frankenRFC723x_sem.txt   draft-ietf-httpbis-semantics-16.txt 
Internet Engineering Task Force (IETF) R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Request for Comments: 7231 Adobe Internet-Draft Adobe
Obsoletes: 2616 J. Reschke, Ed. Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235, M. Nottingham, Ed.
Updates: 2817 greenbytes 7538, 7615, 7694 (if approved) Fastly
Category: Standards Track June 2014 Updates: 3864 (if approved) J. Reschke, Ed.
ISSN: 2070-1721 Intended status: Standards Track greenbytes
Expires: 28 November 2021 27 May 2021
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content HTTP Semantics
draft-ietf-httpbis-semantics-16
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines the semantics of HTTP/1.1 messages, systems. This document describes the overall architecture of HTTP,
as expressed by request methods, request header fields, response establishes common terminology, and defines aspects of the protocol
status codