representation metadata, content to be
interpreted in accordance with the status code, and trailer fields to interpreted in accordance with the status code, and trailer fields to
communicate information collected while sending the content. communicate information collected while sending the content.
3.5. User Agents 3.5. User Agents
The term _user agent_ refers to any of the various client programs The term "user agent" refers to any of the various client programs
that initiate a request. that initiate a request.
The most familiar form of user agent is the general-purpose Web The most familiar form of user agent is the general-purpose Web
browser, but that's only a small percentage of implementations. browser, but that's only a small percentage of implementations.
Other common user agents include spiders (web-traversing robots), Other common user agents include spiders (web-traversing robots),
command-line tools, billboard screens, household appliances, scales, command-line tools, billboard screens, household appliances, scales,
light bulbs, firmware update scripts, mobile apps, and communication light bulbs, firmware update scripts, mobile apps, and communication
devices in a multitude of shapes and sizes. devices in a multitude of shapes and sizes.
Being a user agent does not imply that there is a human user directly Being a user agent does not imply that there is a human user directly
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reporting of errors to the user, it is acceptable for such reporting reporting of errors to the user, it is acceptable for such reporting
to only be observable in an error console or log file. Likewise, to only be observable in an error console or log file. Likewise,
requirements that an automated action be confirmed by the user before requirements that an automated action be confirmed by the user before
proceeding might be met via advance configuration choices, run-time proceeding might be met via advance configuration choices, run-time
options, or simple avoidance of the unsafe action; confirmation does options, or simple avoidance of the unsafe action; confirmation does
not imply any specific user interface or interruption of normal not imply any specific user interface or interruption of normal
processing if the user has already made that choice. processing if the user has already made that choice.
3.6. Origin Server 3.6. Origin Server
The term _origin server_ refers to a program that can originate The term "origin server" refers to a program that can originate
authoritative responses for a given target resource. authoritative responses for a given target resource.
The most familiar form of origin server are large public websites. The most familiar form of origin server are large public websites.
However, like user agents being equated with browsers, it is easy to However, like user agents being equated with browsers, it is easy to
be misled into thinking that all origin servers are alike. Common be misled into thinking that all origin servers are alike. Common
origin servers also include home automation units, configurable origin servers also include home automation units, configurable
networking components, office machines, autonomous robots, news networking components, office machines, autonomous robots, news
feeds, traffic cameras, real-time ad selectors, and video-on-demand feeds, traffic cameras, real-time ad selectors, and video-on-demand
platforms. platforms.
skipping to change at page 19, line 39 skipping to change at page 19, line 34
request > request >
UA ======================================= O UA ======================================= O
< response < response
Figure 1 Figure 1
3.7. Intermediaries 3.7. Intermediaries
HTTP enables the use of intermediaries to satisfy requests through a HTTP enables the use of intermediaries to satisfy requests through a
chain of connections. There are three common forms of HTTP chain of connections. There are three common forms of HTTP
_intermediary_: proxy, gateway, and tunnel. In some cases, a single "intermediary": proxy, gateway, and tunnel. In some cases, a single
intermediary might act as an origin server, proxy, gateway, or intermediary might act as an origin server, proxy, gateway, or
tunnel, switching behavior based on the nature of each request. tunnel, switching behavior based on the nature of each request.
> > > > > > > >
UA =========== A =========== B =========== C =========== O UA =========== A =========== B =========== C =========== O
< < < < < < < <
Figure 2 Figure 2
The figure above shows three intermediaries (A, B, and C) between the The figure above shows three intermediaries (A, B, and C) between the
skipping to change at page 20, line 4 skipping to change at page 19, line 47
> > > > > > > >
UA =========== A =========== B =========== C =========== O UA =========== A =========== B =========== C =========== O
< < < < < < < <
Figure 2 Figure 2
The figure above shows three intermediaries (A, B, and C) between the The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections. travels the whole chain will pass through four separate connections.
Some HTTP communication options might apply only to the connection Some HTTP communication options might apply only to the connection
with the nearest, non-tunnel neighbor, only to the endpoints of the with the nearest, non-tunnel neighbor, only to the endpoints of the
chain, or to all connections along the chain. Although the diagram chain, or to all connections along the chain. Although the diagram
is linear, each participant might be engaged in multiple, is linear, each participant might be engaged in multiple,
simultaneous communications. For example, B might be receiving simultaneous communications. For example, B might be receiving
requests from many clients other than A, and/or forwarding requests requests from many clients other than A, and/or forwarding requests
to servers other than C, at the same time that it is handling A's to servers other than C, at the same time that it is handling A's
request. Likewise, later requests might be sent through a different request. Likewise, later requests might be sent through a different
path of connections, often based on dynamic configuration for load path of connections, often based on dynamic configuration for load
balancing. balancing.
The terms _upstream_ and _downstream_ are used to describe The terms "upstream" and "downstream" are used to describe
directional requirements in relation to the message flow: all directional requirements in relation to the message flow: all
messages flow from upstream to downstream. The terms "inbound" and messages flow from upstream to downstream. The terms "inbound" and
"outbound" are used to describe directional requirements in relation "outbound" are used to describe directional requirements in relation
to the request route: _inbound_ means toward the origin server and to the request route: inbound means "toward the origin server",
_outbound_ means toward the user agent. whereas outbound means "toward the user agent".
A _proxy_ is a message-forwarding agent that is chosen by the client, A "proxy" is a message-forwarding agent that is chosen by the client,
usually via local configuration rules, to receive requests for some usually via local configuration rules, to receive requests for some
type(s) of absolute URI and attempt to satisfy those requests via type(s) of absolute URI and attempt to satisfy those requests via
translation through the HTTP interface. Some translations are translation through the HTTP interface. Some translations are
minimal, such as for proxy requests for "http" URIs, whereas other minimal, such as for proxy requests for "http" URIs, whereas other
requests might require translation to and from entirely different requests might require translation to and from entirely different
application-level protocols. Proxies are often used to group an application-level protocols. Proxies are often used to group an
organization's HTTP requests through a common intermediary for the organization's HTTP requests through a common intermediary for the
sake of security services, annotation services, or shared caching. sake of security services, annotation services, or shared caching.
Some proxies are designed to apply transformations to selected Some proxies are designed to apply transformations to selected
messages or content while they are being forwarded, as described in messages or content while they are being forwarded, as described in
Section 7.7. Section 7.7.
A _gateway_ (a.k.a. _reverse proxy_) is an intermediary that acts as A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
an origin server for the outbound connection but translates received an origin server for the outbound connection but translates received
requests and forwards them inbound to another server or servers. requests and forwards them inbound to another server or servers.
Gateways are often used to encapsulate legacy or untrusted Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through information services, to improve server performance through
_accelerator_ caching, and to enable partitioning or load balancing "accelerator" caching, and to enable partitioning or load balancing
of HTTP services across multiple machines. of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server also apply to All HTTP requirements applicable to an origin server also apply to
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 needs 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, [TLS13]) is used to establish Transport Layer Security (TLS, [TLS13]) is used to establish
confidential communication through a shared firewall proxy. confidential communication through a shared firewall proxy.
The above categories for intermediary only consider those acting as The above categories for intermediary only consider those acting as
participants in the HTTP communication. There are also participants in the HTTP communication. There are also
intermediaries that can act on lower layers of the network protocol intermediaries that can act on lower layers of the network protocol
stack, filtering or redirecting HTTP traffic without the knowledge or stack, filtering or redirecting HTTP traffic without the knowledge or
permission of message senders. Network intermediaries are permission of message senders. Network intermediaries are
indistinguishable (at a protocol level) from an on-path attacker, indistinguishable (at a protocol level) from an on-path attacker,
often introducing security flaws or interoperability problems due to often introducing security flaws or interoperability problems due to
mistakenly violating HTTP semantics. mistakenly violating HTTP semantics.
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.
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
if one of the participants along the chain has a cached response if one of the participants along the chain has a cached response
applicable to that request. The following illustrates the resulting applicable to that request. The following illustrates the resulting
chain if B has a cached copy of an earlier response from O (via C) chain if B has a cached copy of an earlier response from O (via C)
for a request that has not been cached by UA or A. for a request that has not been cached by UA or A.
> > > >
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 [CACHING]. cache behavior and cacheable responses are defined in [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 optimize 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":
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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 | Section |
+------------+------------------------------------+-------+ +------------+------------------------------------+---------+
| 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.
4.2.1. http URI Scheme 4.2.1. http URI Scheme
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subcomponent is empty or not given, TCP port 80 (the reserved port subcomponent is empty or not given, TCP port 80 (the reserved port
for WWW services) is the default. The origin determines who has the for WWW services) is the default. The origin determines who has the
right to respond authoritatively to requests that target the right to respond authoritatively to requests that target the
identified resource, as defined in Section 4.3.2. identified resource, as defined in Section 4.3.2.
A sender MUST NOT generate an "http" URI with an empty host A sender MUST NOT generate an "http" URI with an empty host
identifier. A recipient that processes such a URI reference MUST identifier. A recipient that processes such a URI reference MUST
reject it as invalid. reject it as invalid.
The hierarchical path component and optional query component identify The hierarchical path component and optional query component identify
the target resource within that origin server's name space. the target resource within that origin server's namespace.
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 ([TLS13]) connection that has been secured for establishing a TLS ([TLS13]) 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
confidentiality and integrity protection that is acceptable to both confidentiality and integrity protection that is acceptable to both
client and server. 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 ([URI], Section 3.2.2) component, which includes a host identifier ([URI], Section 3.2.2)
and optional port number ([URI], Section 3.2.3). If the port and optional port number ([URI], Section 3.2.3). If the port
subcomponent is empty or not given, TCP port 443 (the reserved port subcomponent is empty or not given, TCP port 443 (the reserved port
for HTTP over TLS) is the default. The origin determines who has the for HTTP over TLS) is the default. The origin determines who has the
right to respond authoritatively to requests that target the right to respond authoritatively to requests that target the
identified resource, as defined in Section 4.3.3. identified resource, as 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 namespace.
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. Note that the definition of secured responses to those requests. Note that the definition of
what cryptographic mechanisms are acceptable to client and server are what cryptographic mechanisms are acceptable to client and server are
usually negotiated and can change over time. 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, extensions to HTTP that are defined as separate namespaces. However, extensions to HTTP that are defined as
applying to all origins with the same host, such as the Cookie applying to all origins with the same host, such as the Cookie
protocol [COOKIE], allow information set by one service to impact protocol [COOKIE], allow information set by one service to impact
communication with other services within a matching group of host communication with other services within a matching group of host
domains. Such extensions ought to be designed with great care to domains. Such extensions ought to be designed with great care to
prevent information obtained from a secured connection being prevent information obtained from a secured connection being
inadvertently exchanged within an unsecured context. inadvertently exchanged within an unsecured context.
4.2.3. http(s) Normalization and Comparison 4.2.3. http(s) Normalization and Comparison
The "http" and "https" URI are normalized and compared according to URIs with an "http" or "https" scheme are normalized and compared
the methods defined in Section 6 of [URI], using the defaults according to the methods defined in Section 6 of [URI], using the
described above for each scheme. defaults described above for each scheme.
HTTP does not require use of a specific method for determining HTTP does not require the use of a specific method for determining
equivalence. For example, a cache key might be compared as a simple equivalence. For example, a cache key might be compared as a simple
string, after syntax-based normalization, or after scheme-based string, after syntax-based normalization, or after scheme-based
normalization. normalization.
Scheme-based normalization (Section 6.2.3 of [URI]) of "http" and Scheme-based normalization (Section 6.2.3 of [URI]) of "http" and
"https" URIs involves the following additional rules: "https" URIs involves the following additional rules:
o If the port is equal to the default port for a scheme, the normal o If the port is equal to the default port for a scheme, the normal
form is to omit the port subcomponent. form is to omit the port subcomponent.
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Section 4.3.1 defines the concept of an origin as an aid to such Section 4.3.1 defines the concept of an origin as an aid to such
uses, and the subsequent subsections explain how to establish that a uses, and the subsequent subsections explain how to establish that a
peer has the authority to represent an origin. peer has the authority to represent an origin.
See Section 17.1 for security considerations related to establishing See Section 17.1 for security considerations related to establishing
authority. authority.
4.3.1. URI Origin 4.3.1. URI Origin
The _origin_ for a given URI is the triple of scheme, host, and port The "origin" for a given URI is the triple of scheme, host, and port
after normalizing the scheme and host to lowercase and normalizing after normalizing the scheme and host to lowercase and normalizing
the port to remove any leading zeros. If port is elided from the the port to remove any leading zeros. If port is elided from the
URI, the default port for that scheme is used. For example, the URI URI, the default port for that scheme is used. For example, the URI
https://Example.Com/happy.js https://Example.Com/happy.js
would have the origin would have the origin
{ "https", "example.com", "443" } { "https", "example.com", "443" }
which can also be described as the normalized URI prefix with port which can also be described as the normalized URI prefix with port
always present: always present:
https://example.com:443 https://example.com:443
Each origin defines its own namespace and controls how identifiers Each origin defines its own namespace and controls how identifiers
within that namespace are mapped to resources. In turn, how the within that namespace are mapped to resources. In turn, how the
origin responds to valid requests, consistently over time, determines origin responds to valid requests, consistently over time, determines
the semantics that users will associate with a URI, and the the semantics that users will associate with a URI, and the
usefulness of those semantics is what ultimately transforms these usefulness of those semantics is what ultimately transforms these
mechanisms into a "resource" for users to reference and access in the mechanisms into a resource for users to reference and access in the
future. future.
Two origins are distinct if they differ in scheme, host, or port. Two origins are distinct if they differ in scheme, host, or port.
Even when it can be verified that the same entity controls two Even when it can be verified that the same entity controls two
distinct origins, the two namespaces under those origins are distinct distinct origins, the two namespaces under those origins are distinct
unless explicitly aliased by a server authoritative for that origin. unless explicitly aliased by a server authoritative for that origin.
Origin is also used within HTML and related Web protocols, beyond the Origin is also used within HTML and related Web protocols, beyond the
scope of this document, as described in [RFC6454]. scope of this document, as described in [RFC6454].
4.3.2. http origins 4.3.2. http Origins
Although HTTP is independent of the transport protocol, the "http" Although HTTP is independent of the transport protocol, the "http"
scheme (Section 4.2.1) is specific to associating authority with scheme (Section 4.2.1) is specific to associating authority with
whomever controls the origin server listening for TCP connections on whomever controls the origin server listening for TCP connections on
the indicated port of whatever host is identified within the the indicated port of whatever host is identified within the
authority component. This is a very weak sense of authority because authority component. This is a very weak sense of authority because
it depends on both client-specific name resolution mechanisms and it depends on both client-specific name resolution mechanisms and
communication that might not be secured from an on-path attacker. communication that might not be secured from an on-path attacker.
Nevertheless, it is a sufficient minimum for binding "http" Nevertheless, it is a sufficient minimum for binding "http"
identifiers to an origin server for consistent resolution within a identifiers to an origin server for consistent resolution within a
skipping to change at page 29, line 17 skipping to change at page 29, line 17
considered an authoritative answer to the client's request. considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative authoritative response, nor does it imply that an authoritative
response is always necessary (see [CACHING]). For example, the Alt- response is always necessary (see [CACHING]). For example, the Alt-
Svc header field [ALTSVC] allows an origin server to identify other Svc header field [ALTSVC] allows an origin server to identify other
services that are also authoritative for that origin. Access to services that are also authoritative for that origin. Access to
"http" identified resources might also be provided by protocols "http" identified resources might also be provided by protocols
outside the scope of this document. outside the scope of this document.
4.3.3. https origins 4.3.3. https Origins
The "https" scheme (Section 4.2.2) associates authority based on the The "https" scheme (Section 4.2.2) associates authority based on the
ability of a server to use the private key corresponding to a ability of a server to use the private key corresponding to a
certificate that the client considers to be trustworthy for the certificate that the client considers to be trustworthy for the
identified origin server. The client usually relies upon a chain of identified origin server. The client usually relies upon a chain of
trust, conveyed from some prearranged or configured trust anchor, to trust, conveyed from some prearranged or configured trust anchor, to
deem a certificate trustworthy (Section 4.3.4). deem a certificate trustworthy (Section 4.3.4).
In HTTP/1.1 and earlier, a client will only attribute authority to a In HTTP/1.1 and earlier, a client will only attribute authority to a
server when they are communicating over a successfully established server when they are communicating over a successfully established
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client will make a DNS query to check that the origin's host contains client will make a DNS query to check that the origin's host contains
the same server IP address as the established connection. This the same server IP address as the established connection. This
restriction can be removed by the origin server sending an equivalent restriction can be removed by the origin server sending an equivalent
ORIGIN frame [RFC8336]. ORIGIN frame [RFC8336].
The request target's host and port value are passed within each HTTP The request target's host and port value are passed within each HTTP
request, identifying the origin and distinguishing it from other request, identifying the origin and distinguishing it from other
namespaces that might be controlled by the same server (Section 7.2). namespaces that might be controlled by the same server (Section 7.2).
It is the origin's responsibility to ensure that any services It is the origin's responsibility to ensure that any services
provided with control over its certificate's private key are equally provided with control over its certificate's private key are equally
responsible for managing the corresponding "https" namespaces, or at responsible for managing the corresponding "https" namespaces or at
least prepared to reject requests that appear to have been least prepared to reject requests that appear to have been
misdirected (Section 7.4). misdirected (Section 7.4).
An origin server might be unwilling to process requests for certain An origin server might be unwilling to process requests for certain
target URIs even when they have the authority to do so. For example, target URIs even when they have the authority to do so. For example,
when a host operates distinct services on different ports (e.g., 443 when a host operates distinct services on different ports (e.g., 443
and 8000), checking the target URI at the origin server is necessary and 8000), checking the target URI at the origin server is necessary
(even after the connection has been secured) because a network (even after the connection has been secured) because a network
attacker might cause connections for one port to be received at some attacker might cause connections for one port to be received at some
other port. Failing to check the target URI might allow such an other port. Failing to check the target URI might allow such an
skipping to change at page 30, line 40 skipping to change at page 30, line 40
target URI (Section 7.1). target URI (Section 7.1).
If the server responds to such a request with a non-interim HTTP If the server responds to such a request with a non-interim HTTP
response message, as described in Section 15, then that response is response message, as described in Section 15, then that response is
considered an authoritative answer to the client's request. considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative authoritative response, nor does it imply that an authoritative
response is always necessary (see [CACHING]). response is always necessary (see [CACHING]).
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]. The client verification process defined in Section 6 of [RFC6125]. The client
MUST construct a reference identity from the service's host: if the MUST construct a reference identity from the service's host: if the
host is a literal IP address (Section 4.3.5), the reference identity host is a literal IP address (Section 4.3.5), the reference identity
is an IP-ID, otherwise the host is a name and the reference identity is an IP-ID, otherwise the host is a name and the reference identity
is a DNS-ID. is a DNS-ID.
A reference identity of type CN-ID MUST NOT be used by clients. As A reference identity of type CN-ID MUST NOT be used by clients. As
noted in Section 6.2.1 of [RFC6125] a reference identity of type CN- noted in Section 6.2.1 of [RFC6125], a reference identity of type CN-
ID might be used by older clients. ID might be used by older clients.
A client might be specially configured to accept an alternative form A client might be specially configured to accept an alternative form
of server identity verification. For example, a client might be of server identity verification. For example, a client might be
connecting to a server whose address and hostname are dynamic, with connecting to a server whose address and hostname are dynamic, with
an expectation that the service will present a specific certificate an expectation that the service will present a specific certificate
(or a certificate matching some externally defined reference (or a certificate matching some externally defined reference
identity) rather than one matching the target URI's origin. identity) rather than one matching the target URI's origin.
In special cases, it might be appropriate for a client to simply In special cases, it might be appropriate for a client to simply
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If the certificate is not valid for the target URI's origin, a user If the certificate is not valid for the target URI's origin, a user
agent MUST either obtain confirmation from the user before proceeding agent MUST either obtain confirmation from the user before proceeding
(see Section 3.5) or terminate the connection with a bad certificate (see Section 3.5) or terminate the connection with a bad certificate
error. Automated clients MUST log the error to an appropriate audit error. Automated clients MUST log the error to an appropriate audit
log (if available) and SHOULD terminate the connection (with a bad log (if available) and SHOULD terminate the connection (with a bad
certificate error). Automated clients MAY provide a configuration certificate error). Automated clients MAY provide a configuration
setting that disables this check, but MUST provide a setting which setting that disables this check, but MUST provide a setting which
enables it. enables it.
4.3.5. IP-ID reference identity 4.3.5. IP-ID Reference Identity
A server that is identified using an IP address literal in the "host" 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 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 IP version 4 address uses the "IPv4address" ABNF rule, and an IP
version 6 address uses the "IP-literal" production with the version 6 address uses the "IP-literal" production with the
"IPv6address" option; see Section 3.2.2 of [URI]. A reference "IPv6address" option; see Section 3.2.2 of [URI]. A reference
identity of IP-ID contains the decoded bytes of the IP address. 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 An IP version 4 address is 4 octets, and an IP version 6 address is
octets. Use of IP-ID is not defined for any other IP version. The 16 octets. Use of IP-ID is not defined for any other IP version.
iPAddress choice in the certificate subjectAltName extension does not The iPAddress choice in the certificate subjectAltName extension does
explicitly include the IP version and so relies on the length of the not explicitly include the IP version and so relies on the length of
address to distinguish versions; see Section 4.2.1.6 of [RFC5280]. 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 A reference identity of type IP-ID matches if the address is
identical to an iPAddress value of the subjectAltName extension of identical to an iPAddress value of the subjectAltName extension of
the certificate. 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 name/
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
semantics defined by that name. For example, the Date header field semantics defined by that name. For example, the Date header field
is defined in Section 6.6.1 as containing the origination timestamp is defined in Section 6.6.1 as containing the origination timestamp
for the message in which it appears. for the message in which it appears.
skipping to change at page 33, line 7 skipping to change at page 33, line 7
A proxy MUST forward unrecognized header fields unless the field name A proxy MUST forward unrecognized header fields unless the field name
is listed in the Connection header field (Section 7.6.1) or the proxy is listed in the Connection header field (Section 7.6.1) or the proxy
is specifically configured to block, or otherwise transform, such is specifically configured to block, or otherwise transform, such
fields. Other recipients SHOULD ignore unrecognized header and fields. Other recipients SHOULD ignore unrecognized header and
trailer fields. Adhering to these requirements allows HTTP's trailer fields. Adhering to these requirements allows HTTP's
functionality to be extended without updating or removing deployed functionality to be extended without updating or removing deployed
intermediaries. intermediaries.
5.2. Field Lines and Combined Field Value 5.2. Field Lines and Combined Field Value
Field sections are composed of any number of _field lines_, each with Field sections are composed of any number of "field lines", each with
a _field name_ (see Section 5.1) identifying the field, and a _field a "field name" (see Section 5.1) identifying the field, and a "field
line value_ that conveys data for that instance of the field. line value" that conveys data for that instance of the field.
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 field line within that section, concatenated in order, with each field line
value separated by a comma. 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
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(",") and optional whitespace (OWS, defined in Section 5.6.3). For (",") and optional whitespace (OWS, defined in Section 5.6.3). For
consistency, use comma SP. consistency, use comma SP.
The order in which field lines with the same name are received is The order in which field lines with the same name are received is
therefore significant to the interpretation of the field value; a therefore significant to the interpretation of the field value; a
proxy MUST NOT change the order of these field line values when proxy MUST NOT change the order of these field line values when
forwarding a message. forwarding a message.
This means that, aside from the well-known exception noted below, a This means that, aside from the well-known exception noted below, a
sender MUST NOT generate multiple field lines with the same name in a sender MUST NOT generate multiple field lines with the same name in a
message (whether in the headers or trailers), or append a field line message (whether in the headers or trailers) or append a field line
when a field line of the same name already exists in the message, when a field line of the same name already exists in the message,
unless that field's definition allows multiple field line values to unless that field's definition allows multiple field line values to
be recombined as a comma-separated list [i.e., at least one be recombined as a comma-separated list (i.e., at least one
alternative of the field's definition allows a comma-separated list, alternative of the field's definition allows a comma-separated list,
such as an ABNF rule of #(values) defined in Section 5.6.1]. such as an ABNF rule of #(values) defined in Section 5.6.1).
| *Note:* In practice, the "Set-Cookie" header field ([COOKIE]) | *Note:* In practice, the "Set-Cookie" header field ([COOKIE])
| often appears in a response message across multiple field lines | often appears in a response message across multiple field lines
| and does not use the list syntax, violating the above | and does not use the list syntax, violating the above
| requirements on multiple field lines with the same field name. | requirements on multiple field lines with the same field name.
| Since it cannot be combined into a single field value, | Since it cannot be combined into a single field value,
| recipients ought to handle "Set-Cookie" as a special case while | recipients ought to handle "Set-Cookie" as a special case while
| processing fields. (See Appendix A.2.3 of [Kri2001] for | processing fields. (See Appendix A.2.3 of [Kri2001] for
| details.) | details.)
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and interpret those characters; a recipient of CR, LF, or NUL within and interpret those characters; a recipient of CR, LF, or NUL within
a field value MUST either reject the message or replace each of those a field value MUST either reject the message or replace each of those
characters with SP before further processing or forwarding of that characters with SP before further processing or forwarding of that
message. Field values containing other CTL characters are also message. Field values containing other CTL characters are also
invalid; however, recipients MAY retain such characters for the sake invalid; however, recipients MAY retain such characters for the sake
of robustness when they appear within a safe context (e.g., an of robustness when they appear within a safe context (e.g., an
application-specific quoted string that will not be processed by any application-specific quoted string that will not be processed by any
downstream HTTP parser). downstream HTTP parser).
Fields that only anticipate a single member as the field value are Fields that only anticipate a single member as the field value are
referred to as _singleton fields_. referred to as "singleton fields".
Fields that allow multiple members as the field value are referred to Fields that allow multiple members as the field value are referred to
as _list-based fields_. The list operator extension of Section 5.6.1 as "list-based fields". The list operator extension of Section 5.6.1
is used as a common notation for defining field values that can is used as a common notation for defining field values that can
contain multiple members. contain multiple members.
Because commas (",") are used as the 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 erroneously sent with multiple members and a singleton field might be erroneously sent with multiple members and
detecting such errors improves interoperability. Fields that expect detecting such errors improves interoperability. Fields that expect
to contain a comma within a member, such as within an HTTP-date or to contain a comma within a member, such as within an HTTP-date or
URI-reference element, ought to be defined with delimiters around URI-reference element, ought to be defined with delimiters around
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Comments can be included in some HTTP fields by surrounding the Comments can be included in some HTTP fields by surrounding the
comment text with parentheses. Comments are only allowed in fields comment text with parentheses. Comments are only allowed in fields
containing "comment" as part of their field value definition. containing "comment" as part of their field value definition.
comment = "(" *( ctext / quoted-pair / comment ) ")" comment = "(" *( ctext / quoted-pair / comment ) ")"
ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text
5.6.6. Parameters 5.6.6. Parameters
Parameters are instances of name=value pairs; they are often used in Parameters are instances of name/value pairs; they are often used in
field values as a common syntax for appending auxiliary information field values as a common syntax for appending auxiliary information
to an item. Each parameter is usually delimited by an immediately to an item. Each parameter is usually delimited by an immediately
preceding semicolon. preceding semicolon.
parameters = *( OWS ";" OWS [ parameter ] ) parameters = *( OWS ";" OWS [ parameter ] )
parameter = parameter-name "=" parameter-value parameter = parameter-name "=" parameter-value
parameter-name = token parameter-name = token
parameter-value = ( token / quoted-string ) parameter-value = ( token / quoted-string )
Parameter names are case-insensitive. Parameter values might or Parameter names are case-insensitive. Parameter values might or
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accept all three HTTP-date formats. When a sender generates a field accept all three HTTP-date formats. When a sender generates a field
that contains one or more timestamps defined as HTTP-date, the sender that contains one or more timestamps defined as HTTP-date, the sender
MUST generate those timestamps in the IMF-fixdate format. MUST generate those timestamps in the IMF-fixdate format.
An HTTP-date value represents time as an instance of Coordinated An HTTP-date value represents time as an instance of Coordinated
Universal Time (UTC). The first two formats indicate UTC by the Universal Time (UTC). The first two formats indicate UTC by the
three-letter abbreviation for Greenwich Mean Time, "GMT", a three-letter abbreviation for Greenwich Mean Time, "GMT", a
predecessor of the UTC name; values in the asctime format are assumed predecessor of the UTC name; values in the asctime format are assumed
to be in UTC. to be in UTC.
A _clock_ is an implementation capable of providing a reasonable A "clock" is an implementation capable of providing a reasonable
approximation of the current instant in UTC. A clock implementation approximation of the current instant in UTC. A clock implementation
ought to use NTP ([RFC5905]), or some similar protocol, to ought to use NTP ([RFC5905]), or some similar protocol, to
synchronize with UTC. synchronize with UTC.
Preferred format: Preferred format:
IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
; fixed length/zone/capitalization subset of the format ; fixed length/zone/capitalization subset of the format
; see Section 3.3 of [RFC5322] ; see Section 3.3 of [RFC5322]
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two-digit year, MUST interpret a timestamp that appears to be more two-digit year, MUST interpret a timestamp that appears to be more
than 50 years in the future as representing the most recent year in than 50 years in the future as representing the most recent year in
the past that had the same last two digits. the past that had the same last two digits.
Recipients of timestamp values are encouraged to be robust in parsing Recipients of timestamp values are encouraged to be robust in parsing
timestamps unless otherwise restricted by the field definition. For timestamps unless otherwise restricted by the field definition. For
example, messages are occasionally forwarded over HTTP from a non- example, messages are occasionally forwarded over HTTP from a non-
HTTP source that might generate any of the date and time HTTP source that might generate any of the date and time
specifications defined by the Internet Message Format. specifications defined by the Internet Message Format.
| *Note:* HTTP requirements for the date/time stamp format apply | *Note:* HTTP requirements for timestamp formats apply only to
| only to their usage within the protocol stream. | their usage within the protocol stream. Implementations are
| Implementations are not required to use these formats for user | not required to use these formats for user presentation,
| presentation, request logging, etc. | request logging, etc.
6. Message Abstraction 6. Message Abstraction
Each major version of HTTP defines its own syntax for communicating Each major version of HTTP defines its own syntax for communicating
messages. This section defines an abstract data type for HTTP messages. This section defines an abstract data type for HTTP
messages based on a generalization of those message characteristics, messages based on a generalization of those message characteristics,
common structure, and capacity for conveying semantics. This common structure, and capacity for conveying semantics. This
abstraction is used to define requirements on senders and recipients abstraction is used to define requirements on senders and recipients
that are independent of the HTTP version, such that a message in one that are independent of the HTTP version, such that a message in one
version can be relayed through other versions without changing its version can be relayed through other versions without changing its
meaning. meaning.
A _message_ consists of control data to describe and route the A "message" consists of the following:
message, a headers lookup table of key/value pairs for extending that
control data and conveying additional information about the sender, o control data to describe and route the message,
message, content, or context, a potentially unbounded stream of
content, and a trailers lookup table of key/value pairs for o a headers lookup table of name/value pairs for extending that
communicating information obtained while sending the content. control data and conveying additional information about the
sender, message, content, or context,
o a potentially unbounded stream of content, and
o a trailers lookup table of name/value pairs for 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, potentially content, the content is sent after the header section, potentially
followed by a trailer section that might contain fields for the followed by a trailer section that might contain fields for 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 optional metadata that was semantics, and trailer fields provide optional metadata that was
unknown prior to sending the content. 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). However, a client MUST retain knowledge of the via prior messages). However, a client MUST retain knowledge of the
request when parsing, interpreting, or caching a corresponding request when parsing, interpreting, or caching a corresponding
response. For example, responses to the HEAD method look just like 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 the beginning of a response to GET but cannot be parsed in the same
manner. 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 trailer section after the content. An chunked transfer coding as a trailer section after the content. An
equivalent feature is present in HTTP/2 and HTTP/3 within the header equivalent feature is present in HTTP/2 and HTTP/3 within the header
block that terminates each stream. block that terminates each stream.
6.1. Framing and Completeness 6.1. Framing and Completeness
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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
compatibility, this implicit framing is also allowed in HTTP/1.1. compatibility, this implicit framing is also allowed in HTTP/1.1.
However, implicit framing can fail to distinguish an incomplete However, implicit framing can fail to distinguish an incomplete
response if the connection closes early. For that reason, almost all response if the connection closes early. For that reason, almost all
modern implementations use explicit framing in the form of length- modern implementations use explicit framing in the form of length-
delimited sequences of message data. delimited sequences of message data.
A message is considered _complete_ when all of the octets indicated A message is considered "complete" when all of the octets indicated
by its framing are available. Note that, when no explicit framing is by its framing are available. Note that, when no explicit framing is
used, a response message that is ended by the underlying connection's used, a response message that is ended by the underlying connection's
close is considered complete even though it might be close is considered complete even though it might be
indistinguishable from an incomplete response, unless a transport- indistinguishable from an incomplete response, unless a transport-
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),
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implements SHOULD process the message as if it were in the highest implements SHOULD process the message as if it were in the highest
minor version within that major version to which the recipient is minor version within that major version to which the recipient is
conformant. A recipient can assume that a message with a higher conformant. A recipient can assume that a message with a higher
minor version, when sent to a recipient that has not yet indicated minor version, when sent to a recipient that has not yet indicated
support for that higher version, is sufficiently backwards-compatible support for that higher version, is sufficiently backwards-compatible
to be safely processed by any implementation of the same major to be safely processed by any 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 or 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 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
HTTP messages often transfer a complete or partial representation as HTTP messages often transfer a complete or partial representation as
the message _content_: a stream of octets sent after the header the message "content": a stream of octets sent after the header
section, as delineated by the message framing. section, as delineated by the message framing.
This abstract definition of content reflects the data after it has This abstract definition of content reflects the data after it has
been extracted from the message framing. For example, an HTTP/1.1 been extracted from the message framing. For example, an HTTP/1.1
message body (Section 6 of [HTTP/1.1]) might consist of a stream of message body (Section 6 of [HTTP/1.1]) might consist of a stream of
data encoded with the chunked transfer coding -- a sequence of data data encoded with the chunked transfer coding -- a sequence of data
chunks, one zero-length chunk, and a trailer section -- whereas the chunks, one zero-length chunk, and a trailer section -- whereas the
content of that same message includes only the data stream after the content of that same message includes only the data stream after the
transfer coding has been decoded; it does not include the chunk transfer coding has been decoded; it does not include the chunk
lengths, chunked framing syntax, nor the trailer fields lengths, chunked framing syntax, nor the trailer fields
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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 by HTTP, but a more 7. Otherwise, the content is unidentified by HTTP, but a more
specific identifier might be supplied within the content itself. specific identifier might be supplied within the content itself.
6.5. Trailer Fields 6.5. Trailer Fields
Fields (Section 5) that are located within a _trailer section_ are Fields (Section 5) that are located within a "trailer section" are
referred to as "trailer fields" (or just "trailers", colloquially). referred to as "trailer fields" (or just "trailers", colloquially).
Trailer fields can be useful for supplying message integrity checks, Trailer fields can be useful for supplying message integrity checks,
digital signatures, delivery metrics, or post-processing status digital signatures, delivery metrics, or post-processing status
information. 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
A trailer section is 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 transfer coding in HTTP/1.1 allows a trailer
be sent after the content (Section 7.1.2 of [HTTP/1.1]). section to be sent after the content (Section 7.1.2 of [HTTP/1.1]).
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
trailers. trailers.
Trailer fields can be difficult to process by intermediaries that Trailer fields can be difficult to process by intermediaries that
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field value. 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. Trailer fields that might be generated more as a list of members. Trailer fields that might be generated more
than once during a message MUST be defined as a list-based field even than once during a message MUST be defined as a list-based field even
if each member value is only processed once per field line received. if each member value is only processed once per field line received.
At the end of a message, a recipient MAY treat the set of received At the end of a message, a recipient MAY treat the set of received
trailer fields as a data structure of key/value pairs, similar to trailer fields as a data structure of name/value pairs, similar to
(but separate from) the header fields. Additional processing (but separate from) the header fields. Additional processing
expectations, if any, can be defined within the field specification expectations, if any, can be defined within the field specification
for a field intended for use in trailers. for a field intended for use in trailers.
6.6. Message Metadata 6.6. Message Metadata
Fields that describe the message itself, such as when and how the Fields that describe the message itself, such as when and how the
message has been generated, can appear in both requests and message has been generated, can appear in both requests and
responses. responses.
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7.1. Determining the Target Resource 7.1. Determining the Target Resource
Although HTTP is used in a wide variety of applications, most clients Although HTTP is used in a wide variety of applications, most clients
rely on the same resource identification mechanism and configuration rely on the same resource identification mechanism and configuration
techniques as general-purpose Web browsers. Even when communication techniques as general-purpose Web browsers. Even when communication
options are hard-coded in a client's configuration, we can think of options are hard-coded in a client's configuration, we can think of
their combined effect as a URI reference (Section 4.1). their combined effect as a URI reference (Section 4.1).
A URI reference is resolved to its absolute form in order to obtain A URI reference is resolved to its absolute form in order to obtain
the _target URI_. The target URI excludes the reference's fragment the "target URI". The target URI excludes the reference's fragment
component, if any, since fragment identifiers are reserved for component, if any, since fragment identifiers are reserved for
client-side processing ([URI], Section 3.5). client-side processing ([URI], Section 3.5).
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 o 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 o For OPTIONS (Section 9.3.7), the request target can be a single
asterisk ("*"). asterisk ("*").
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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, Section 3.3 is specific to each major protocol version. For example, Section 3.3
of [HTTP/1.1] defines how a server determines the target URI of an of [HTTP/1.1] defines how a server determines the target URI of an
HTTP/1.1 request. HTTP/1.1 request.
| *Note:* Previous specifications defined the recomposed target | *Note:* Previous specifications defined the recomposed target
| URI as a distinct concept, the _effective request URI_. | URI as a distinct concept, the "effective request URI".
7.2. Host and :authority 7.2. Host and :authority
The "Host" header field in a request provides the host and port The "Host" header field in a request provides the host and port
information from the target URI, enabling the origin server to information from the target URI, enabling the origin server to
distinguish among resources while servicing requests for multiple distinguish among resources while servicing requests for multiple
host names. host names.
In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], the Host header field is, in In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], 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
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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.
All responses, regardless of the status code (including interim All responses, regardless of the status code (including interim
responses) can be sent at any time after a request is received, even responses) can be sent at any time after a request is received, even
if the request is not yet complete. A response can complete before if the request is not yet complete. A response can complete before
its corresponding request is complete (Section 6.1). Likewise, its corresponding request is complete (Section 6.1). Likewise,
clients are not expected to wait any specific amount of time for a clients are not expected to wait any specific amount of time for a
response. Clients (including intermediaries) might abandon a request response. Clients (including intermediaries) might abandon a request
if the response is not forthcoming within a reasonable period of if the response is not received within a reasonable period of time.
time.
A client that receives a response while it is still sending the A client that receives a response while it is still sending the
associated request SHOULD continue sending that request, unless it associated request SHOULD continue sending that request unless it
receives an explicit indication to the contrary (see, e.g., receives an explicit indication to the contrary (see, e.g.,
Section 9.5 of [HTTP/1.1] and Section 6.4 of [HTTP/2]). Section 9.5 of [HTTP/1.1] and Section 6.4 of [HTTP/2]).
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 Intermediaries are expected to forward messages even when protocol
elements are not recognized (e.g., new methods, status codes, or elements are not recognized (e.g., new methods, status codes, or
field names), since that preserves extensibility for downstream field names) since that preserves extensibility for downstream
recipients. 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
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or forwarding downstream. However, senders and recipients cannot or forwarding downstream. However, senders and recipients cannot
rely on incremental delivery of partial messages, since some rely on incremental delivery of partial messages, since some
implementations will buffer or delay message forwarding for the sake implementations will buffer or delay message forwarding for the sake
of network efficiency, security checks, or content transformations. of network efficiency, security checks, or content transformations.
7.6.1. Connection 7.6.1. Connection
The "Connection" header field allows the sender to list desired The "Connection" header field allows the sender to list desired
control options for the current connection. control options for the current connection.
Connection = #connection-option
connection-option = token
Connection options are case-insensitive.
When a field aside from Connection is used to supply control When a field aside from Connection is used to supply control
information for or about the current connection, the sender MUST list information for or about the current connection, the sender MUST list
the corresponding field name within the Connection header field. the corresponding field name within the Connection header field.
Note that some versions of HTTP prohibit the use of fields for such Note that some versions of HTTP prohibit the use of fields for such
information, and therefore do not allow the Connection field. information, and therefore do not allow the Connection field.
Intermediaries MUST parse a received Connection header field before a Intermediaries MUST parse a received Connection header field before a
message is forwarded and, for each connection-option in this field, message is forwarded and, for each connection-option in this field,
remove any header or trailer field(s) from the message with the same remove any header or trailer field(s) from the message with the same
name as the connection-option, and then remove the Connection header name as the connection-option, and then remove the Connection header
field itself (or replace it with the intermediary's own connection field itself (or replace it with the intermediary's own control
options for the forwarded message). options for the forwarded message).
Hence, the Connection header field provides a declarative way of Hence, the Connection header field provides a declarative way of
distinguishing fields that are only intended for the immediate distinguishing fields that are only intended for the immediate
recipient ("hop-by-hop") from those fields that are intended for all recipient ("hop-by-hop") from those fields that are intended for all
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 fields that are
semantics are known to require removal before forwarding, whether or known to require removal before forwarding, whether or not they
not they appear as a Connection option, after applying those fields' 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 [HTTP/1.1]) o Proxy-Connection (Appendix C.2.2 of [HTTP/1.1])
o Keep-Alive (Section 19.7.1 of [RFC2068]) o Keep-Alive (Section 19.7.1 of [RFC2068])
o TE (Section 10.1.4) o TE (Section 10.1.4)
o Transfer-Encoding (Section 6.1 of [HTTP/1.1]) o Transfer-Encoding (Section 6.1 of [HTTP/1.1])
o Upgrade (Section 7.8) o Upgrade (Section 7.8)
The Connection header field's value has the following grammar:
Connection = #connection-option
connection-option = token
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]).
Connection options do not always correspond to a field present in the Connection options do not always correspond to a field present in 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 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 a new connection option that does not correspond to a When defining a new connection option that does not correspond to a
field, specification authors ought to reserve the corresponding field field, specification authors ought to reserve the corresponding field
name anyway in order to avoid later collisions. Such reserved field name anyway in order to avoid later collisions. Such reserved field
names are registered in the Hypertext Transfer Protocol (HTTP) Field names are registered in the "Hypertext Transfer Protocol (HTTP) Field
Name Registry (Section 16.3.1). 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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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
formats in order to save cache space or to reduce the amount of formats in order to save cache space or to reduce the amount of
traffic on a slow link. However, operational problems might occur traffic on a slow link. However, operational problems might occur
when these transformations are applied to content intended for when these transformations are applied to content intended for
critical applications, such as medical imaging or scientific data critical applications, such as medical imaging or scientific data
analysis, particularly when integrity checks or digital signatures analysis, particularly when integrity checks or digital signatures
are used to ensure that the content received is identical to the are used to ensure that the content received is identical to the
original. original.
An HTTP-to-HTTP proxy is called a _transforming proxy_ if it is An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
designed or configured to modify messages in a semantically designed or configured to modify messages in a semantically
meaningful way (i.e., modifications, beyond those required by normal meaningful way (i.e., modifications, beyond those required by normal
HTTP processing, that change the message in a way that would be HTTP processing, that change the message in a way that would be
significant to the original sender or potentially significant to significant to the original sender or potentially significant to
downstream recipients). For example, a transforming proxy might be downstream recipients). For example, a transforming proxy might be
acting as a shared annotation server (modifying responses to include acting as a shared annotation server (modifying responses to include
references to a local annotation database), a malware filter, a references to a local annotation database), a malware filter, a
format transcoder, or a privacy filter. Such transformations are format transcoder, or a privacy filter. Such transformations are
presumed to be desired by whichever client (or client organization) presumed to be desired by whichever client (or client organization)
chose the proxy. chose the proxy.
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received when forwarding the request. A proxy MUST NOT change the received when forwarding the request. A proxy MUST NOT change the
host name if the target URI contains a fully qualified domain name. host name if the target URI contains a fully qualified domain name.
A proxy MUST NOT modify the "absolute-path" and "query" parts of the A proxy MUST NOT modify the "absolute-path" and "query" parts of the
received target URI when forwarding it to the next inbound server received target URI when forwarding it to the next inbound server
except as required by that forwarding protocol. For example, a proxy except as required by that forwarding protocol. For example, a proxy
forwarding a request to an origin server via HTTP/1.1 will replace an forwarding a request to an origin server via HTTP/1.1 will replace an
empty path with "/" (Section 3.2.1 of [HTTP/1.1]) or "*" empty path with "/" (Section 3.2.1 of [HTTP/1.1]) or "*"
(Section 3.2.4 of [HTTP/1.1]), depending on the request method. (Section 3.2.4 of [HTTP/1.1]), depending on the request method.
A proxy MUST NOT transform the content (Section 6.4) of a message A proxy MUST NOT transform the content (Section 6.4) of a response
that contains a no-transform cache-control response directive message that contains a no-transform cache directive (Section 5.2.2.6
(Section 5.2 of [CACHING]). Note that this does not include changes of [CACHING]). Note that this does not apply to message
to the message body that do not affect the content, such as transfer transformations that do not change the content, such as the addition
codings (Section 7 of [HTTP/1.1]). or removal of transfer codings (Section 7 of [HTTP/1.1]).
A proxy MAY transform the content of a message that does not contain A proxy MAY transform the content of a message that does not contain
a no-transform cache-control directive. A proxy that transforms the a no-transform cache directive. A proxy that transforms the content
content of a 200 (OK) response can inform downstream recipients that of a 200 (OK) response can inform downstream recipients that a
a transformation has been applied by changing the response status transformation has been applied by changing the response status code
code to 203 (Non-Authoritative Information) (Section 15.3.4). to 203 (Non-Authoritative Information) (Section 15.3.4).
A proxy SHOULD NOT modify header fields that provide information A proxy SHOULD NOT modify header fields that provide information
about the endpoints of the communication chain, the resource state, about the endpoints of the communication chain, the resource state,
or the selected representation (other than the content) unless the or the selected representation (other than the content) unless the
field's definition specifically allows such modification or the field's definition specifically allows such modification or the
modification is deemed necessary for privacy or security. modification is deemed necessary for privacy or security.
7.8. Upgrade 7.8. Upgrade
The "Upgrade" header field is intended to provide a simple mechanism The "Upgrade" header field is intended to provide a simple mechanism
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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 the message context. 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
media type, depending on its definition within the media type media type, depending on its definition within the media type
registry. Parameter values might or might not be case-sensitive, registry. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. depending on the semantics of the parameter name.
For example, the following media types are equivalent in describing For example, the following media types are equivalent in describing
HTML text data encoded in the UTF-8 character encoding scheme, but HTML text data encoded in the UTF-8 character encoding scheme, but
the first is preferred for consistency (the "charset" parameter value the first is preferred for consistency (the "charset" parameter value
is defined as being case-insensitive in [RFC2046], Section 4.1.2): is defined as being case-insensitive in [RFC2046], Section 4.1.2):
text/html;charset=utf-8 text/html;charset=utf-8
Text/HTML;Charset="utf-8" Text/HTML;Charset="utf-8"
text/html; charset="utf-8" text/html; charset="utf-8"
text/html;charset=UTF-8 text/html;charset=UTF-8
Media types ought to be registered with IANA according to the Media types ought to be registered with IANA according to the
procedures defined in [BCP13]. procedures defined in [BCP13].
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 ([RFC6365], Section 1.3) of a textual representation. encoding scheme ([RFC6365], Section 2) of a textual representation.
In the fields defined by this document, charset names appear either In the fields defined by this document, charset names appear either
in parameters (Content-Type), or, for Accept-Encoding, in the form of in parameters (Content-Type), or, for Accept-Encoding, in the form of
a plain token. In both cases, charset names are matched case- a plain token. In both cases, charset names are matched case-
insensitively. 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-
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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.
Unlike Transfer-Encoding (Section 6.1 of [HTTP/1.1]), the codings Unlike Transfer-Encoding (Section 6.1 of [HTTP/1.1]), the codings
listed in Content-Encoding are a characteristic of the listed in Content-Encoding are a characteristic of the
representation; the representation is defined in terms of the coded representation; the representation is defined in terms of the coded
form, and all other metadata about the representation is about the form, and all other metadata about the representation is about the
coded form unless otherwise noted in the metadata definition. coded form unless otherwise noted in the metadata definition.
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8.6. Content-Length 8.6. Content-Length
The "Content-Length" header field indicates the associated The "Content-Length" header field indicates the associated
representation's data length as a decimal non-negative integer number representation's data length as a decimal non-negative integer number
of octets. When transferring a representation as content, Content- of octets. When transferring a representation as content, Content-
Length refers specifically to the amount of data enclosed so that it Length refers specifically to the amount of data enclosed so that it
can be used to delimit framing (e.g., Section 6.2 of [HTTP/1.1]). In can be used to delimit framing (e.g., Section 6.2 of [HTTP/1.1]). In
other cases, Content-Length indicates the selected representation's other cases, Content-Length indicates the selected representation's
current length, which can be used by recipients to estimate transfer current length, which can be used by recipients to estimate transfer
time or compare to previously stored representations. time or to compare with previously stored representations.
Content-Length = 1*DIGIT Content-Length = 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
A user agent SHOULD send Content-Length in a request when the method A user agent SHOULD send Content-Length in a request when the method
defines a meaning for enclosed content and it is not sending defines a meaning for enclosed content and it is not sending
Transfer-Encoding. For example, a user agent normally sends Content- Transfer-Encoding. For example, a user agent normally sends Content-
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If the message is forwarded by a downstream intermediary, a Content- If the message is forwarded by a downstream intermediary, a Content-
Length field value that is inconsistent with the received message Length field value that is inconsistent with the received message
framing might cause a security failure due to request smuggling or framing might cause a security failure due to request smuggling or
response splitting. response splitting.
As a result, a sender MUST NOT forward a message with a Content- As a result, a sender MUST NOT forward a message with a Content-
Length header field value that is known to be incorrect. Length header field value that is known to be incorrect.
Likewise, a sender MUST NOT forward a message with a Content-Length Likewise, a sender MUST NOT forward a message with a Content-Length
header field value that does not match the ABNF above, with one header field value that does not match the ABNF above, with one
exception: A recipient of a Content-Length header field value exception: a recipient of a Content-Length header field value
consisting of the same decimal value repeated as a comma-separated consisting of the same decimal value repeated as a comma-separated
list (e.g, "Content-Length: 42, 42"), MAY either reject the message list (e.g, "Content-Length: 42, 42") MAY either reject the message as
as invalid or replace that invalid field value with a single instance invalid or replace that invalid field value with a single instance of
of the decimal value, since this likely indicates that a duplicate the decimal value, since this likely indicates that a duplicate was
was generated or combined by an upstream message processor. 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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and the origin server accepts that PUT (without redirection), then and the origin server accepts that PUT (without redirection), then
the new state of that resource is expected to be consistent with the the new state of that resource is expected to be consistent with the
one representation supplied in that PUT; the Content-Location cannot one representation supplied in that PUT; the Content-Location cannot
be used as a form of reverse content selection identifier to update be used as a form of reverse content selection identifier to update
only one of the negotiated representations. If the user agent had only one of the negotiated representations. If the user agent had
wanted the latter semantics, it would have applied the PUT directly wanted the latter semantics, it would have applied the PUT directly
to the Content-Location URI. to the Content-Location URI.
8.8. Validator Fields 8.8. Validator Fields
Resource metadata is referred to as a _validator_ if it can be used Resource metadata is referred to as a "validator" if it can be used
within a precondition (Section 13.1) to make a conditional request within a precondition (Section 13.1) to make a conditional request
(Section 13). Validator fields convey a current validator for the (Section 13). Validator fields convey a current validator for the
selected representation (Section 3.2). selected representation (Section 3.2).
In responses to safe requests, validator fields describe the selected In responses to safe requests, validator fields describe the selected
representation chosen by the origin server while handling the representation chosen by the origin server while handling the
response. Note that, depending on the method and status code response. Note that, depending on the method and status code
semantics, the selected representation for a given response is not semantics, the selected representation for a given response is not
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 the entity-tag can be used as a validator in later conditional that the entity tag can be used as a validator in later conditional
requests to prevent the "lost update" problem. requests to prevent the "lost update" problem.
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], that are beyond the Distributed Authoring and Versioning [WEBDAV], that are beyond the
scope of this specification. scope of this specification.
8.8.1. Weak versus Strong 8.8.1. Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are Validators come in two flavors: strong or weak. Weak validators are
easy to generate but are far less useful for comparisons. Strong easy to generate but are far less useful for comparisons. Strong
validators are ideal for comparisons but can be very difficult (and validators are ideal for comparisons but can be very difficult (and
occasionally impossible) to generate efficiently. Rather than impose occasionally impossible) to generate efficiently. Rather than impose
that all forms of resource adhere to the same strength of validator, that all forms of resource adhere to the same strength of validator,
HTTP exposes the type of validator in use and imposes restrictions on HTTP exposes the type of validator in use and imposes restrictions on
when weak validators can be used as preconditions. when weak validators can be used as preconditions.
A _strong validator_ is representation metadata that changes value A "strong validator" is representation metadata that changes value
whenever a change occurs to the representation data that would be whenever a change occurs to the representation data that would be
observable in the content of a 200 (OK) response to GET. observable in the content of a 200 (OK) response to GET.
A strong validator might change for reasons other than a change to A strong validator might change for reasons other than a change to
the representation data, such as when a semantically significant part the representation data, such as when a semantically significant part
of the representation metadata is changed (e.g., Content-Type), but of the representation metadata is changed (e.g., Content-Type), but
it is in the best interests of the origin server to only change the it is in the best interests of the origin server to only change the
value when it is necessary to invalidate the stored responses held by value when it is necessary to invalidate the stored responses held by
remote caches and authoring tools. remote caches and authoring tools.
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accessible to GET. A collision-resistant hash function applied to accessible to GET. A collision-resistant hash function applied to
the representation data is also sufficient if the data is available the representation data is also sufficient if the data is available
prior to the response header fields being sent and the digest does prior to the response header fields being sent and the digest does
not need to be recalculated every time a validation request is not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that received. However, if a resource has distinct representations that
differ only in their metadata, such as might occur with content differ only in their metadata, such as might occur with content
negotiation over media types that happen to share the same data negotiation over media types that happen to share the same data
format, then the origin server needs to incorporate additional format, then the origin server needs to incorporate additional
information in the validator to distinguish those representations. information in the validator to distinguish those representations.
In contrast, a _weak validator_ is representation metadata that might In contrast, a "weak validator" is representation metadata that might
not change for every change to the representation data. This not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated weakness might be due to limitations in how the value is calculated
(e.g., clock resolution), an inability to ensure uniqueness for all (e.g., clock resolution), an inability to ensure uniqueness for all
possible representations of the resource, or a desire of the resource possible representations of the resource, or a desire of the resource
owner to group representations by some self-determined set of owner to group representations by some self-determined set of
equivalency rather than unique sequences of data. equivalency rather than unique sequences of data.
An origin server SHOULD change a weak entity-tag whenever it An origin server SHOULD change a weak entity tag whenever it
considers prior representations to be unacceptable as a substitute considers prior representations to be unacceptable as a substitute
for the current representation. In other words, a weak entity-tag for the current representation. In other words, a weak entity tag
ought to change whenever the origin server wants caches to invalidate ought to change whenever the origin server wants caches to invalidate
old responses. old responses.
For example, the representation of a weather report that changes in For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality). adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only Likewise, a representation's modification time, if defined with only
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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.
8.8.3. ETag 8.8.3. ETag
The "ETag" field in a response provides the current entity-tag for The "ETag" field in a response provides the current entity tag for
the selected representation, as determined at the conclusion of the selected representation, as determined at the conclusion of
handling the request. An entity-tag is an opaque validator for handling the request. An entity tag is an opaque validator for
differentiating between multiple representations of the same differentiating between multiple representations of the same
resource, regardless of whether those multiple representations are resource, regardless of whether those multiple representations are
due to resource state changes over time, content negotiation due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time, resulting in multiple representations being valid at the same time,
or both. An entity-tag consists of an opaque quoted string, possibly or both. An entity tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator. prefixed by a weakness indicator.
ETag = entity-tag ETag = entity-tag
entity-tag = [ weak ] opaque-tag entity-tag = [ weak ] opaque-tag
weak = %s"W/" weak = %s"W/"
opaque-tag = DQUOTE *etagc DQUOTE opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text ; VCHAR except double quotes, plus obs-text
| *Note:* Previously, opaque-tag was defined to be a quoted- | *Note:* Previously, opaque-tag was defined to be a quoted-
| string ([RFC2616], Section 3.11); thus, some recipients might | string ([RFC2616], Section 3.11); thus, some recipients might
| perform backslash unescaping. Servers therefore ought to avoid | perform backslash unescaping. Servers therefore ought to avoid
| 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
generated while sending the content. generated while sending the content.
8.8.3.1. Generation 8.8.3.1. Generation
The principle behind entity-tags is that only the service author The principle behind entity tags is that only the service author
knows the implementation of a resource well enough to select the most knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets that any such mechanism can be mapped to a simple sequence of octets
for easy comparison. Since the value is opaque, there is no need for for easy comparison. Since the value is opaque, there is no need for
the client to be aware of how each entity-tag is constructed. the client to be aware of how each entity tag is constructed.
For example, a resource that has implementation-specific versioning For example, a resource that has implementation-specific versioning
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 substantially reduce evaluating cache freshness ([CACHING]) can substantially reduce
unnecessary transfers and significantly improve service availability, unnecessary transfers and significantly improve service availability,
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" | W/"2" | no match | no match |
| W/"1" | "1" | no match | match | | W/"1" | "1" | no match | match |
| "1" | "1" | 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
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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 [HTTP/1.1]) apply only during message | codings (Section 7 of [HTTP/1.1]) apply only during message
| transfer and do not result in distinct entity-tags. | transfer and do not result in distinct entity tags.
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
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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 | Section |
+---------+--------------------------------------------+-------+ | Name | | |
| GET | Transfer a current representation of the | 9.3.1 | +---------+--------------------------------------------+---------+
| | target resource. | | | GET | Transfer a current representation of the | 9.3.1 |
| HEAD | Same as GET, but do not transfer the | 9.3.2 | | | target resource. | |
| | 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. | | | POST | Perform resource-specific processing on | 9.3.3 |
| PUT | Replace all current representations of the | 9.3.4 | | | the request content. | |
| | target resource with the request content. | | | PUT | Replace all current representations of the | 9.3.4 |
| DELETE | Remove all current representations of the | 9.3.5 | | | target resource with the request content. | |
| | target resource. | | | DELETE | Remove all current representations of the | 9.3.5 |
| CONNECT | Establish a tunnel to the server | 9.3.6 | | | target resource. | |
| | identified by the target resource. | | | CONNECT | Establish a tunnel to the server | 9.3.6 |
| OPTIONS | Describe the communication options for the | 9.3.7 | | | identified by the target resource. | |
| | target resource. | | | OPTIONS | Describe the communication options for the | 9.3.7 |
| TRACE | Perform a message loop-back test along the | 9.3.8 | | | target resource. | |
| | path to the 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. An origin server that receives a methods can change dynamically. An origin server that receives a
request method that is unrecognized or not implemented SHOULD respond request method that is unrecognized or not implemented SHOULD respond
with the 501 (Not Implemented) status code. An origin server that with the 501 (Not Implemented) status code. An origin server that
receives a request method that is recognized and implemented, but not receives a request method that is recognized and implemented, but not
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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
from including behavior that is potentially harmful, that is not from including behavior that is potentially harmful, that is not
entirely read-only, or that causes side effects while invoking a safe entirely read-only, or that causes side effects while invoking a safe
method. What is important, however, is that the client did not method. What is important, however, is that the client did not
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parameters, such as "page?do=delete". If the purpose of such a parameters, such as "page?do=delete". If the purpose of such a
resource is to perform an unsafe action, then the resource owner MUST resource is to perform an unsafe action, then the resource owner MUST
disable or disallow that action when it is accessed using a safe disable or disallow that action when it is accessed using a safe
request method. Failure to do so will result in unfortunate side request method. Failure to do so will result in unfortunate side
effects when automated processes perform a GET on every URI reference effects when automated processes perform a GET on every URI reference
for the sake of link maintenance, pre-fetching, building a search for the sake of link maintenance, pre-fetching, building a search
index, etc. index, etc.
9.2.2. Idempotent Methods 9.2.2. Idempotent Methods
A request method is considered _idempotent_ if the intended effect on A request method is considered "idempotent" if the intended effect on
the server of multiple identical requests with that method is the the server of multiple identical requests with that method is the
same as the effect for a single such request. Of the request methods same as the effect for a single such request. Of the request methods
defined by this specification, PUT, DELETE, and safe request methods defined by this specification, PUT, DELETE, and safe request methods
are idempotent. are idempotent.
Like the definition of safe, the idempotent property only applies to Like the definition of safe, the idempotent property only applies to
what has been requested by the user; a server is free to log each what has been requested by the user; a server is free to log each
request separately, retain a revision control history, or implement request separately, retain a revision control history, or implement
other non-idempotent side effects for each idempotent request. other non-idempotent side effects for each idempotent request.
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automatically retry a POST request if the underlying transport automatically retry a POST request if the underlying transport
connection closed before any part of a response is received, connection closed before any part of a response is received,
particularly if an idle persistent connection was used. 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 to 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 that does not do so cannot be cached. For additional
requirements see [CACHING]. requirements see [CACHING].
This specification defines caching semantics for GET, HEAD, and POST, This specification defines caching semantics for GET, HEAD, and POST,
although the overwhelming majority of cache implementations only although the overwhelming majority of cache implementations only
support GET and HEAD. support GET and HEAD.
9.3. Method Definitions 9.3. Method Definitions
9.3.1. GET 9.3.1. GET
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result of successfully processing a POST request, the origin server result of successfully processing a POST request, the origin server
SHOULD send a 201 (Created) response containing a Location header SHOULD send a 201 (Created) response containing a Location header
field that provides an identifier for the primary resource created field that provides an identifier for the primary resource created
(Section 10.2.2) and a representation that describes the status of (Section 10.2.2) and a representation that describes the status of
the request while referring to the new resource(s). the request while referring to the new resource(s).
Responses to POST requests are only cacheable when they include Responses to POST requests are only cacheable when they include
explicit freshness information (see Section 4.2.1 of [CACHING]) and a explicit freshness information (see Section 4.2.1 of [CACHING]) and a
Content-Location header field that has the same value as the POST's Content-Location header field that has the same value as the POST's
target URI (Section 8.7). A cached POST response can be reused to target URI (Section 8.7). A cached POST response can be reused to
satisfy a later GET or HEAD request, but not a POST request, since satisfy a later GET or HEAD request. In contrast, a POST request
POST is required to be written through to the origin server, because cannot be satisfied by a cached POST response because POST is
it is unsafe; see Section 4 of [CACHING]. potentially unsafe; see Section 4 of [CACHING].
If the result of processing a POST would be equivalent to a If the result of processing a POST would be equivalent to a
representation of an existing resource, an origin server MAY redirect representation of an existing resource, an origin server MAY redirect
the user agent to that resource by sending a 303 (See Other) response the user agent to that resource by sending a 303 (See Other) response
with the existing resource's identifier in the Location field. This with the existing resource's identifier in the Location field. This
has the benefits of providing the user agent a resource identifier has the benefits of providing the user agent a resource identifier
and transferring the representation via a method more amenable to and transferring the representation via a method more amenable to
shared caching, though at the cost of an extra request if the user shared caching, though at the cost of an extra request if the user
agent does not already have the representation cached. agent does not already have the representation cached.
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of the resource state). 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 sent (and retains in memory) is the result of the representation it sent (and retains in memory) is the result of
the PUT, and thus doesn't need to be retrieved again from the origin the PUT, and thus it doesn't need to be retrieved again from the
server. The new validator(s) received in the response can be used origin server. The new validator(s) received in the response can be
for future conditional requests in order to prevent accidental used 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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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. The back to the client as the content of a 200 (OK) response. The
"message/http" (Section 10.1 of [HTTP/1.1]) format is one way to do "message/http" format (Section 10.1 of [HTTP/1.1]) is one way to do
so. The final recipient is either the origin server or the first so. The final recipient is either the origin server or the first
server to 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 [COOKIE] in a TRACE request. The final (Section 11) or cookies [COOKIE] 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
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The Expect field value is case-insensitive. The Expect field value is case-insensitive.
The only expectation defined by this specification is "100-continue" The only expectation defined by this specification is "100-continue"
(with no defined parameters). (with no defined parameters).
A server that receives an Expect field value containing a member A server that receives an Expect field value containing a member
other than 100-continue MAY respond with a 417 (Expectation Failed) other than 100-continue MAY respond with a 417 (Expectation Failed)
status code to indicate that the unexpected expectation cannot be status code to indicate that the unexpected expectation cannot be
met. met.
A _100-continue_ expectation informs recipients that the client is A "100-continue" expectation informs recipients that the client is
about to send (presumably large) content in this request and wishes about to send (presumably large) content in this request and wishes
to receive a 100 (Continue) interim response if the method, target to receive a 100 (Continue) interim response if the method, target
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).
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o A server that sends a 100 (Continue) response MUST ultimately send o A server that sends a 100 (Continue) response MUST ultimately send
a final status code, once it receives and processes the request a final status code, once it receives and processes the request
content, 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 o 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 [HTTP/1.1]) or continue the connection (e.g., see Section 9.6 of [HTTP/1.1]) or continue
reading the request content. reading the request content.
An origin server MUST, upon receiving an HTTP/1.1 (or later) request Upon receiving an HTTP/1.1 (or later) request that has a method,
that has a method, target URI, and complete header section that target URI, and complete header section that contains a 100-continue
contains a 100-continue expectation and an indication that request expectation and an indication that request content will follow, an
content will follow, either send an immediate response with a final origin server MUST send either:
status code, if that status can be determined by examining just the
method, target URI, and header fields, or send an immediate 100
(Continue) response to encourage the client to send the request
content. The origin server MUST NOT wait for the content before
sending the 100 (Continue) response.
A proxy MUST, upon receiving an HTTP/1.1 (or later) request that has o an immediate response with a final status code, if that status can
a method, target URI, and complete header section that contains a be determined by examining just the method, target URI, and header
100-continue expectation and indicates a request content will follow, fields, or
either send an immediate response with a final status code, if that
status can be determined by examining just the method, target URI, o an immediate 100 (Continue) response to encourage the client to
and header fields, or begin forwarding the request toward the origin send the request content.
server by sending a corresponding request-line and header section to
the next inbound server. If the proxy believes (from configuration The origin server MUST NOT wait for the content before sending the
or past interaction) that the next inbound server only supports 100 (Continue) response.
HTTP/1.0, the proxy MAY generate an immediate 100 (Continue) response
to encourage the client to begin sending the content. Upon receiving an HTTP/1.1 (or later) request that has a method,
target URI, and complete header section that contains a 100-continue
expectation and indicates a request content will follow, a proxy MUST
either:
o send an immediate response with a final status code, if that
status can be determined by examining just the method, target URI,
and header fields, or
o forward the request toward the origin server by sending a
corresponding request-line and header section to the next inbound
server.
If the proxy believes (from configuration or past interaction) that
the next inbound server only supports HTTP/1.0, the proxy MAY
generate an immediate 100 (Continue) response to encourage the client
to begin sending the content.
10.1.2. From 10.1.2. From
The "From" header field contains an Internet email address for a The "From" header field contains an Internet email address for a
human user who controls the requesting user agent. The address ought human user who controls the requesting user agent. The address ought
to be machine-usable, as defined by "mailbox" in Section 3.4 of to be machine-usable, as defined by "mailbox" in Section 3.4 of
[RFC5322]: [RFC5322]:
From = mailbox From = mailbox
mailbox = <mailbox, see [RFC5322], Section 3.4> mailbox = <mailbox, see [RFC5322], Section 3.4>
An example is: An example is:
From: webmaster@example.org From: spider-admin@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.
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information disclosure in Referer ought to restrict their changes to information disclosure in Referer ought to restrict their changes to
specific edits, such as replacing internal domain names with specific edits, such as replacing internal domain names with
pseudonyms or truncating the query and/or path components. An pseudonyms or truncating the query and/or path components. An
intermediary SHOULD NOT modify or delete the Referer header field intermediary SHOULD NOT modify or delete the Referer header field
when the field value shares the same scheme and host as the target when the field value shares the same scheme and host as the target
URI. URI.
10.1.4. TE 10.1.4. TE
The "TE" header field describes capabilities of the client with The "TE" header field describes capabilities of the client with
regard to transfer encodings and trailer sections. regard to transfer codings and trailer sections.
A TE field with a "trailers" member sent in a request indicates that As described in Section 6.5, a TE field with a "trailers" member sent
the client will not discard trailer fields, as described in in a request indicates that the client will not discard trailer
Section 6.5. fields.
TE is also used within HTTP/1.1 to advise servers about what transfer TE is also used within HTTP/1.1 to advise servers about which
codings the client is able to accept in a response. As of transfer codings the client is able to accept in a response. As of
publication, only HTTP/1.1 uses transfer codings (see Section 7 of publication, only HTTP/1.1 uses transfer codings (see Section 7 of
[HTTP/1.1]). [HTTP/1.1]).
The TE field value is a list of members, with each member (aside from The TE field value is a list of members, with each member (aside from
"trailers") consisting of a transfer coding name token with an "trailers") consisting of a transfer coding name token with an
optional weight indicating the client's relative preference for that optional weight indicating the client's relative preference for that
transfer coding (Section 12.4.2) and optional parameters for that transfer coding (Section 12.4.2) and optional parameters for that
transfer coding. transfer coding.
TE = #t-codings TE = #t-codings
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11. HTTP Authentication 11. HTTP Authentication
11.1. Authentication Scheme 11.1. Authentication Scheme
HTTP provides a general framework for access control and HTTP provides a general framework for access control and
authentication, via an extensible set of challenge-response authentication, via an extensible set of challenge-response
authentication schemes, which can be used by a server to challenge a authentication schemes, which can be used by a server to challenge a
client request and by a client to provide authentication information. client request and by a client to provide authentication information.
It uses a case-insensitive token to identify the authentication It uses a case-insensitive token to identify the authentication
scheme scheme:
auth-scheme = token auth-scheme = token
Aside from the general framework, this document does not specify any Aside from the general framework, this document does not specify any
authentication schemes. New and existing authentication schemes are authentication schemes. New and existing authentication schemes are
specified independently and ought to be registered within the specified independently and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry". "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
For example, the "basic" and "digest" authentication schemes are For example, the "basic" and "digest" authentication schemes are
defined by RFC 7617 and RFC 7616, respectively. defined by [RFC7617] and [RFC7616], respectively.
11.2. Authentication Parameters 11.2. Authentication Parameters
The authentication scheme is followed by additional information The authentication scheme is followed by additional information
necessary for achieving authentication via that scheme as either a necessary for achieving authentication via that scheme as either a
comma-separated list of parameters or a single sequence of characters comma-separated list of parameters or a single sequence of characters
capable of holding base64-encoded information. capable of holding base64-encoded information.
token68 = 1*( ALPHA / DIGIT / token68 = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"=" "-" / "." / "_" / "~" / "+" / "/" ) *"="
The token68 syntax allows the 66 unreserved URI characters ([URI]), The token68 syntax allows the 66 unreserved URI characters ([URI]),
plus a few others, so that it can hold a base64, base64url (URL and plus a few others, so that it can hold a base64, base64url (URL and
filename safe alphabet), base32, or base16 (hex) encoding, with or filename safe alphabet), base32, or base16 (hex) encoding, with or
without padding, but excluding whitespace ([RFC4648]). without padding, but excluding whitespace ([RFC4648]).
Authentication parameters are name=value pairs, where the name token Authentication parameters are name/value pairs, where the name token
is matched case-insensitively and each parameter name MUST only occur is matched case-insensitively and each parameter name MUST only occur
once per challenge. once per challenge.
auth-param = token BWS "=" BWS ( token / quoted-string ) auth-param = token BWS "=" BWS ( token / quoted-string )
Parameter values can be expressed either as "token" or as "quoted- Parameter values can be expressed either as "token" or as "quoted-
string" (Section 5.6). Authentication scheme definitions need to string" (Section 5.6). Authentication scheme definitions need to
accept both notations, both for senders and recipients, to allow accept both notations, both for senders and recipients, to allow
recipients to use generic parsing components regardless of the recipients to use generic parsing components regardless of the
authentication scheme. authentication scheme.
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encapsulation, and with additional header fields specifying encapsulation, and with additional header fields specifying
authentication information. However, such additional mechanisms are authentication information. However, such additional mechanisms are
not defined by this specification. not defined by this specification.
Note that various custom mechanisms for user authentication use the Note that various custom mechanisms for user authentication use the
Set-Cookie and Cookie header fields, defined in [COOKIE], for passing Set-Cookie and Cookie header fields, defined in [COOKIE], for passing
tokens related to authentication. tokens related to authentication.
11.5. Establishing a Protection Space (Realm) 11.5. Establishing a Protection Space (Realm)
The _realm_ authentication parameter is reserved for use by The "realm" authentication parameter is reserved for use by
authentication schemes that wish to indicate a scope of protection. authentication schemes that wish to indicate a scope of protection.
A _protection space_ is defined by the origin (see Section 4.3.1) of A "protection space" is defined by the origin (see Section 4.3.1) of
the server being accessed, in combination with the realm value if the server being accessed, in combination with the realm value if
present. These realms allow the protected resources on a server to present. These realms allow the protected resources on a server to
be partitioned into a set of protection spaces, each with its own be partitioned into a set of protection spaces, each with its own
authentication scheme and/or authorization database. The realm value authentication scheme and/or authorization database. The realm value
is a string, generally assigned by the origin server, that can have is a string, generally assigned by the origin server, that can have
additional semantics specific to the authentication scheme. Note additional semantics specific to the authentication scheme. Note
that a response can have multiple challenges with the same auth- that a response can have multiple challenges with the same auth-
scheme but with different realms. scheme but with different realms.
The protection space determines the domain over which credentials can The protection space determines the domain over which credentials can
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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: Basic realm="simple", Newauth realm="apps", WWW-Authenticate: Basic realm="simple", Newauth realm="apps",
type=1, title="Login to \"apps\"" type=1, title="Login to \"apps\""
This header field contains two challenges; one for the "Basic" scheme This header field contains two challenges, one for the "Basic" scheme
with a realm value of "simple", and another for the "Newauth" scheme with a realm value of "simple" and another for the "Newauth" scheme
with a realm value of "apps", and two additional parameters "type" with a realm value of "apps". It also contains two additional
and "title". parameters, "type" and "title".
Some user agents do not recognise this form, however. As a result, Some user agents do not recognize 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
| the header field value and thus is harmless. | the header field value and thus is harmless.
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require otherwise, such as credentials that vary according to a require otherwise, such as credentials that vary according to a
challenge value or using synchronized clocks). challenge value or using synchronized clocks).
A proxy forwarding a request MUST NOT modify any Authorization header A proxy forwarding a request MUST NOT modify any Authorization header
fields in that request. See Section 3.5 of [CACHING] for details of fields in that request. See Section 3.5 of [CACHING] for details of
and requirements pertaining to handling of the Authorization header and requirements pertaining to handling of the Authorization header
field by HTTP caches. field by HTTP caches.
11.6.3. Authentication-Info 11.6.3. Authentication-Info
HTTP authentication schemes can use the Authentication-Info response HTTP authentication schemes can use the "Authentication-Info"
field to communicate information after the client's authentication response field to communicate information after the client's
credentials have been accepted. This information can include a authentication credentials have been accepted. This information can
finalization message from the server (e.g., it can contain the server include a finalization message from the server (e.g., it can contain
authentication). the server authentication).
The field value is a list of parameters (name/value pairs), using the The field value is a list of parameters (name/value pairs), using the
"auth-param" syntax defined in Section 11.3. This specification only "auth-param" syntax defined in Section 11.3. This specification only
describes the generic format; authentication schemes using describes the generic format; authentication schemes using
Authentication-Info will define the individual parameters. The Authentication-Info will define the individual parameters. The
"Digest" Authentication Scheme, for instance, defines multiple "Digest" Authentication Scheme, for instance, defines multiple
parameters in Section 3.5 of [RFC7616]. parameters in Section 3.5 of [RFC7616].
Authentication-Info = #auth-param Authentication-Info = #auth-param
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only to the next inbound proxy that demanded authentication using the only to the next inbound proxy that demanded authentication using the
Proxy-Authenticate header field. When multiple proxies are used in a Proxy-Authenticate header field. When multiple proxies are used in a
chain, the Proxy-Authorization header field is consumed by the first chain, the Proxy-Authorization header field is consumed by the first
inbound proxy that was expecting to receive credentials. A proxy MAY inbound proxy that was expecting to receive credentials. A proxy MAY
relay the credentials from the client request to the next proxy if relay the credentials from the client request to the next proxy if
that is the mechanism by which the proxies cooperatively authenticate that is the mechanism by which the proxies cooperatively authenticate
a given request. a given request.
11.7.3. Proxy-Authentication-Info 11.7.3. Proxy-Authentication-Info
The Proxy-Authentication-Info response header field is equivalent to The "Proxy-Authentication-Info" response header field is equivalent
Authentication-Info, except that it applies to proxy authentication to Authentication-Info, except that it applies to proxy
(Section 11.3) and its semantics are defined by the authentication authentication (Section 11.3) and its semantics are defined by the
scheme indicated by the Proxy-Authorization header field authentication scheme indicated by the Proxy-Authorization header
(Section 11.7.2) of the corresponding request: field (Section 11.7.2) of the corresponding request:
Proxy-Authentication-Info = #auth-param Proxy-Authentication-Info = #auth-param
However, unlike Authentication-Info, the Proxy-Authentication-Info However, unlike Authentication-Info, the Proxy-Authentication-Info
header field applies only to the next outbound client on the response header field applies only to the next outbound client on the response
chain. This is because only the client that chose a given proxy is chain. This is because only the client that chose a given proxy is
likely to have the credentials necessary for authentication. likely to have the credentials necessary for authentication.
However, when multiple proxies are used within the same However, when multiple proxies are used within the same
administrative domain, such as office and regional caching proxies administrative domain, such as office and regional caching proxies
within a large corporate network, it is common for credentials to be within a large corporate network, it is common for credentials to be
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that information; for example, in different formats, languages, or that information; for example, in different formats, languages, or
encodings. Likewise, different users or user agents might have encodings. Likewise, different users or user agents might have
differing capabilities, characteristics, or preferences that could differing capabilities, characteristics, or preferences that could
influence which representation, among those available, would be best influence which representation, among those available, would be best
to deliver. For this reason, HTTP provides mechanisms for content to deliver. For this reason, HTTP provides mechanisms for content
negotiation. negotiation.
This specification defines three patterns of content negotiation that This specification defines three patterns of content negotiation that
can be made visible within the protocol: "proactive" negotiation, can be made visible within the protocol: "proactive" negotiation,
where the server selects the representation based upon the user where the server selects the representation based upon the user
agent's stated preferences, "reactive" negotiation, where the server agent's stated preferences; "reactive" negotiation, where the server
provides a list of representations for the user agent to choose from, provides a list of representations for the user agent to choose from;
and "request content" negotiation, where the user agent selects the and "request content" negotiation, where the user agent selects the
representation for a future request based upon the server's stated representation for a future request based upon the server's stated
preferences in past responses. preferences in past responses.
Other patterns of content negotiation include "conditional content", Other patterns of content negotiation include "conditional content",
where the representation consists of multiple parts that are where the representation consists of multiple parts that are
selectively rendered based on user agent parameters, "active selectively rendered based on user agent parameters, "active
content", where the representation contains a script that makes content", where the representation contains a script that makes
additional (more specific) requests based on the user agent additional (more specific) requests based on the user agent
characteristics, and "Transparent Content Negotiation" ([RFC2295]), characteristics, and "Transparent Content Negotiation" ([RFC2295]),
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The consistency with which an origin server responds to requests, The consistency with which an origin server responds to requests,
over time and over the varying dimensions of content negotiation, and over time and over the varying dimensions of content negotiation, and
thus the "sameness" of a resource's observed representations over thus the "sameness" of a resource's observed representations over
time, is determined entirely by whatever entity or algorithm selects time, is determined entirely by whatever entity or algorithm selects
or generates those responses. or generates those responses.
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 (when "best guess" to the user agent along with the first response (when
that "best guess" is good enough for the user, this avoids the round that "best guess" is good enough for the user, this avoids the round-
trip delay of a subsequent request). In order to improve the trip delay of a subsequent request). 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 o 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
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The request header fields Accept, Accept-Charset, Accept-Encoding, The request header fields Accept, Accept-Charset, Accept-Encoding,
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 content (regardless of the status code) is performed by selection of content (regardless of the status code) is performed by
the user agent after receiving an initial response. The mechanism the user agent after receiving an initial response. The mechanism
for reactive negotiation might be as simple as a list of references for reactive negotiation might be as simple as a list of references
to alternative representations. to alternative representations.
If the user agent is not satisfied by the initial response content, If the user agent is not satisfied by the initial response content,
it can perform a GET request on one or more of the alternative it can perform a GET request on one or more of the alternative
resources to obtain a different representation. Selection of such resources to obtain a different representation. Selection of such
alternatives might be performed automatically (by the user agent) or alternatives might be performed automatically (by the user agent) or
manually (e.g., by the user selecting from a hypertext menu). manually (e.g., by the user selecting from a hypertext menu).
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list of alternatives to the user agent, which degrades user-perceived list of alternatives to the user agent, which degrades user-perceived
latency if transmitted in the header section, and needing a second latency if transmitted in the header section, and needing a second
request to obtain an alternate representation. Furthermore, this request to obtain an alternate representation. Furthermore, this
specification does not define a mechanism for supporting automatic specification does not define a mechanism for supporting automatic
selection, though it does not prevent such a mechanism from being selection, though it does not prevent such a mechanism from being
developed. developed.
12.3. Request Content Negotiation 12.3. Request Content Negotiation
When content negotiation preferences are sent in a server's response, When content negotiation preferences are sent in a server's response,
the listed preferences are called _request content negotiation_ the listed preferences are called "request content negotiation"
because they intend to influence selection of an appropriate content because they intend to influence selection of an appropriate content
for subsequent requests to that resource. For example, the Accept for subsequent requests to that resource. For example, the Accept
(Section 12.5.1) and Accept-Encoding (Section 12.5.3) header fields (Section 12.5.1) and Accept-Encoding (Section 12.5.3) header fields
can be sent in a response to indicate preferred media types and can be sent in a response to indicate preferred media types and
content codings for subsequent requests to that resource. content codings for subsequent requests to that resource.
Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch" Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch"
response header field which allows discovery of which content types response header field, which allows discovery of which content types
are accepted in PATCH requests. are accepted in PATCH requests.
12.4. Content Negotiation Field Features 12.4. Content Negotiation Field Features
12.4.1. Absence 12.4.1. Absence
For each of the content negotiation fields, a request that does not For each of the content negotiation fields, a request that does not
contain the field implies that the sender has no preference on that contain the field implies that the sender has no preference on that
dimension of negotiation. dimension of negotiation.
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12.4.3. Wildcard Values 12.4.3. Wildcard Values
Most of these header fields, where indicated, define a wildcard value Most of these header fields, where indicated, define a wildcard value
("*") to select unspecified values. If no wildcard is present, ("*") to select unspecified values. If no wildcard is present,
values that are not explicitly mentioned in the field are considered values that are not explicitly mentioned in the field are considered
unacceptable. Within Vary, the wildcard value means that the unacceptable. Within Vary, the wildcard value means that the
variance is unlimited. variance is unlimited.
| *Note:* In practice, using wildcards in content negotiation has | *Note:* In practice, using wildcards in content negotiation has
| limited practical value, because it is seldom useful to say, | limited practical value because it is seldom useful to say, for
| for example, "I prefer image/* more or less than (some other | example, "I prefer image/* more or less than (some other
| specific value)". Clients can explicitly request a 406 (Not | specific value)". By sending Accept: */*;q=0, clients can
| Acceptable) response if a more preferred format is not | explicitly request a 406 (Not Acceptable) response if a more
| available by sending Accept: */*;q=0, but they still need to be | preferred format is not available, but they still need to be
| able to handle a different response, since the server is | able to handle a different response since the server is allowed
| allowed to ignore their preference. | to ignore their preference.
12.5. Content Negotiation Fields 12.5. Content Negotiation Fields
12.5.1. Accept 12.5.1. Accept
The "Accept" header field can be used by user agents to specify their The "Accept" header field can be used by user agents to specify their
preferences regarding response media types. For example, Accept preferences regarding response media types. For example, Accept
header fields can be used to indicate that the request is header fields can be used to indicate that the request is
specifically limited to a small set of desired types, as in the case specifically limited to a small set of desired types, as in the case
of a request for an in-line image. of a request for an in-line image.
When sent by a server in a response, Accept provides information When sent by a server in a response, Accept provides information
about what content types are preferred in the content of a subsequent about which content types are preferred in the content of a
request to the same resource. subsequent request to the same resource.
Accept = #( media-range [ weight ] ) Accept = #( media-range [ weight ] )
media-range = ( "*/*" media-range = ( "*/*"
/ ( type "/" "*" ) / ( type "/" "*" )
/ ( type "/" subtype ) / ( type "/" subtype )
) parameters ) parameters
The asterisk "*" character is used to group media types into ranges, The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all with "*/*" indicating all media types and "type/*" indicating all
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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 17.13). Most general- | fingerprinting far too easy (Section 17.13). 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.
12.5.3. Accept-Encoding 12.5.3. Accept-Encoding
The "Accept-Encoding" header field can be used to indicate The "Accept-Encoding" header field can be used to indicate
preferences regarding the use of content codings (Section 8.4.1). preferences regarding the use of content codings (Section 8.4.1).
When sent by a user agent in a request, Accept-Encoding indicates the When sent by a user agent in a request, Accept-Encoding indicates the
content codings acceptable in a response. content codings acceptable in a response.
When sent by a server in a response, Accept-Encoding provides When sent by a server in a response, Accept-Encoding provides
information about what content codings are preferred in the content information about which content codings are preferred in the content
of a subsequent request to the same resource. of a subsequent request to the same resource.
An "identity" token is used as a synonym for "no encoding" in order An "identity" token is used as a synonym for "no encoding" in order
to communicate when no encoding is preferred. to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ weight ] ) Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*" codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value (weight) Each codings value MAY be given an associated quality value (weight)
representing the preference for that encoding, as defined in representing the preference for that encoding, as defined in
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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".)
A representation could be encoded with multiple content codings. A representation could be encoded with multiple content codings.
However, most content codings are alternative ways to accomplish the However, most content codings are alternative ways to accomplish the
same purpose (e.g., data compression). When selecting between same purpose (e.g., data compression). When selecting between
multiple content codings that have the same purpose, the acceptable multiple content codings that have the same purpose, the acceptable
content coding with the highest non-zero qvalue is preferred. content coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a field value that is empty An Accept-Encoding header field with a field value that is empty
implies that the user agent does not want any content coding in implies that the user agent does not want any content coding in
response. If an Accept-Encoding header field is present in a request response. If a non-empty Accept-Encoding header field is present in
and none of the available representations for the response have a a request and none of the available representations for the response
content coding that is listed as acceptable, the origin server SHOULD have a content coding that is listed as acceptable, the origin server
send a response without any content coding. SHOULD send a response without any content coding unless the identity
coding is indicated as unacceptable.
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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include an Accept-Encoding header field in that response, allowing include an Accept-Encoding header field in that response, allowing
clients to distinguish between issues related to content codings and clients to distinguish between issues related to content codings and
media types. In order to avoid confusion with issues related to media types. In order to avoid confusion with issues related to
media types, servers that fail a request with a 415 status for media types, servers that fail a request with a 415 status for
reasons unrelated to content codings MUST NOT include the Accept- reasons unrelated to content codings MUST NOT include the Accept-
Encoding header field. Encoding header field.
The most common use of Accept-Encoding is in responses with a 415 The most common use of Accept-Encoding is in responses with a 415
(Unsupported Media Type) status code, in response to optimistic use (Unsupported Media Type) status code, in response to optimistic use
of a content coding by clients. However, the header field can also of a content coding by clients. However, the header field can also
be used to indicate to clients that content codings are supported, to be used to indicate to clients that content codings are supported in
optimize future interactions. For example, a resource might include order to optimize future interactions. For example, a resource might
it in a 2xx (Successful) response when the request content was big include it in a 2xx (Successful) response when the request content
enough to justify use of a compression coding but the client failed was big enough to justify use of a compression coding but the client
do so. failed do so.
12.5.4. Accept-Language 12.5.4. Accept-Language
The "Accept-Language" header field can be used by user agents to The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the indicate the set of natural languages that are preferred in the
response. Language tags are defined in Section 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>
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response when it wishes that response to be selectively reused for response when it wishes that response to be selectively reused for
subsequent requests. Generally, that is the case when the response subsequent requests. Generally, that is the case when the response
content has been tailored to better fit the preferences expressed by content has been tailored to better fit the preferences expressed by
those selecting header fields, such as when an origin server has those selecting header fields, such as when an origin server has
selected the response's language based on the request's selected the response's language based on the request's
Accept-Language header field. Accept-Language header field.
Vary might be elided when an origin server considers variance in Vary might be elided when an origin server considers variance in
content selection to be less significant than Vary's performance content selection to be less significant than Vary's performance
impact on caching, particularly when reuse is already limited by impact on caching, particularly when reuse is already limited by
Cache-Control response directives (Section 5.2 of [CACHING]). cache response directives (Section 5.2 of [CACHING]).
There is no need to send the Authorization field name in Vary because There is no need to send the Authorization field name in Vary because
reuse of that response for a different user is prohibited by the reuse of that response for a different user is prohibited by the
field definition (Section 11.6.2). Likewise, if the response content field definition (Section 11.6.2). Likewise, if the response content
has been selected or influenced by network region but the origin has been selected or influenced by network region, but the origin
server wants the cached response to be reused even if recipients move server wants the cached response to be reused even if recipients move
from one region to another, then there is no need for the origin from one region to another, then there is no need for the origin
server to indicate such variance in Vary. server to indicate such variance in Vary.
13. Conditional Requests 13. Conditional Requests
A conditional request is an HTTP request with one or more request A conditional request is an HTTP request with one or more request
header fields that indicate a precondition to be tested before header fields that indicate a precondition to be tested before
applying the request method to the target resource. Section 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
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implementation is signaled by some other property of the target implementation is signaled by some other property of the target
resource. This encourages a focus on mutually agreed deployment of resource. This encourages a focus on mutually agreed deployment of
common standards. 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). In general, it can be used with prevent the "lost update" problem). In general, it can be used with
any method that involves the selection or modification of a any method that involves the selection or modification of a
representation to abort the request if the selected representation's representation to abort the request if the selected representation's
current entity-tag is not a member within the If-Match field value. current entity tag is not a member within the If-Match field value.
When an origin server receives a request that selects a When an origin server receives a request that selects a
representation and that request includes an If-Match header field, representation and that request includes an If-Match header field,
the origin server MUST evaluate the If-Match condition as per the origin server MUST evaluate the If-Match condition per
Section 13.2 prior to performing the method. Section 13.2 prior to performing the method.
To evaluate a received If-Match header field: To evaluate a received If-Match header field:
1. If the field value is "*", the condition is true if the origin 1. If the field value is "*", the condition is true if the origin
server has a current representation for the target resource. server has a current representation for the target resource.
2. If the field value is a list of entity-tags, the condition is 2. If the field value is a list of entity tags, the condition is
true if any of the listed tags match the entity-tag of the true if any of the listed tags match the entity tag of the
selected representation. selected representation.
3. Otherwise, the condition is false. 3. Otherwise, the condition is false.
An origin server that evaluates an If-Match condition MUST NOT An origin server that evaluates an If-Match condition MUST NOT
perform the requested method if the condition evaluates to false. perform the requested method if the condition evaluates to false.
Instead, the origin server MAY indicate that the conditional request Instead, the origin server MAY indicate that the conditional request
failed by responding with a 412 (Precondition Failed) status code. failed by responding with a 412 (Precondition Failed) status code.
Alternatively, if the request is a state-changing operation that Alternatively, if the request is a state-changing operation that
appears to have already been applied to the selected representation, appears to have already been applied to the selected representation,
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(i.e., the change requested by the user agent has already succeeded, (i.e., the change requested by the user agent has already succeeded,
but the user agent might not be aware of it, perhaps because the but the user agent might not be aware of it, perhaps because the
prior response was lost or an equivalent change was made by some prior response was lost or an equivalent change was made by some
other user agent). other user agent).
Allowing an origin server to send a success response when a change Allowing an origin server to send a success response when a change
request appears to have already been applied is more efficient for request appears to have already been applied is more efficient for
many authoring use cases, but comes with some risk if multiple user many authoring use cases, but comes with some risk if multiple user
agents are making change requests that are very similar but not agents are making change requests that are very similar but not
cooperative. For example, multiple user agents writing to a common cooperative. For example, multiple user agents writing to a common
resource as a semaphore (e.g., a non-atomic increment) are likely to resource as a semaphore (e.g., a nonatomic increment) are likely to
collide and potentially lose important state transitions. For those collide and potentially lose important state transitions. For those
kinds of resources, an origin server is better off being stringent in kinds of resources, an origin server is better off being stringent in
sending 412 for every failed precondition on an unsafe method. In sending 412 for every failed precondition on an unsafe method. In
other cases, excluding the ETag field from a success response might other cases, excluding the ETag field from a success response might
encourage the user agent to perform a GET as its next request to encourage the user agent to perform a GET as its next request to
eliminate confusion about the resource's current state. eliminate confusion about the resource's current state.
A client MAY send an If-Match header field in a GET request to A client MAY send an If-Match header field in a GET request to
indicate that it would prefer a 412 (Precondition Failed) response if indicate that it would prefer a 412 (Precondition Failed) response if
the selected representation does not match. However, this is only the selected representation does not match. However, this is only
useful in range requests (Section 14), for completing a previously useful in range requests (Section 14) for completing a previously
received partial representation, when there is no desire for a new received partial representation when there is no desire for a new
representation. If-Range (Section 13.1.5) is better suited for range representation. If-Range (Section 13.1.5) is better suited for range
requests when the client prefers to receive a new representation. requests when the client prefers to receive a new representation.
A cache or intermediary MAY ignore If-Match because its A cache or intermediary MAY ignore If-Match because its
interoperability features are only necessary for an origin server. interoperability features are only necessary for an origin server.
Note that an If-Match header field with a list value containing "*" Note that an If-Match header field with a list value containing "*"
and other values (including other instances of "*") is syntactically and other values (including other instances of "*") is syntactically
invalid (therefore not allowed to be generated) and furthermore is invalid (therefore not allowed to be generated) and furthermore is
unlikely to be interoperable. unlikely to be interoperable.
13.1.2. If-None-Match 13.1.2. If-None-Match
The "If-None-Match" header field makes the request method conditional The "If-None-Match" header field makes the request method conditional
on a recipient cache or origin server either not having any current on a recipient cache or origin server either not having any current
representation of the target resource, when the field value is "*", representation of the target resource, when the field value is "*",
or having a selected representation with an entity-tag that does not or having a selected representation with an entity tag that does not
match any of those listed in the field value. match any of those listed in the field value.
A recipient MUST use the weak comparison function when comparing A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (Section 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.
If-None-Match can also be used with a value of "*" to prevent an If-None-Match can also be used with a value of "*" to prevent an
unsafe request method (e.g., PUT) from inadvertently modifying an unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client existing representation of the target resource when the client
believes that the resource does not have a current representation believes that the resource does not have a current representation
(Section 9.2.1). This is a variation on the "lost update" problem (Section 9.2.1). This is a variation on the "lost update" problem
that might arise if more than one client attempts to create an that might arise if more than one client attempts to create an
initial representation for the target resource. initial representation for the target resource.
When an origin server receives a request that selects a When an origin server receives a request that selects a
representation and that request includes an If-None-Match header representation and that request includes an If-None-Match header
field, the origin server MUST evaluate the If-None-Match condition as field, the origin server MUST evaluate the If-None-Match condition
per Section 13.2 prior to performing the method. per Section 13.2 prior to performing the method.
To evaluate a received If-None-Match header field: To evaluate a received If-None-Match header field:
1. If the field value is "*", the condition is false if the origin 1. If the field value is "*", the condition is false if the origin
server has a current representation for the target resource. server has a current representation for the target resource.
2. If the field value is a list of entity-tags, the condition is 2. If the field value is a list of entity tags, the condition is
false if one of the listed tags matches the entity-tag of the false if one of the listed tags matches the entity tag of the
selected representation. selected representation.
3. Otherwise, the condition is true. 3. Otherwise, the condition is true.
An origin server that evaluates an If-None-Match condition MUST NOT An origin server that evaluates an If-None-Match condition MUST NOT
perform the requested method if the condition evaluates to false; perform the requested method if the condition evaluates to false;
instead, the origin server MUST respond with either a) the 304 (Not instead, the origin server MUST respond with either a) the 304 (Not
Modified) status code if the request method is GET or HEAD or b) the Modified) status code if the request method is GET or HEAD or b) the
412 (Precondition Failed) status code for all other request methods. 412 (Precondition Failed) status code for all other request methods.
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HEAD. HEAD.
A recipient MUST ignore the If-Modified-Since header field if the A recipient MUST ignore the If-Modified-Since header field if the
resource does not have a modification date available. resource does not have a modification date available.
A recipient MUST interpret an If-Modified-Since field value's A recipient MUST interpret an If-Modified-Since field value's
timestamp in terms of the origin server's clock. timestamp in terms of the origin server's clock.
If-Modified-Since is typically used for two distinct purposes: 1) to If-Modified-Since is typically used for two distinct purposes: 1) to
allow efficient updates of a cached representation that does not have allow efficient updates of a cached representation that does not have
an entity-tag and 2) to limit the scope of a web traversal to an entity tag and 2) to limit the scope of a web traversal to
resources that have recently changed. resources that have recently changed.
When used for cache updates, a cache will typically use the value of When used for cache updates, a cache will typically use the value of
the cached message's Last-Modified header field to generate the field the cached message's Last-Modified header field to generate the field
value of If-Modified-Since. This behavior is most interoperable for value of If-Modified-Since. This behavior is most interoperable for
cases where clocks are poorly synchronized or when the server has cases where clocks are poorly synchronized or when the server has
chosen to only honor exact timestamp matches (due to a problem with chosen to only honor exact timestamp matches (due to a problem with
Last-Modified dates that appear to go "back in time" when the origin Last-Modified dates that appear to go "back in time" when the origin
server's clock is corrected or a representation is restored from an server's clock is corrected or a representation is restored from an
archived backup). However, caches occasionally generate the field archived backup). However, caches occasionally generate the field
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window, a user agent will generate an If-Modified-Since field value window, a user agent will generate an If-Modified-Since field value
based on either its own clock or a Date header field received from based on either its own clock or a Date header field received from
the server in a prior response. Origin servers that choose an exact the server in a prior response. Origin servers that choose an exact
timestamp match based on the selected representation's Last-Modified timestamp match based on the selected representation's Last-Modified
header field will not be able to help the user agent limit its data header field will not be able to help the user agent limit its data
transfers to only those changed during the specified window. transfers to only those changed during the specified window.
When an origin server receives a request that selects a When an origin server receives a request that selects a
representation and that request includes an If-Modified-Since header representation and that request includes an If-Modified-Since header
field without an If-None-Match header field, the origin server SHOULD field without an If-None-Match header field, the origin server SHOULD
evaluate the If-Modified-Since condition as per Section 13.2 prior to evaluate the If-Modified-Since condition per Section 13.2 prior to
performing the method. performing the method.
To evaluate a received If-Modified-Since header field: To evaluate a received If-Modified-Since header field:
1. If the selected representation's last modification date is 1. If the selected representation's last modification date is
earlier or equal to the date provided in the field value, the earlier or equal to the date provided in the field value, the
condition is false. condition is false.
2. Otherwise, the condition is true. 2. Otherwise, the condition is true.
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Requirements on cache handling of a received If-Modified-Since header Requirements on cache handling of a received If-Modified-Since header
field are defined in Section 4.3.2 of [CACHING]. field are defined in Section 4.3.2 of [CACHING].
13.1.4. If-Unmodified-Since 13.1.4. If-Unmodified-Since
The "If-Unmodified-Since" header field makes the request method The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field value. being earlier than or equal to the date provided in the field value.
This field accomplishes the same purpose as If-Match for cases where This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation. the user agent does not have an entity tag for the representation.
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
An example of the field is: An example of the field is:
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-
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A recipient MUST ignore the If-Unmodified-Since header field if the A recipient MUST ignore the If-Unmodified-Since header field if the
resource does not have a modification date available. resource does not have a modification date available.
A recipient MUST interpret an If-Unmodified-Since field value's A recipient MUST interpret an If-Unmodified-Since field value's
timestamp in terms of the origin server's clock. timestamp in terms of the origin server's clock.
If-Unmodified-Since is most often used with state-changing methods If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
multiple user agents might be acting in parallel on a resource that multiple user agents might be acting in parallel on a resource that
does not supply entity-tags with its representations (i.e., to does not supply entity tags with its representations (i.e., to
prevent the "lost update" problem). In general, it can be used with prevent the "lost update" problem). In general, it can be used with
any method that involves the selection or modification of a any method that involves the selection or modification of a
representation to abort the request if the selected representation's representation to abort the request if the selected representation's
last modification date has changed since the date provided in the If- last modification date has changed since the date provided in the If-
Unmodified-Since field value. Unmodified-Since field value.
When an origin server receives a request that selects a When an origin server receives a request that selects a
representation and that request includes an If-Unmodified-Since representation and that request includes an If-Unmodified-Since
header field without an If-Match header field, the origin server MUST header field without an If-Match header field, the origin server MUST
evaluate the If-Unmodified-Since condition as per Section 13.2 prior evaluate the If-Unmodified-Since condition per Section 13.2 prior to
to performing the method. performing the method.
To evaluate a received If-Unmodified-Since header field: To evaluate a received If-Unmodified-Since header field:
1. If the selected representation's last modification date is 1. If the selected representation's last modification date is
earlier than or equal to the date provided in the field value, earlier than or equal to the date provided in the field value,
the condition is true. the condition is true.
2. Otherwise, the condition is false. 2. Otherwise, the condition is false.
An origin server that evaluates an If-Unmodified-Since condition MUST An origin server that evaluates an If-Unmodified-Since condition MUST
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request appears to have already been applied is more efficient for request appears to have already been applied is more efficient for
many authoring use cases, but comes with some risk if multiple user many authoring use cases, but comes with some risk if multiple user
agents are making change requests that are very similar but not agents are making change requests that are very similar but not
cooperative. In those cases, an origin server is better off being cooperative. In those cases, an origin server is better off being
stringent in sending 412 for every failed precondition on an unsafe stringent in sending 412 for every failed precondition on an unsafe
method. method.
A client MAY send an If-Unmodified-Since header field in a GET A client MAY send an If-Unmodified-Since header field in a GET
request to indicate that it would prefer a 412 (Precondition Failed) request to indicate that it would prefer a 412 (Precondition Failed)
response if the selected representation has been modified. However, response if the selected representation has been modified. However,
this is only useful in range requests (Section 14), for completing a this is only useful in range requests (Section 14) for completing a
previously received partial representation, when there is no desire previously received partial representation when there is no desire
for a new representation. If-Range (Section 13.1.5) is better suited for a new representation. If-Range (Section 13.1.5) is better suited
for range requests when the client prefers to receive a new for range requests when the client prefers to receive a new
representation. representation.
A cache or intermediary MAY ignore If-Unmodified-Since because its A cache or intermediary MAY ignore If-Unmodified-Since because its
interoperability features are only necessary for an origin server. interoperability features are only necessary for an origin server.
13.1.5. If-Range 13.1.5. If-Range
The "If-Range" header field provides a special conditional request The "If-Range" header field provides a special conditional request
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examining the first three characters for a DQUOTE. examining the first three characters for a DQUOTE.
A client MUST NOT generate an If-Range header field in a request that A client MUST NOT generate an If-Range header field in a request that
does not contain a Range header field. A server MUST ignore an If- does not contain a Range header field. A server MUST ignore an If-
Range header field received in a request that does not contain a Range header field received in a request that does not contain a
Range header field. An origin server MUST ignore an If-Range header Range header field. An origin server MUST ignore an If-Range header
field received in a request for a target resource that does not field received in a request for a target resource that does not
support Range requests. support Range requests.
A client MUST NOT generate an If-Range header field containing an A client MUST NOT generate an If-Range header field containing an
entity-tag that is marked as weak. A client MUST NOT generate an If- entity tag that is marked as weak. A client MUST NOT generate an If-
Range header field containing an HTTP-date unless the client has no Range header field containing an HTTP-date unless the client has no
entity-tag for the corresponding representation and the date is a entity tag for the corresponding representation and the date is a
strong validator in the sense defined by Section 8.8.2.2. strong validator in the sense defined by Section 8.8.2.2.
A server that receives an If-Range header field on a Range request A server that receives an If-Range header field on a Range request
MUST evaluate the condition as per Section 13.2 prior to performing MUST evaluate the condition per Section 13.2 prior to performing the
the method. method.
To evaluate a received If-Range header field containing an HTTP-date: To evaluate a received If-Range header field containing an HTTP-date:
1. If the HTTP-date validator provided is not a strong validator in 1. If the HTTP-date validator provided is not a strong validator in
the sense defined by Section 8.8.2.2, the condition is false. the sense defined by Section 8.8.2.2, the condition is false.
2. If the HTTP-date validator provided exactly matches the 2. If the HTTP-date validator provided exactly matches the
Last-Modified field value for the selected representation, the Last-Modified field value for the selected representation, the
condition is true. condition is true.
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field value for the selected representation using the strong field value for the selected representation using the strong
comparison function (Section 8.8.3.2), the condition is true. comparison function (Section 8.8.3.2), the condition is true.
2. Otherwise, the condition is false. 2. Otherwise, the condition is false.
A recipient of an If-Range header field MUST ignore the Range header A recipient of an If-Range header field MUST ignore the Range header
field if the If-Range condition evaluates to false. Otherwise, the field if the If-Range condition evaluates to false. Otherwise, the
recipient SHOULD process the Range header field as requested. recipient SHOULD process the Range header field as requested.
Note that the If-Range comparison is by exact match, including when Note that the If-Range comparison is by exact match, including when
the validator is an HTTP-date, and so differs from the "earlier than the validator is an HTTP-date, and so it differs from the "earlier
or equal to" comparison used when evaluating an If-Unmodified-Since than or equal to" comparison used when evaluating an
conditional. If-Unmodified-Since conditional.
13.2. Evaluation of Preconditions 13.2. Evaluation of Preconditions
13.2.1. When to Evaluate 13.2.1. When to Evaluate
Except when excluded below, a recipient cache or origin server MUST Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully evaluate received request preconditions after it has successfully
performed its normal request checks and just before it would process performed its normal request checks and just before it would process
the request content (if any) or perform the action associated with the request content (if any) or perform the action associated with
the request method. A server MUST ignore all received preconditions the request method. A server MUST ignore all received preconditions
if its response to the same request without those conditions, prior if its response to the same request without those conditions, prior
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specification, and it MUST forward them if the request is forwarded, specification, and it MUST forward them if the request is forwarded,
since the generating client intends that they be evaluated by a since the generating client intends that they be evaluated by a
server that can provide a current representation. Likewise, a server server that can provide a current representation. Likewise, a server
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
preconditions are evaluated or the consequences of their evaluation. preconditions are evaluated or the consequences of their evaluation.
For example, the "immutable" cache directive (defined by [RFC8246]) For example, the immutable cache directive (defined by [RFC8246])
instructs caches to forgo forwarding conditional requests when they instructs caches to forgo forwarding conditional requests when they
hold a fresh response. hold a fresh response.
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.
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recipients not implementing this feature (or not supporting it for recipients not implementing this feature (or not supporting it for
the target resource) can respond as if it is a normal GET request the target resource) can respond as if it is a normal GET request
without impacting interoperability. Partial responses are indicated without impacting interoperability. Partial responses are indicated
by a distinct status code to not be mistaken for full responses by by a distinct status code to not be mistaken for full responses by
caches that might not implement the feature. caches that might not implement the feature.
14.1. Range Units 14.1. Range Units
Representation data can be partitioned into subranges when there are Representation data can be partitioned into subranges when there are
addressable structural units inherent to that data's content coding addressable structural units inherent to that data's content coding
or media type. For example, octet (a.k.a., byte) boundaries are a or media type. For example, octet (a.k.a. byte) boundaries are a
structural unit common to all representation data, allowing structural unit common to all representation data, allowing
partitions of the data to be identified as a range of bytes at some partitions of the data to be identified as a range of bytes at some
offset from the start or end of that data. offset from the start or end of that data.
This general notion of a _range unit_ is used in the Accept-Ranges This general notion of a "range unit" is used in the Accept-Ranges
(Section 14.3) response header field to advertise support for range (Section 14.3) response header field to advertise support for range
requests, the Range (Section 14.2) request header field to delineate requests, the Range (Section 14.2) request header field to delineate
the parts of a representation that are requested, and the the parts of a representation that are requested, and the
Content-Range (Section 14.4) header field to describe which part of a Content-Range (Section 14.4) header field to describe which part of a
representation is being transferred. representation is being transferred.
range-unit = token range-unit = token
All range unit names are case-insensitive and ought to be registered All range unit names are case-insensitive and ought to be registered
within the "HTTP Range Unit Registry", as defined in Section 16.5.1. within the "HTTP Range Unit Registry", as defined in Section 16.5.1.
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suffix-range = "-" suffix-length suffix-range = "-" suffix-length
suffix-length = 1*DIGIT suffix-length = 1*DIGIT
To provide for extensibility, the other-range rule is a mostly To provide for extensibility, the other-range rule is a mostly
unconstrained grammar that allows application-specific or future unconstrained grammar that allows application-specific or future
range units to define additional range specifiers. range units to define additional range specifiers.
other-range = 1*( %x21-2B / %x2D-7E ) other-range = 1*( %x21-2B / %x2D-7E )
; 1*(VCHAR excluding comma) ; 1*(VCHAR excluding comma)
A ranges-specifier is invalid if it contains any range-spec that is
invalid or undefined for the indicated range-unit.
A valid ranges-specifier is "satisfiable" if it contains at least one
range-spec that is satisfiable, as defined by the indicated
range-unit. Otherwise, the ranges-specifier is "unsatisfiable".
14.1.2. Byte Ranges 14.1.2. Byte Ranges
The "bytes" range unit is used to express subranges of a The "bytes" range unit is used to express subranges of a
representation data's octet sequence. Each byte range is expressed representation data's octet sequence. Each byte range is expressed
as an integer range at some offset, relative to either the beginning as an integer range at some offset, relative to either the beginning
(int-range) or end (suffix-range) of the representation data. Byte (int-range) or end (suffix-range) of the representation data. Byte
ranges do not use the other-range specifier. ranges do not use the other-range specifier.
The first-pos value in a bytes int-range gives the offset of the The first-pos value in a bytes int-range gives the offset of the
first byte in a range. The last-pos value gives the offset of the first byte in a range. The last-pos value gives the offset of the
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bytes=500-999 bytes=500-999
A client can limit the number of bytes requested without knowing the A client can limit the number of bytes requested without knowing the
size of the selected representation. If the last-pos value is size of the selected representation. If the last-pos value is
absent, or if the value is greater than or equal to the current absent, or if the value is greater than or equal to the current
length of the representation data, the byte range is interpreted as length of the representation data, the byte range is interpreted as
the remainder of the representation (i.e., the server replaces the the remainder of the representation (i.e., the server replaces the
value of last-pos with a value that is one less than the current value of last-pos with a value that is one less than the current
length of the selected representation). length of the selected representation).
A client can request the last N bytes (N > 0) of the selected A client can refer to 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): o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500 bytes=-500
skipping to change at page 139, line 21 skipping to change at page 139, line 29
o The first, middle, and last 1000 bytes: o 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 o 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 For a GET request, a valid bytes range-spec is satisfiable if it is
first-pos that is less than the current length of the representation, either:
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
unsatisfiable.
If the selected representation has zero length, the only satisfiable o an int-range with a first-pos that is less than the current length
form of range-spec is a suffix-range with a non-zero suffix-length. of the selected representation or
o a suffix-range with a non-zero suffix-length.
When a selected representation has zero length, the only satisfiable
form of range-spec in a GET request is a suffix-range with a non-zero
suffix-length.
In the byte-range syntax, first-pos, last-pos, and suffix-length are In the byte-range syntax, first-pos, last-pos, and suffix-length are
expressed as decimal number of octets. Since there is no predefined expressed as decimal number of octets. Since there is no predefined
limit to the length of content, recipients MUST anticipate limit to the length of content, recipients MUST anticipate
potentially large decimal numerals and prevent parsing errors due to potentially large decimal numerals and prevent parsing errors due to
integer conversion overflows. integer conversion overflows.
14.2. Range 14.2. Range
The "Range" header field on a GET request modifies the method The "Range" header field on a GET request modifies the method
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selected representation. selected representation.
Range = ranges-specifier Range = ranges-specifier
A server MAY ignore the Range header field. However, origin servers A server MAY ignore the Range header field. However, origin servers
and intermediate caches ought to support byte ranges when possible, and intermediate caches ought to support byte ranges when possible,
since they support efficient recovery from partially failed transfers since they support efficient recovery from partially failed transfers
and partial retrieval of large representations. and partial retrieval of large representations.
A server MUST ignore a Range header field received with a request A server MUST ignore a Range header field received with a request
method which is unrecognized or for which range handling is not method that is unrecognized or for which range handling is not
defined. For this specification, GET is the only method for which defined. For this specification, GET is the only method for which
range handling is defined. range handling is defined.
An origin server MUST ignore a Range header field that contains a An origin server MUST ignore a Range header field that contains a
range unit it does not understand. A proxy MAY discard a Range range unit it does not understand. A proxy MAY discard a Range
header field that contains a range unit it does not understand. header field that contains a range unit it does not understand.
A server that supports range requests MAY ignore or reject a Range A server that supports range requests MAY ignore or reject a Range
header field that consists of more than two overlapping ranges, or a header field that contains an invalid ranges-specifier
set of many small ranges that are not listed in ascending order, (Section 14.1.1), a ranges-specifier with more than two overlapping
since both are indications of either a broken client or a deliberate ranges, or a set of many small ranges that are not listed in
denial-of-service attack (Section 17.15). A client SHOULD NOT ascending order, since these are indications of either a broken
request multiple ranges that are inherently less efficient to process client or a deliberate denial-of-service attack (Section 17.15). A
and transfer than a single range that encompasses the same data. client SHOULD NOT request multiple ranges that are inherently less
efficient to process and transfer than a single range that
encompasses the same data.
A server that supports range requests MAY ignore a Range header field A server that supports range requests MAY ignore a Range header field
when the selected representation has no content (i.e., the selected when the selected representation has no content (i.e., the selected
representation's data is of zero length). representation's data is of zero length).
A client that is requesting multiple ranges SHOULD list those ranges A client that is requesting multiple ranges SHOULD list those ranges
in ascending order (the order in which they would typically be in ascending order (the order in which they would typically be
received in a complete representation) unless there is a specific received in a complete representation) unless there is a specific
need to request a later part earlier. For example, a user agent need to request a later part earlier. For example, a user agent
processing a large representation with an internal catalog of parts processing a large representation with an internal catalog of parts
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The Range header field is evaluated after evaluating the precondition The Range header field is evaluated after evaluating the precondition
header fields defined in Section 13.1, and only if the result in header fields defined in Section 13.1, and only if the result in
absence of the Range header field would be a 200 (OK) response. In absence of the Range header field would be a 200 (OK) response. In
other words, Range is ignored when a conditional GET would result in other words, Range is ignored when a conditional GET would result in
a 304 (Not Modified) response. a 304 (Not Modified) response.
The If-Range header field (Section 13.1.5) can be used as a The If-Range header field (Section 13.1.5) can be used as a
precondition to applying the Range header field. precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are header field for the target resource, the received Range field-value
valid and satisfiable (as defined in Section 14.1.2), the server contains a valid ranges-specifier with a range-unit supported for
SHOULD send a 206 (Partial Content) response with a content that target resource, and that ranges-specifier is satisfiable with
containing one or more partial representations that correspond to the respect to the selected representation, the server SHOULD send a 206
satisfiable ranges requested. (Partial Content) response with content containing one or more
partial representations that correspond to the satisfiable
range-spec(s) requested.
The above does not imply that a server will send all requested The above does not imply that a server will send all requested
ranges. In some cases, it may only be possible (or efficient) to ranges. In some cases, it may only be possible (or efficient) to
send a portion of the requested ranges first, while expecting the send a portion of the requested ranges first, while expecting the
client to re-request the remaining portions later if they are still client to re-request the remaining portions later if they are still
desired (see Section 15.3.7). desired (see Section 15.3.7).
If all of the preconditions are true, the server supports the Range If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are header field for the target resource, the received Range field-value
invalid or unsatisfiable, the server SHOULD send a 416 (Range Not contains a valid ranges-specifier, and either the range-unit is not
Satisfiable) response. supported for that target resource or the ranges-specifier is
unsatisfiable with respect to the selected representation, the server
SHOULD send a 416 (Range Not Satisfiable) response.
14.3. Accept-Ranges 14.3. Accept-Ranges
The "Accept-Ranges" field in a response indicates whether an upstream The "Accept-Ranges" field in a response indicates whether an upstream
server supports range requests for the target resource. server supports range requests for the target resource.
Accept-Ranges = acceptable-ranges Accept-Ranges = acceptable-ranges
acceptable-ranges = 1#range-unit acceptable-ranges = 1#range-unit
For example, a server that supports byte-range requests For example, a server that supports byte-range requests
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preferred to be sent as a header field because the information is preferred to be sent as a header field because the information is
particularly useful for restarting large information transfers that particularly useful for restarting large information transfers that
have failed in mid-content (before the trailer section is received). have failed in mid-content (before the trailer section is received).
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 (Section 14.6), and sent in 416 (Range Not
responses to provide information about the selected representation. Satisfiable) responses to provide information about the selected
representation.
Content-Range = range-unit SP Content-Range = range-unit SP
( range-resp / unsatisfied-range ) ( range-resp / unsatisfied-range )
range-resp = incl-range "/" ( complete-length / "*" ) range-resp = incl-range "/" ( complete-length / "*" )
incl-range = first-pos "-" last-pos incl-range = first-pos "-" last-pos
unsatisfied-range = "*/" complete-length unsatisfied-range = "*/" complete-length
complete-length = 1*DIGIT complete-length = 1*DIGIT
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specifically defined for partial updates (for example, the PATCH specifically defined for partial updates (for example, the PATCH
method defined in [RFC5789]). method defined in [RFC5789]).
14.6. Media Type multipart/byteranges 14.6. Media Type multipart/byteranges
When a 206 (Partial Content) response message includes the content of When a 206 (Partial Content) response message includes the content of
multiple ranges, they are transmitted as body parts in a multipart multiple ranges, they are transmitted as body parts in a multipart
message body ([RFC2046], Section 5.1) with the media type of message body ([RFC2046], Section 5.1) with the media type of
"multipart/byteranges". "multipart/byteranges".
The multipart/byteranges media type includes one or more body parts, The "multipart/byteranges" media type includes one or more body
each with its own Content-Type and Content-Range fields. The parts, each with its own Content-Type and Content-Range fields. The
required boundary parameter specifies the boundary string used to required boundary parameter specifies the boundary string used to
separate each body part. separate each body part.
Implementation Notes: Implementation Notes:
1. Additional CRLFs might precede the first boundary string in the 1. Additional CRLFs might precede the first boundary string in the
body. body.
2. Although [RFC2046] permits the boundary string to be quoted, some 2. Although [RFC2046] permits the boundary string to be quoted, some
existing implementations handle a quoted boundary string existing implementations handle a quoted boundary string
incorrectly. incorrectly.
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
x-byteranges , which is almost (but not quite) compatible with "multipart/x-byteranges", which is almost (but not quite)
this type. compatible with 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
skipping to change at page 145, line 33 skipping to change at page 145, line 47
...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
Optional parameters: N/A Optional parameters: N/A
Encoding considerations: only "7bit", "8bit", or "binary" are Encoding considerations: only "7bit", "8bit", or "binary" are
permitted permitted
Security considerations: see Section 17 Security considerations: see Section 17
Interoperability considerations: N/A Interoperability considerations: N/A
Published specification: This specification (see Section 14.6). Published specification: RFC 9110 (see Section 14.6)
Applications that use this media type: HTTP components supporting Applications that use this media type: HTTP components supporting
multiple ranges in a single request. multiple ranges in a single request
Fragment identifier considerations: N/A Fragment identifier considerations: N/A
Additional information: Deprecated alias names for this type: N/A Additional information: Deprecated alias names for this type: N/A
Magic number(s): N/A Magic number(s): N/A
File extension(s): N/A File extension(s): N/A
Macintosh file type code(s): N/A Macintosh file type code(s): N/A
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Request) status code. The response message will usually contain a Request) status code. The response message will usually contain a
representation that explains the status. representation that explains the status.
Values outside the range 100..599 are invalid. Implementations often Values outside the range 100..599 are invalid. Implementations often
use three-digit integer values outside of that range (i.e., 600..999) use three-digit integer values outside of that range (i.e., 600..999)
for internal communication of non-HTTP status (e.g., library errors). for internal communication of non-HTTP status (e.g., library errors).
A client that receives a response with an invalid status code SHOULD 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. 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
affecting the protocol. affecting the protocol.
Responses with status codes that are defined as heuristically Responses with status codes that are defined as heuristically
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definition or explicit cache controls [CACHING]; all other status definition or explicit cache controls [CACHING]; all other status
codes are not heuristically cacheable. codes are not heuristically cacheable.
Additional status codes, outside the scope of this specification, Additional status codes, outside the scope of this specification,
have been specified for use in HTTP. All such status codes ought to have been specified for use in HTTP. All such status codes ought to
be registered within the "Hypertext Transfer Protocol (HTTP) Status be registered within the "Hypertext Transfer Protocol (HTTP) Status
Code Registry", as described in Section 16.2. Code Registry", as described in Section 16.2.
15.2. Informational 1xx 15.2. Informational 1xx
The _1xx (Informational)_ class of status code indicates an interim The 1xx (Informational) class of status code indicates an interim
response for communicating connection status or request progress response for communicating connection status or request progress
prior to completing the requested action and sending a final prior to completing the requested action and sending a final
response. Since HTTP/1.0 did not define any 1xx status codes, a response. Since HTTP/1.0 did not define any 1xx status codes, a
server MUST NOT send a 1xx response to an HTTP/1.0 client. server MUST NOT send a 1xx response to an HTTP/1.0 client.
A 1xx response is terminated by the end of the header section; it A 1xx response is terminated by the end of the header section; it
cannot contain content or trailers. cannot contain content or trailers.
A client MUST be able to parse one or more 1xx responses received A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one. A prior to a final response, even if the client does not expect one. A
user agent MAY ignore unexpected 1xx responses. user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself requested A proxy MUST forward 1xx responses unless the proxy itself requested
the generation of the 1xx response. For example, if a proxy adds an the generation of the 1xx response. For example, if a proxy adds an
"Expect: 100-continue" header field when it forwards a request, then "Expect: 100-continue" header field when it forwards a request, then
it need not forward the corresponding 100 (Continue) response(s). it need not forward the corresponding 100 (Continue) response(s).
15.2.1. 100 Continue 15.2.1. 100 Continue
The _100 (Continue)_ status code indicates that the initial part of a The 100 (Continue) status code indicates that the initial part of a
request has been received and has not yet been rejected by the request has been received and has not yet been rejected by the
server. The server intends to send a final response after the server. The server intends to send a final response after the
request has been fully received and acted upon. request has been fully received and acted upon.
When the request contains an Expect header field that includes a When the request contains an Expect header field that includes a
100-continue expectation, the 100 response indicates that the server 100-continue expectation, the 100 response indicates that the server
wishes to receive the request content, as described in wishes to receive the request content, as described in
Section 10.1.1. The client ought to continue sending the request and Section 10.1.1. The client ought to continue sending the request and
discard the 100 response. discard the 100 response.
If the request did not contain an Expect header field containing the If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim 100-continue expectation, the client can simply discard this interim
response. response.
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 in effect after this response. protocol(s) will be in effect after this 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
client's request was successfully received, understood, and accepted. 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 | | HEAD | the target resource, like GET, but without |
| | transferring the representation data | | | transferring the representation data |
| POST | the status of, or results obtained from, | | POST | the status of, or results obtained from, |
| | the action | | | the action |
| PUT, DELETE | the status of the action | | PUT, DELETE | the status of the action |
| OPTIONS | communication options for the target | | OPTIONS | communication options for the target |
| | resource | | | resource |
| TRACE | the request message as received by the | | TRACE | the request message as received by the |
| | server returning the trace | | | server returning the trace |
+----------------+--------------------------------------------+ +----------------+--------------------------------------------+
Table 6 Table 6
Aside from responses to CONNECT, a 200 response is expected to Aside from responses to CONNECT, a 200 response is expected to
contain message content unless the message framing explicitly contain message content unless the message framing explicitly
indicates that the content has zero length. If some aspect of the indicates that the content has zero length. If some aspect of the
request indicates a preference for no content upon success, the request indicates a preference for no content upon success, the
origin server ought to send a _204 (No Content)_ response instead. origin server ought to send a 204 (No Content) response instead. For
For CONNECT, there is no content because the successful result is a CONNECT, there is no content because the successful result is a
tunnel, which begins immediately after the 200 response header tunnel, which begins immediately after the 200 response header
section. section.
A 200 response is heuristically cacheable; i.e., unless otherwise A 200 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
In 200 responses to GET or HEAD, an origin server SHOULD send any In 200 responses to GET or HEAD, an origin server SHOULD send any
available validator fields (Section 8.8) for the selected available validator fields (Section 8.8) for the selected
representation, with both a strong entity-tag and a Last-Modified representation, with both a strong entity tag and a Last-Modified
date being preferred. date being preferred.
In 200 responses to state-changing methods, any validator fields In 200 responses to state-changing methods, any validator fields
(Section 8.8) sent in the response convey the current validators for (Section 8.8) sent in the response convey the current validators for
the new representation formed as a result of successfully applying the new representation formed as a result of successfully applying
the request semantics. Note that the PUT method (Section 9.3.4) has the request semantics. Note that the PUT method (Section 9.3.4) has
additional requirements that might preclude sending such validators. additional requirements that might preclude sending such validators.
15.3.2. 201 Created 15.3.2. 201 Created
The _201 (Created)_ status code indicates that the request has been The 201 (Created) status code indicates that the request has been
fulfilled and has resulted in one or more new resources being fulfilled and has resulted in one or more new resources being
created. The primary resource created by the request is identified created. The primary resource created by the request is identified
by either a Location header field in the response or, if no Location by either a Location header field in the response or, if no Location
header field is received, by the target URI. header field is received, by the target URI.
The 201 response content typically describes and links to the The 201 response content typically describes and links to the
resource(s) created. Any validator fields (Section 8.8) sent in the resource(s) created. Any validator fields (Section 8.8) sent in the
response convey the current validators for a new representation response convey the current validators for a new representation
created by the request. Note that the PUT method (Section 9.3.4) has created by the request. Note that the PUT method (Section 9.3.4) has
additional requirements that might preclude sending such validators. additional requirements that might preclude sending such validators.
15.3.3. 202 Accepted 15.3.3. 202 Accepted
The _202 (Accepted)_ status code indicates that the request has been The 202 (Accepted) status code indicates that the request has been
accepted for processing, but the processing has not been completed. accepted for processing, but the processing has not been completed.
The request might or might not eventually be acted upon, as it might The request might or might not eventually be acted upon, as it might
be disallowed when processing actually takes place. There is no be disallowed when processing actually takes place. There is no
facility in HTTP for re-sending a status code from an asynchronous facility in HTTP for re-sending a status code from an asynchronous
operation. operation.
The 202 response is intentionally noncommittal. Its purpose is to The 202 response is intentionally noncommittal. Its purpose is to
allow a server to accept a request for some other process (perhaps a allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist requiring that the user agent's connection to the server persist
until the process is completed. The representation sent with this until the process is completed. The representation sent with this
response ought to describe the request's current status and point to response ought to describe the request's current status and point to
(or embed) a status monitor that can provide the user with an (or embed) a status monitor that can provide the user with an
estimate of when the request will be fulfilled. estimate of when the request will be fulfilled.
15.3.4. 203 Non-Authoritative Information 15.3.4. 203 Non-Authoritative Information
The _203 (Non-Authoritative Information)_ status code indicates that The 203 (Non-Authoritative Information) status code indicates that
the request was successful but the enclosed content has been modified the request was successful but the enclosed content has been modified
from that of the origin server's 200 (OK) response by a transforming from that of the origin server's 200 (OK) response by a transforming
proxy (Section 7.7). This status code allows the proxy to notify proxy (Section 7.7). This status code allows the proxy to notify
recipients when a transformation has been applied, since that recipients when a transformation has been applied, since that
knowledge might impact later decisions regarding the content. For knowledge might impact later decisions regarding the content. For
example, future cache validation requests for the content might only example, future cache validation requests for the content might only
be applicable along the same request path (through the same proxies). be applicable along the same request path (through the same proxies).
A 203 response is heuristically cacheable; i.e., unless otherwise A 203 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.3.5. 204 No Content 15.3.5. 204 No Content
The _204 (No Content)_ status code indicates that the server has The 204 (No Content) status code indicates that the server has
successfully fulfilled the request and that there is no additional successfully fulfilled the request and that there is no additional
content to send in the response content. Metadata in the response content to send in the response content. Metadata in the response
header fields refer to the target resource and its selected header fields refer to the target resource and its selected
representation after the requested action was applied. representation after the requested action was applied.
For example, if a 204 status code is received in response to a PUT For example, if a 204 status code is received in response to a PUT
request and the response contains an ETag field, then the PUT was request and the response contains an ETag field, then the PUT was
successful and the ETag field value contains the entity-tag for the successful and the ETag field value contains the entity tag for the
new representation of that target resource. new representation of that target resource.
The 204 response allows a server to indicate that the action has been The 204 response allows a server to indicate that the action has been
successfully applied to the target resource, while implying that the successfully applied to the target resource, while implying that the
user agent does not need to traverse away from its current "document user agent does not need to traverse away from its current "document
view" (if any). The server assumes that the user agent will provide view" (if any). The server assumes that the user agent will provide
some indication of the success to its user, in accord with its own some indication of the success to its user, in accord with its own
interface, and apply any new or updated metadata in the response to interface, and apply any new or updated metadata in the response to
its active representation. its active representation.
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A 204 response is terminated by the end of the header section; it A 204 response is terminated by the end of the header section; it
cannot contain content or trailers. cannot contain content or trailers.
A 204 response is heuristically cacheable; i.e., unless otherwise A 204 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.3.6. 205 Reset Content 15.3.6. 205 Reset Content
The _205 (Reset Content)_ status code indicates that the server has The 205 (Reset Content) status code indicates that the server has
fulfilled the request and desires that the user agent reset the fulfilled the request and desires that the user agent reset the
"document view", which caused the request to be sent, to its original "document view", which caused the request to be sent, to its original
state as received from the origin server. state as received from the origin server.
This response is intended to support a common data entry use case This response is intended to support a common data entry use case
where the user receives content that supports data entry (a form, where the user receives content that supports data entry (a form,
notepad, canvas, etc.), enters or manipulates data in that space, notepad, canvas, etc.), enters or manipulates data in that space,
causes the entered data to be submitted in a request, and then the causes the entered data to be submitted in a request, and then the
data entry mechanism is reset for the next entry so that the user can data entry mechanism is reset for the next entry so that the user can
easily initiate another input action. easily initiate another input action.
Since the 205 status code implies that no additional content will be Since the 205 status code implies that no additional content will be
provided, a server MUST NOT generate content in a 205 response. provided, a server MUST NOT generate content in a 205 response.
15.3.7. 206 Partial Content 15.3.7. 206 Partial Content
The _206 (Partial Content)_ status code indicates that the server is The 206 (Partial Content) status code indicates that the server is
successfully fulfilling a range request for the target resource by successfully fulfilling a range request for the target resource by
transferring one or more parts of the selected representation. transferring one or more parts of the selected representation.
A server that supports range requests (Section 14) will usually A server that supports range requests (Section 14) will usually
attempt to satisfy all of the requested ranges, since sending less attempt to satisfy all of the requested ranges, since sending less
data will likely result in another client request for the remainder. data will likely result in another client request for the remainder.
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
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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.
A sender that generates a 206 response to a request with an If-Range A sender that generates a 206 response to a request with an If-Range
header field SHOULD NOT generate other representation header fields header field SHOULD NOT generate other representation header fields
beyond those required, because the client already has a prior beyond those required because the client already has a prior response
response containing those header fields. Otherwise, a sender MUST containing those header fields. Otherwise, a sender MUST generate
generate all of the representation header fields that would have been all of the representation header fields that would have been sent in
sent in a 200 (OK) response to the same request. a 200 (OK) response to the same 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
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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
To avoid confusion with single-part responses, a server MUST NOT parameter. To avoid confusion with single-part responses, a server
generate a Content-Range header field in the HTTP header section of a MUST NOT generate a Content-Range header field in the HTTP header
multiple part response (this field will be sent in each part section of a multiple part response (this field will be sent in each
instead). part 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
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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
if multiple ranges were requested and only one range was found to be part if multiple ranges were requested and only one range was found
satisfiable or only one range remained after coalescing. A client to be satisfiable or only one range remained after coalescing. A
that cannot process a multipart/byteranges response MUST NOT generate client that cannot process a "multipart/byteranges" response MUST NOT
a request that asks for multiple ranges. generate a request that asks for multiple ranges.
A server that generates a multipart response SHOULD send the parts in A server that generates a multipart response SHOULD send the parts in
the same order that the corresponding range-spec appeared in the the same order that the corresponding range-spec appeared in the
received Range header field, excluding those ranges that were deemed received Range header field, excluding those ranges that were deemed
unsatisfiable or that were coalesced into other ranges. A client unsatisfiable or that were coalesced into other ranges. A client
that receives a multipart response MUST inspect the Content-Range that receives a multipart response MUST inspect the Content-Range
header field present in each body part in order to determine which header field present in each body part in order to determine which
range is contained in that body part; a client cannot rely on range is contained in that body part; a client cannot rely on
receiving the same ranges that it requested, nor the same order that receiving the same ranges that it requested, nor the same order that
it requested. it requested.
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The combined response content consists of the union of partial The combined response content consists of the union of partial
content ranges within the new response and all of the matching stored content ranges within the new response and all of the matching stored
responses. If the union consists of the entire range of the responses. If the union consists of the entire range of the
representation, then the client MUST process the combined response as representation, then the client MUST process the combined response as
if it were a complete 200 (OK) response, including a Content-Length if it were a complete 200 (OK) response, including a Content-Length
header field that reflects the complete length. Otherwise, the header field that reflects the complete length. Otherwise, the
client MUST process the set of continuous ranges as one of the client MUST process the set of continuous ranges as one of the
following: an incomplete 200 (OK) response if the combined response following: an incomplete 200 (OK) response if the combined response
is a prefix of the representation, a single 206 (Partial Content) is a prefix of the representation, a single 206 (Partial Content)
response containing multipart/byteranges content, or multiple 206 response containing "multipart/byteranges" content, or multiple 206
(Partial Content) responses, each with one continuous range that is (Partial Content) responses, each with one continuous range that is
indicated by a Content-Range header field. indicated by a Content-Range header field.
15.4. Redirection 3xx 15.4. Redirection 3xx
The _3xx (Redirection)_ class of status code indicates that further The 3xx (Redirection) class of status code indicates that further
action needs to be taken by the user agent in order to fulfill the action needs to be taken by the user agent in order to fulfill the
request. There are several types of redirects: request. There are several types of redirects:
1. Redirects that indicate this resource might be available at a 1. Redirects that indicate this resource might be available at a
different URI, as provided by the Location header field, as in different URI, as provided by the Location header field, as in
the status codes 301 (Moved Permanently), 302 (Found), 307 the status codes 301 (Moved Permanently), 302 (Found), 307
(Temporary Redirect), and 308 (Permanent Redirect). (Temporary Redirect), and 308 (Permanent Redirect).
2. Redirection that offers a choice among matching resources capable 2. Redirection that offers a choice among matching resources capable
of representing this resource, as in the 300 (Multiple Choices) of representing this resource, as in the 300 (Multiple Choices)
skipping to change at page 156, line 32 skipping to change at page 156, line 38
| and 302 (Found) were originally defined as method-preserving | and 302 (Found) were originally defined as method-preserving
| ([HTTP/1.0], Section 9.3) to match their implementation at | ([HTTP/1.0], Section 9.3) to match their implementation at
| CERN; 303 (See Other) was defined for a redirection that | CERN; 303 (See Other) was defined for a redirection that
| changed its method to GET. However, early user agents split on | changed its method to GET. However, early user agents split on
| whether to redirect POST requests as POST (according to then- | whether to redirect POST requests as POST (according to then-
| current specification) or as GET (the safer alternative when | current specification) or as GET (the safer alternative when
| redirected to a different site). Prevailing practice | redirected to a different site). Prevailing practice
| eventually converged on changing the method to GET. 307 | eventually converged on changing the method to GET. 307
| (Temporary Redirect) and 308 (Permanent Redirect) [RFC7538] | (Temporary Redirect) and 308 (Permanent Redirect) [RFC7538]
| were later added to unambiguously indicate method-preserving | were later added to unambiguously indicate method-preserving
| redirects, and 301/302 have been adjusted to allow a POST | redirects, and status codes 301 and 302 have been adjusted to
| request to be redirected as GET. | allow a POST request to be redirected as GET.
If a Location header field (Section 10.2.2) is provided, the user If a Location header field (Section 10.2.2) is provided, the user
agent MAY automatically redirect its request to the URI referenced by agent MAY automatically redirect its request to the URI referenced by
the Location field value, even if the specific status code is not the Location field value, even if the specific status code is not
understood. Automatic redirection needs to be done with care for understood. Automatic redirection needs to be done with care for
methods not known to be safe, as defined in Section 9.2.1, since the methods not known to be safe, as defined in Section 9.2.1, since the
user might not wish to redirect an unsafe request. user might not wish to redirect an unsafe request.
When automatically following a redirected request, the user agent When automatically following a redirected request, the user agent
SHOULD resend the original request message with the following SHOULD resend the original request message with the following
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1. Connection-specific header fields (see Section 7.6.1), 1. Connection-specific header fields (see Section 7.6.1),
2. Header fields specific to the client's proxy configuration, 2. Header fields specific to the client's proxy configuration,
including (but not limited to) Proxy-Authorization, including (but not limited to) Proxy-Authorization,
3. Origin-specific header fields (if any), including (but not 3. Origin-specific header fields (if any), including (but not
limited to) Host, limited to) Host,
4. Validating header fields that were added by the 4. Validating header fields that were added by the
implementation's cache (e.g., If-None-Match, implementation's cache (e.g., If-None-Match,
If-Modified-Since), If-Modified-Since), and
5. Resource-specific header fields, including (but not limited 5. Resource-specific header fields, including (but not limited
to) Referer, Origin, Authorization, and Cookie. to) Referer, Origin, Authorization, and Cookie.
3. Consider removing header fields that were not automatically 3. Consider removing header fields that were not automatically
generated by the implementation (i.e., those present in the generated by the implementation (i.e., those present in the
request because they were added by the calling context) where request because they were added by the calling context) where
there are security implications; this includes but is not limited there are security implications; this includes but is not limited
to Authorization and Cookie. to Authorization and Cookie.
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A client SHOULD detect and intervene in cyclical redirections (i.e., A client SHOULD detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops). "infinite" redirection loops).
| *Note:* An earlier version of this specification recommended a | *Note:* An earlier version of this specification recommended a
| maximum of five redirections ([RFC2068], Section 10.3). | maximum of five redirections ([RFC2068], Section 10.3).
| Content developers need to be aware that some clients might | Content developers need to be aware that some clients might
| implement such a fixed limitation. | implement such a fixed limitation.
15.4.1. 300 Multiple Choices 15.4.1. 300 Multiple Choices
The _300 (Multiple Choices)_ status code indicates that the target The 300 (Multiple Choices) status code indicates that the target
resource has more than one representation, each with its own more resource has more than one representation, each with its own more
specific identifier, and information about the alternatives is being specific identifier, and information about the alternatives is being
provided so that the user (or user agent) can select a preferred provided so that the user (or user agent) can select a preferred
representation by redirecting its request to one or more of those representation by redirecting its request to one or more of those
identifiers. In other words, the server desires that the user agent identifiers. In other words, the server desires that the user agent
engage in reactive negotiation to select the most appropriate engage in reactive negotiation to select the most appropriate
representation(s) for its needs (Section 12). representation(s) for its needs (Section 12).
If the server has a preferred choice, the server SHOULD generate a If the server has a preferred choice, the server SHOULD generate a
Location header field containing a preferred choice's URI reference. Location header field containing a preferred choice's URI reference.
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| 406 responses and be transferred in responses to the HEAD | 406 responses and be transferred in responses to the HEAD
| method. However, lack of deployment and disagreement over | method. However, lack of deployment and disagreement over
| syntax led to both URI and Alternates (a subsequent proposal) | syntax led to both URI and Alternates (a subsequent proposal)
| being dropped from this specification. It is possible to | being dropped from this specification. It is possible to
| communicate the list as a Link header field value [RFC8288] | communicate the list as a Link header field value [RFC8288]
| whose members have a relationship of "alternate", though | whose members have a relationship of "alternate", though
| deployment is a chicken-and-egg problem. | deployment is a chicken-and-egg problem.
15.4.2. 301 Moved Permanently 15.4.2. 301 Moved Permanently
The _301 (Moved Permanently)_ status code indicates that the target The 301 (Moved Permanently) status code indicates that the target
resource has been assigned a new permanent URI and any future resource has been assigned a new permanent URI and any future
references to this resource ought to use one of the enclosed URIs. references to this resource ought to use one of the enclosed URIs.
The server is suggesting that a user agent with link-editing The server is suggesting that a user agent with link-editing
capability can permanently replace references to the target URI with capability can permanently replace references to the target URI with
one of the new references sent by the server. However, this one of the new references sent by the server. However, this
suggestion is usually ignored unless the user agent is actively suggestion is usually ignored unless the user agent is actively
editing references (e.g., engaged in authoring content), the editing references (e.g., engaged in authoring content), the
connection is secured, and the origin server is a trusted authority connection is secured, and the origin server is a trusted authority
for the content being edited. for the content being edited.
skipping to change at page 159, line 22 skipping to change at page 159, line 35
| request method from POST to GET for the subsequent request. If | request method from POST to GET for the subsequent request. If
| this behavior is undesired, the 308 (Permanent Redirect) status | this behavior is undesired, the 308 (Permanent Redirect) status
| code can be used instead. | code can be used instead.
A 301 response is heuristically cacheable; i.e., unless otherwise A 301 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.4.3. 302 Found 15.4.3. 302 Found
The _302 (Found)_ status code indicates that the target resource The 302 (Found) status code indicates that the target resource
resides temporarily under a different URI. Since the redirection resides temporarily under a different URI. Since the redirection
might be altered on occasion, the client ought to continue to use the might be altered on occasion, the client ought to continue to use the
target URI for future requests. target URI for future requests.
The server SHOULD generate a Location header field in the response The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server's use the Location field value for automatic redirection. The server's
response content usually contains a short hypertext note with a response content usually contains a short hypertext note with a
hyperlink to the different URI(s). hyperlink to the different URI(s).
| *Note:* For historical reasons, a user agent MAY change the | *Note:* For historical reasons, a user agent MAY change the
| request method from POST to GET for the subsequent request. If | request method from POST to GET for the subsequent request. If
| this behavior is undesired, the 307 (Temporary Redirect) status | this behavior is undesired, the 307 (Temporary Redirect) status
| code can be used instead. | code can be used instead.
15.4.4. 303 See Other 15.4.4. 303 See Other
The _303 (See Other)_ status code indicates that the server is The 303 (See Other) status code indicates that the server is
redirecting the user agent to a different resource, as indicated by a redirecting the user agent to a different resource, as indicated by a
URI in the Location header field, which is intended to provide an URI in the Location header field, which is intended to provide an
indirect response to the original request. A user agent can perform indirect response to the original request. A user agent can perform
a retrieval request targeting that URI (a GET or HEAD request if a retrieval request targeting that URI (a GET or HEAD request if
using HTTP), which might also be redirected, and present the eventual using HTTP), which might also be redirected, and present the eventual
result as an answer to the original request. Note that the new URI result as an answer to the original request. Note that the new URI
in the Location header field is not considered equivalent to the in the Location header field is not considered equivalent to the
target URI. target URI.
This status code is applicable to any HTTP method. It is primarily This status code is applicable to any HTTP method. It is primarily
skipping to change at page 160, line 28 skipping to change at page 160, line 40
answers to the questions of what can be represented, what answers to the questions of what can be represented, what
representations are adequate, and what might be a useful description representations are adequate, and what might be a useful description
are outside the scope of HTTP. are outside the scope of HTTP.
Except for responses to a HEAD request, the representation of a 303 Except for responses to a HEAD request, the representation of a 303
response ought to contain a short hypertext note with a hyperlink to response ought to contain a short hypertext note with a hyperlink to
the same URI reference provided in the Location header field. the same URI reference provided in the Location header field.
15.4.5. 304 Not Modified 15.4.5. 304 Not Modified
The _304 (Not Modified)_ status code indicates that a conditional GET The 304 (Not Modified) status code indicates that a conditional GET
or HEAD request has been received and would have resulted in a 200 or HEAD request has been received and would have resulted in a 200
(OK) response if it were not for the fact that the condition (OK) response if it were not for the fact that the condition
evaluated to false. In other words, there is no need for the server evaluated to false. In other words, there is no need for the server
to transfer a representation of the target resource because the to transfer a representation of the target resource because the
request indicates that the client, which made the request request indicates that the client, which made the request
conditional, already has a valid representation; the server is conditional, already has a valid representation; the server is
therefore redirecting the client to make use of that stored therefore redirecting the client to make use of that stored
representation as if it were the content of a 200 (OK) response. representation as if it were the content of a 200 (OK) response.
The server generating a 304 response MUST generate any of the The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK) following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date, response to the same request:
ETag, Expires, and Vary.
o Content-Location, Date, ETag, and Vary
o Cache-Control and Expires (see [CACHING])
Since the goal of a 304 response is to minimize information transfer Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field). response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in Requirements on a cache that receives a 304 response are defined in
Section 4.3.4 of [CACHING]. If the conditional request originated Section 4.3.4 of [CACHING]. If the conditional request originated
with an outbound client, such as a user agent with its own cache with an outbound client, such as a user agent with its own cache
sending a conditional GET to a shared proxy, then the proxy SHOULD sending a conditional GET to a shared proxy, then the proxy SHOULD
forward the 304 response to that client. forward the 304 response to that client.
A 304 response is terminated by the end of the header section; it A 304 response is terminated by the end of the header section; it
cannot contain content or trailers. cannot contain content or trailers.
15.4.6. 305 Use Proxy 15.4.6. 305 Use Proxy
The _305 (Use Proxy)_ status code was defined in a previous version The 305 (Use Proxy) status code was defined in a previous version of
of this specification and is now deprecated (Appendix B of this specification and is now deprecated (Appendix B of [RFC7231]).
[RFC7231]).
15.4.7. 306 (Unused) 15.4.7. 306 (Unused)
The 306 status code was defined in a previous version of this The 306 status code was defined in a previous version of this
specification, is no longer used, and the code is reserved. specification, is no longer used, and the code is reserved.
15.4.8. 307 Temporary Redirect 15.4.8. 307 Temporary Redirect
The _307 (Temporary Redirect)_ status code indicates that the target The 307 (Temporary Redirect) status code indicates that the target
resource resides temporarily under a different URI and the user agent resource resides temporarily under a different URI and the user agent
MUST NOT change the request method if it performs an automatic MUST NOT change the request method if it performs an automatic
redirection to that URI. Since the redirection can change over time, redirection to that URI. Since the redirection can change over time,
the client ought to continue using the original target URI for future the client ought to continue using the original target URI for future
requests. requests.
The server SHOULD generate a Location header field in the response The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server's use the Location field value for automatic redirection. The server's
response content usually contains a short hypertext note with a response content usually contains a short hypertext note with a
hyperlink to the different URI(s). hyperlink to the different URI(s).
15.4.9. 308 Permanent Redirect 15.4.9. 308 Permanent Redirect
The _308 (Permanent Redirect)_ status code indicates that the target The 308 (Permanent Redirect) status code indicates that the target
resource has been assigned a new permanent URI and any future resource has been assigned a new permanent URI and any future
references to this resource ought to use one of the enclosed URIs. references to this resource ought to use one of the enclosed URIs.
The server is suggesting that a user agent with link-editing The server is suggesting that a user agent with link-editing
capability can permanently replace references to the target URI with capability can permanently replace references to the target URI with
one of the new references sent by the server. However, this one of the new references sent by the server. However, this
suggestion is usually ignored unless the user agent is actively suggestion is usually ignored unless the user agent is actively
editing references (e.g., engaged in authoring content), the editing references (e.g., engaged in authoring content), the
connection is secured, and the origin server is a trusted authority connection is secured, and the origin server is a trusted authority
for the content being edited. for the content being edited.
skipping to change at page 162, line 16 skipping to change at page 162, line 29
containing a preferred URI reference for the new permanent URI. The containing a preferred URI reference for the new permanent URI. The
user agent MAY use the Location field value for automatic user agent MAY use the Location field value for automatic
redirection. The server's response content usually contains a short redirection. The server's response content usually contains a short
hypertext note with a hyperlink to the new URI(s). hypertext note with a hyperlink to the new URI(s).
A 308 response is heuristically cacheable; i.e., unless otherwise A 308 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
| *Note:* This status code is much younger (June 2014) than its | *Note:* This status code is much younger (June 2014) than its
| sibling codes, and thus might not be recognized everywhere. | sibling codes and thus might not be recognized everywhere. See
| See Section 4 of [RFC7538] for deployment considerations. | Section 4 of [RFC7538] for deployment considerations.
15.5. Client Error 4xx 15.5. Client Error 4xx
The _4xx (Client Error)_ class of status code indicates that the The 4xx (Client Error) class of status code indicates that the client
client seems to have erred. Except when responding to a HEAD seems to have erred. Except when responding to a HEAD request, the
request, the server SHOULD send a representation containing an server SHOULD send a representation containing an explanation of the
explanation of the error situation, and whether it is a temporary or error situation, and whether it is a temporary or permanent
permanent condition. These status codes are applicable to any condition. These status codes are applicable to any request method.
request method. User agents SHOULD display any included User agents SHOULD display any included representation to the user.
representation to the user.
15.5.1. 400 Bad Request 15.5.1. 400 Bad Request
The _400 (Bad Request)_ status code indicates that the server cannot The 400 (Bad Request) status code indicates that the server cannot or
or will not process the request due to something that is perceived to will not process the request due to something that is perceived to be
be a client error (e.g., malformed request syntax, invalid request a client error (e.g., malformed request syntax, invalid request
message framing, or deceptive request routing). message framing, or deceptive request routing).
15.5.2. 401 Unauthorized 15.5.2. 401 Unauthorized
The _401 (Unauthorized)_ status code indicates that the request has The 401 (Unauthorized) status code indicates that the request has not
not been applied because it lacks valid authentication credentials been applied because it lacks valid authentication credentials for
for the target resource. The server generating a 401 response MUST the target resource. The server generating a 401 response MUST send
send a WWW-Authenticate header field (Section 11.6.1) containing at a WWW-Authenticate header field (Section 11.6.1) containing at least
least one challenge applicable to the target resource. one challenge applicable to the target resource.
If the request included authentication credentials, then the 401 If the request included authentication credentials, then the 401
response indicates that authorization has been refused for those response indicates that authorization has been refused for those
credentials. The user agent MAY repeat the request with a new or credentials. The user agent MAY repeat the request with a new or
replaced Authorization header field (Section 11.6.2). If the 401 replaced Authorization header field (Section 11.6.2). If the 401
response contains the same challenge as the prior response, and the response contains the same challenge as the prior response, and the
user agent has already attempted authentication at least once, then user agent has already attempted authentication at least once, then
the user agent SHOULD present the enclosed representation to the the user agent SHOULD present the enclosed representation to the
user, since it usually contains relevant diagnostic information. user, since it usually contains relevant diagnostic information.
15.5.3. 402 Payment Required 15.5.3. 402 Payment Required
The _402 (Payment Required)_ status code is reserved for future use. The 402 (Payment Required) status code is reserved for future use.
15.5.4. 403 Forbidden 15.5.4. 403 Forbidden
The _403 (Forbidden)_ status code indicates that the server The 403 (Forbidden) status code indicates that the server understood
understood the request but refuses to fulfill it. A server that the request but refuses to fulfill it. A server that wishes to make
wishes to make public why the request has been forbidden can describe public why the request has been forbidden can describe that reason in
that reason in the response content (if any). the response content (if any).
If authentication credentials were provided in the request, the If authentication credentials were provided in the request, the
server considers them insufficient to grant access. The client server considers them insufficient to grant access. The client
SHOULD NOT automatically repeat the request with the same SHOULD NOT automatically repeat the request with the same
credentials. The client MAY repeat the request with new or different credentials. The client MAY repeat the request with new or different
credentials. However, a request might be forbidden for reasons credentials. However, a request might be forbidden for reasons
unrelated to the credentials. unrelated to the credentials.
An origin server that wishes to "hide" the current existence of a An origin server that wishes to "hide" the current existence of a
forbidden target resource MAY instead respond with a status code of forbidden target resource MAY instead respond with a status code of
404 (Not Found). 404 (Not Found).
15.5.5. 404 Not Found 15.5.5. 404 Not Found
The _404 (Not Found)_ status code indicates that the origin server The 404 (Not Found) status code indicates that the origin server did
did not find a current representation for the target resource or is not find a current representation for the target resource or is not
not willing to disclose that one exists. A 404 status code does not willing to disclose that one exists. A 404 status code does not
indicate whether this lack of representation is temporary or indicate whether this lack of representation is temporary or
permanent; the 410 (Gone) status code is preferred over 404 if the permanent; the 410 (Gone) status code is preferred over 404 if the
origin server knows, presumably through some configurable means, that origin server knows, presumably through some configurable means, that
the condition is likely to be permanent. the condition is likely to be permanent.
A 404 response is heuristically cacheable; i.e., unless otherwise A 404 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.5.6. 405 Method Not Allowed 15.5.6. 405 Method Not Allowed
The _405 (Method Not Allowed)_ status code indicates that the method The 405 (Method Not Allowed) status code indicates that the method
received in the request-line is known by the origin server but not received in the request-line is known by the origin server but not
supported by the target resource. The origin server MUST generate an supported by the target resource. The origin server MUST generate an
Allow header field in a 405 response containing a list of the target Allow header field in a 405 response containing a list of the target
resource's currently supported methods. resource's currently supported methods.
A 405 response is heuristically cacheable; i.e., unless otherwise A 405 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.5.7. 406 Not Acceptable 15.5.7. 406 Not Acceptable
The _406 (Not Acceptable)_ status code indicates that the target The 406 (Not Acceptable) status code indicates that the target
resource does not have a current representation that would be resource does not have a current representation that would be
acceptable to the user agent, according to the proactive negotiation acceptable to the user agent, according to the proactive negotiation
header fields received in the request (Section 12.1), and the server header fields received in the request (Section 12.1), and the server
is unwilling to supply a default representation. is unwilling to supply a default representation.
The server SHOULD generate content containing a list of available The server SHOULD generate content containing a list of available
representation characteristics and corresponding resource identifiers representation characteristics and corresponding resource identifiers
from which the user or user agent can choose the one most from which the user or user agent can choose the one most
appropriate. A user agent MAY automatically select the most appropriate. A user agent MAY automatically select the most
appropriate choice from that list. However, this specification does appropriate choice from that list. However, this specification does
not define any standard for such automatic selection, as described in not define any standard for such automatic selection, as described in
Section 15.4.1. Section 15.4.1.
15.5.8. 407 Proxy Authentication Required 15.5.8. 407 Proxy Authentication Required
The _407 (Proxy Authentication Required)_ status code is similar to The 407 (Proxy Authentication Required) status code is similar to 401
401 (Unauthorized), but it indicates that the client needs to (Unauthorized), but it indicates that the client needs to
authenticate itself in order to use a proxy for this request. The authenticate itself in order to use a proxy for this request. The
proxy MUST send a Proxy-Authenticate header field (Section 11.7.1) proxy MUST send a Proxy-Authenticate header field (Section 11.7.1)
containing a challenge applicable to that proxy for the request. The containing a challenge applicable to that proxy for the request. The
client MAY repeat the request with a new or replaced client MAY repeat the request with a new or replaced
Proxy-Authorization header field (Section 11.7.2). Proxy-Authorization header field (Section 11.7.2).
15.5.9. 408 Request Timeout 15.5.9. 408 Request Timeout
The _408 (Request Timeout)_ status code indicates that the server did The 408 (Request Timeout) status code indicates that the server did
not receive a complete request message within the time that it was not receive a complete request message within the time that it was
prepared to wait. prepared to wait.
If the client has an outstanding request in transit, it MAY repeat If the client has an outstanding request in transit, it MAY repeat
that request. If the current connection is not usable (e.g., as it that request. If the current connection is not usable (e.g., as it
would be in HTTP/1.1, because request delimitation is lost), a new would be in HTTP/1.1 because request delimitation is lost), a new
connection will be used. connection will be used.
15.5.10. 409 Conflict 15.5.10. 409 Conflict
The _409 (Conflict)_ status code indicates that the request could not The 409 (Conflict) status code indicates that the request could not
be completed due to a conflict with the current state of the target be completed due to a conflict with the current state of the target
resource. This code is used in situations where the user might be resource. This code is used in situations where the user might be
able to resolve the conflict and resubmit the request. The server able to resolve the conflict and resubmit the request. The server
SHOULD generate content that includes enough information for a user SHOULD generate content that includes enough information for a user
to recognize the source of the conflict. to recognize the source of the conflict.
Conflicts are most likely to occur in response to a PUT request. For Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being example, if versioning were being used and the representation being
PUT included changes to a resource that conflict with those made by PUT included changes to a resource that conflict with those made by
an earlier (third-party) request, the origin server might use a 409 an earlier (third-party) request, the origin server might use a 409
response to indicate that it can't complete the request. In this response to indicate that it can't complete the request. In this
case, the response representation would likely contain information case, the response representation would likely contain information
useful for merging the differences based on the revision history. useful for merging the differences based on the revision history.
15.5.11. 410 Gone 15.5.11. 410 Gone
The _410 (Gone)_ status code indicates that access to the target The 410 (Gone) status code indicates that access to the target
resource is no longer available at the origin server and that this resource is no longer available at the origin server and that this
condition is likely to be permanent. If the origin server does not condition is likely to be permanent. If the origin server does not
know, or has no facility to determine, whether or not the condition know, or has no facility to determine, whether or not the condition
is permanent, the status code 404 (Not Found) ought to be used is permanent, the status code 404 (Not Found) ought to be used
instead. instead.
The 410 response is primarily intended to assist the task of web The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common remote links to that resource be removed. Such an event is common
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is not necessary to mark all permanently unavailable resources as is not necessary to mark all permanently unavailable resources as
"gone" or to keep the mark for any length of time -- that is left to "gone" or to keep the mark for any length of time -- that is left to
the discretion of the server owner. the discretion of the server owner.
A 410 response is heuristically cacheable; i.e., unless otherwise A 410 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.5.12. 411 Length Required 15.5.12. 411 Length Required
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
more conditions given in the request header fields evaluated to false conditions given in the request header fields evaluated to false when
when tested on the server (Section 13). This response status code tested on the server (Section 13). This response status code allows
allows the client to place preconditions on the current resource the client to place preconditions on the current resource state (its
state (its current representations and metadata) and, thus, prevent current representations and metadata) and, thus, prevent the request
the request method from being applied if the target resource is in an method from being applied if the target resource is in an unexpected
unexpected state. 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
is refusing to process a request because the request content is refusing to process a request because the request content is larger
larger than the server is willing or able to process. The server MAY 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
Retry-After header field to indicate that it is temporary and after Retry-After header field to indicate that it is temporary and after
what time the client MAY try again. what time the client MAY try again.
15.5.15. 414 URI Too Long 15.5.15. 414 URI Too Long
The _414 (URI Too Long)_ status code indicates that the server is The 414 (URI Too Long) status code indicates that the server is
refusing to service the request because the target URI is longer than refusing to service the request because the target URI is longer than
the server is willing to interpret. This rare condition is only the server is willing to interpret. This rare condition is only
likely to occur when a client has improperly converted a POST request likely to occur when a client has improperly converted a POST request
to a GET request with long query information, when the client has to a GET request with long query information, when the client has
descended into a "black hole" of redirection (e.g., a redirected URI descended into an infinite loop of redirection (e.g., a redirected
prefix that points to a suffix of itself) or when the server is under URI prefix that points to a suffix of itself) or when the server is
attack by a client attempting to exploit potential security holes. under attack by a client attempting to exploit potential security
holes.
A 414 response is heuristically cacheable; i.e., unless otherwise A 414 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.5.16. 415 Unsupported Media Type 15.5.16. 415 Unsupported Media Type
The _415 (Unsupported Media Type)_ status code indicates that the The 415 (Unsupported Media Type) status code indicates that the
origin server is refusing to service the request because the content origin server is refusing to service the request because the content
is in a format not supported by this method on the target resource. is in a format not supported by this method on the target resource.
The format problem might be due to the request's indicated The format problem might be due to the request's indicated
Content-Type or Content-Encoding, or as a result of inspecting the Content-Type or Content-Encoding, or as a result of inspecting the
data directly. data directly.
If the problem was caused by an unsupported content coding, the If the problem was caused by an unsupported content coding, the
Accept-Encoding response header field (Section 12.5.3) ought to be Accept-Encoding response header field (Section 12.5.3) ought to be
used to indicate what (if any) content codings would have been used to indicate which (if any) content codings would have been
accepted in the request. accepted in the request.
On the other hand, if the cause was an unsupported media type, the On the other hand, if the cause was an unsupported media type, the
Accept response header field (Section 12.5.1) can be used to indicate Accept response header field (Section 12.5.1) can be used to indicate
what media types would have been accepted in the request. which media types would have been accepted in the request.
15.5.17. 416 Range Not Satisfiable 15.5.17. 416 Range Not Satisfiable
The _416 (Range Not Satisfiable)_ status code indicates that the set The 416 (Range Not Satisfiable) status code indicates that the set of
of ranges in the request's Range header field (Section 14.2) has been 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.
A server that generates a 416 response to a byte-range request SHOULD A server that generates a 416 response to a byte-range request SHOULD
generate a Content-Range header field specifying the current length generate a Content-Range header field specifying the current length
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A server that generates a 416 response to a byte-range request SHOULD A server that generates a 416 response to a byte-range request SHOULD
generate a Content-Range header field specifying the current length generate a Content-Range header field specifying the current length
of the selected representation (Section 14.4). of the selected representation (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.
15.5.18. 417 Expectation Failed 15.5.18. 417 Expectation Failed
The _417 (Expectation Failed)_ status code indicates that the The 417 (Expectation Failed) status code indicates that the
expectation given in the request's Expect header field expectation given in the request's Expect header field
(Section 10.1.1) could not be met by at least one of the inbound (Section 10.1.1) could not be met by at least one of the inbound
servers. servers.
15.5.19. 418 (Unused) 15.5.19. 418 (Unused)
[RFC2324] was an April 1 RFC that lampooned the various ways HTTP was [RFC2324] was an April 1 RFC that lampooned the various ways HTTP was
abused; one such abuse was the definition of an application-specific abused; one such abuse was the definition of an application-specific
418 status code. In the intervening years, this status code has been 418 status code, which has been deployed as a joke often enough for
widely implemented as an "Easter Egg", and therefore is effectively the code to be unusable for any future use.
consumed by this use.
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
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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 [ALTSVC]. target resource's origin, or via an alternative service [ALTSVC].
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
The 422 (Unprocessable Content) status code indicates that the server The 422 (Unprocessable Content) status code indicates that the server
understands the content type of the request content (hence a 415 understands the content type of the request content (hence a 415
(Unsupported Media Type) status code is inappropriate), and the (Unsupported Media Type) status code is inappropriate), and the
syntax of the request content is correct, but was unable to process syntax of the request content is correct, but it was unable to
the contained instructions. For example, this status code can be process the contained instructions. For example, this status code
sent if an XML request content contains well-formed (i.e., can be sent if an XML request content contains well-formed (i.e.,
syntactically correct), but semantically erroneous XML instructions. syntactically correct), but semantically erroneous XML instructions.
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
server is aware that it has erred or is incapable of performing the 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 status codes are applicable to any request
method. method.
15.6.1. 500 Internal Server Error 15.6.1. 500 Internal Server Error
The _500 (Internal Server Error)_ status code indicates that the The 500 (Internal Server Error) status code indicates that the server
server encountered an unexpected condition that prevented it from encountered an unexpected condition that prevented it from fulfilling
fulfilling the request. the request.
15.6.2. 501 Not Implemented 15.6.2. 501 Not Implemented
The _501 (Not Implemented)_ status code indicates that the server The 501 (Not Implemented) status code indicates that the server does
does not support the functionality required to fulfill the request. not support the functionality required to fulfill the request. This
This is the appropriate response when the server does not recognize is the appropriate response when the server does not recognize the
the request method and is not capable of supporting it for any request method and is not capable of supporting it for any resource.
resource.
A 501 response is heuristically cacheable; i.e., unless otherwise A 501 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [CACHING]). Section 4.2.2 of [CACHING]).
15.6.3. 502 Bad Gateway 15.6.3. 502 Bad Gateway
The _502 (Bad Gateway)_ status code indicates that the server, while The 502 (Bad Gateway) status code indicates that the server, while
acting as a gateway or proxy, received an invalid response from an acting as a gateway or proxy, received an invalid response from an
inbound server it accessed while attempting to fulfill the request. inbound server it accessed while attempting to fulfill the request.
15.6.4. 503 Service Unavailable 15.6.4. 503 Service Unavailable
The _503 (Service Unavailable)_ status code indicates that the server The 503 (Service Unavailable) status code indicates that the server
is currently unable to handle the request due to a temporary overload is currently unable to handle the request due to a temporary overload
or scheduled maintenance, which will likely be alleviated after some or scheduled maintenance, which will likely be alleviated after some
delay. The server MAY send a Retry-After header field delay. The server MAY send a Retry-After header field
(Section 10.2.3) to suggest an appropriate amount of time for the (Section 10.2.3) to suggest an appropriate amount of time for the
client to wait before retrying the request. client to wait before retrying the request.
| *Note:* The existence of the 503 status code does not imply | *Note:* The existence of the 503 status code does not imply
| that a server has to use it when becoming overloaded. Some | that a server has to use it when becoming overloaded. Some
| servers might simply refuse the connection. | servers might simply refuse the connection.
15.6.5. 504 Gateway Timeout 15.6.5. 504 Gateway Timeout
The _504 (Gateway Timeout)_ status code indicates that the server, The 504 (Gateway Timeout) status code indicates that the server,
while acting as a gateway or proxy, did not receive a timely response while acting as a gateway or proxy, did not receive a timely response
from an upstream server it needed to access in order to complete the from an upstream server it needed to access in order to complete the
request. request.
15.6.6. 505 HTTP Version Not Supported 15.6.6. 505 HTTP Version Not Supported
The _505 (HTTP Version Not Supported)_ status code indicates that the The 505 (HTTP Version Not Supported) status code indicates that the
server does not support, or refuses to support, the major version of server does not support, or refuses to support, the major version of
HTTP that was used in the request message. The server is indicating HTTP that was used in the request message. The server is indicating
that it is unable or unwilling to complete the request using the same that it is unable or unwilling to complete the request using the same
major version as the client, as described in Section 2.5, other than major version as the client, as described in Section 2.5, other than
with this error message. The server SHOULD generate a representation with this error message. The server SHOULD generate a representation
for the 505 response that describes why that version is not supported for the 505 response that describes why that version is not supported
and what other protocols are supported by that server. and what other protocols are supported by that server.
16. Extending HTTP 16. Extending HTTP
HTTP defines a number of generic extension points that can be used to HTTP defines a number of generic extension points that can be used to
introduce capabilities to the protocol without introducing a new introduce capabilities to the protocol without introducing a new
version, including methods, status codes, field names, and further version, including methods, status codes, field names, and further
extensibility points within defined fields, such as authentication extensibility points within defined fields, such as authentication
schemes and cache-directives (see Cache-Control extensions in schemes and cache directives (see Cache-Control extensions in
Section 5.2.3 of [CACHING]). Because the semantics of HTTP are not Section 5.2.3 of [CACHING]). Because the semantics of HTTP are not
versioned, these extension points are persistent; the version of the versioned, these extension points are persistent; the version of the
protocol in use does not affect their semantics. protocol in use does not affect their semantics.
Version-independent extensions are discouraged from depending on or Version-independent extensions are discouraged from depending on or
interacting with the specific version of the protocol in use. When interacting with the specific version of the protocol in use. When
this is unavoidable, careful consideration needs to be given to how this is unavoidable, careful consideration needs to be given to how
the extension can interoperate across versions. the extension can interoperate across versions.
Additionally, specific versions of HTTP might have their own Additionally, specific versions of HTTP might have their own
extensibility points, such as transfer-codings in HTTP/1.1 extensibility points, such as transfer codings in HTTP/1.1
(Section 6.1 of [HTTP/1.1]) and HTTP/2 ([HTTP/2]) SETTINGS or frame (Section 6.1 of [HTTP/1.1]) and HTTP/2 SETTINGS or frame types
types. These extension points are specific to the version of the ([HTTP/2]). These extension points are specific to the version of
protocol they occur within. the protocol they occur within.
Version-specific extensions cannot override or modify the semantics Version-specific extensions cannot override or modify the semantics
of a version-independent mechanism or extension point (like a method of a version-independent mechanism or extension point (like a method
or header field) without explicitly being allowed by that protocol or header field) without explicitly being allowed by that protocol
element. For example, the CONNECT method (Section 9.3.6) allows element. For example, the CONNECT method (Section 9.3.6) allows
this. this.
These guidelines assure that the protocol operates correctly and These guidelines assure that the protocol operates correctly and
predictably, even when parts of the path implement different versions predictably, even when parts of the path implement different versions
of HTTP. of HTTP.
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The definition of a new status code ought to explain the request The definition of a new status code ought to explain the request
conditions that would cause a response containing that status code conditions that would cause a response containing that status code
(e.g., combinations of request header fields and/or method(s)) along (e.g., combinations of request header fields and/or method(s)) along
with any dependencies on response header fields (e.g., what fields with any dependencies on response header fields (e.g., what fields
are required, what fields can modify the semantics, and what field are required, what fields can modify the semantics, and what field
semantics are further refined when used with the new status code). semantics are further refined when used with the new status code).
By default, a status code applies only to the request corresponding By default, a status code applies only to the request corresponding
to the response it occurs within. If a status code applies to a to the response it occurs within. If a status code applies to a
larger scope of applicability -- for example, all requests to the larger scope of applicability -- for example, all requests to the
resource in question, or all requests to a server -- this must be resource in question or all requests to a server -- this must be
explicitly specified. When doing so, it should be noted that not all explicitly specified. When doing so, it should be noted that not all
clients can be expected to consistently apply a larger scope, because clients can be expected to consistently apply a larger scope because
they might not understand the new status code. they might not understand the new status code.
The definition of a new final status code ought to specify whether or The definition of a new final status code ought to specify whether or
not it is heuristically cacheable. Note that all final status codes not it is heuristically cacheable. Note that any response with a
can be cached if the response they occur in has explicit freshness final status code can be cached if the response has explicit
information; however, those status codes that are defined as being freshness information. A status code defined as heuristically
heuristically cacheable are allowed to be cached without explicit cacheable is allowed to be cached without explicit freshness
freshness information. Likewise, the definition of a status code can information. Likewise, the definition of a status code can place
place constraints upon cache behavior, if the 'must-understand' cache constraints upon cache behavior if the must-understand cache
directive is used. See [CACHING] for more information. directive is used. See [CACHING] for more information.
Finally, the definition of a new status code ought to indicate Finally, the definition of a new status code ought to indicate
whether the content has any implied association with an identified whether the content has any implied association with an identified
resource (Section 6.4.2). resource (Section 6.4.2).
16.3. Field Extensibility 16.3. Field Extensibility
HTTP's most widely used extensibility point is the definition of new HTTP's most widely used extensibility point is the definition of new
header and trailer fields. header and trailer fields.
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Any party can request registration of an 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@w3.org" located there or by sending an email to the "ietf-http-wg@w3.org"
mailing list. mailing list.
Field names are registered on the advice of a Designated Expert Field names are registered on the advice of a designated expert
(appointed by the IESG or their delegate). Fields with the status (appointed by the IESG or their delegate). Fields with the status
'permanent' are Specification Required ([RFC8126], Section 4.6). 'permanent' are Specification Required ([RFC8126], Section 4.6).
Registration requests consist of the following information: Registration requests consist of the following information:
Field name: Field name:
The requested field name. It MUST conform to the field-name The requested field name. It MUST conform to the field-name
syntax defined in Section 5.1, and SHOULD be restricted to just syntax defined in Section 5.1, and it SHOULD be restricted to just
letters, digits, and hyphen ('-') characters, with the first letters, digits, and hyphen ('-') characters, with the first
character being a letter. character being a letter.
Status: Status:
"permanent" or "provisional". "permanent", "provisional", "deprecated", or "obsoleted".
Specification document(s): Specification document(s):
Reference to the document that specifies the field, preferably Reference to the document that specifies the field, preferably
including a URI that can be used to retrieve a copy of the including a URI that can be used to retrieve a copy of the
document. Optional but encouraged for provisional registrations. document. Optional but encouraged for provisional registrations.
An indication of the relevant section(s) can also be included, but An indication of the relevant section(s) can also be included, but
is not required. is not required.
And, optionally: And optionally:
Comments: Additional information, such as about reserved entries. Comments: Additional information, such as about reserved entries.
The Expert(s) can define additional fields to be collected in the The expert(s) can define additional fields to be collected in the
registry, in consultation with the community. registry, in consultation with the community.
Standards-defined names have a status of "permanent". Other names Standards-defined names have a status of "permanent". Other names
can also be registered as permanent, if the Expert(s) find that they can also be registered as permanent if the expert(s) finds that they
are in use, in consultation with the community. Other names should are in use, in consultation with the community. Other names should
be registered as "provisional". be registered as "provisional".
Provisional entries can be removed by the Expert(s) if -- in Provisional entries can be removed by the expert(s) if -- in
consultation with the community -- the Expert(s) find that they are consultation with the community -- the expert(s) find that they are
not in use. The Experts can change a provisional entry's status to not in use. The expert(s) can change a provisional entry's status to
permanent at any time. permanent at any time.
Note that names can be registered by third parties (including the Note that names can be registered by third parties (including the
Expert(s)), if the Expert(s) determines that an unregistered name is expert(s)) if the expert(s) determines that an unregistered name is
widely deployed and not likely to be registered in a timely manner widely deployed and not likely to be registered in a timely manner
otherwise. otherwise.
16.3.2. Considerations for New Fields 16.3.2. Considerations for New Fields
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 o 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
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(see Section 6.5.1). (see Section 6.5.1).
o Whether it is appropriate or even required to list the field name o Whether it is appropriate or even required to list the field name
in the Connection header field (i.e., if the field is to be hop- in the Connection header field (i.e., if the field is to be hop-
by-hop; see Section 7.6.1). by-hop; see Section 7.6.1).
o Whether the field introduces any additional security o 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 behavior is not appropriate:
o If it is appropriate to list the field name in a Vary response o 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 o 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 o 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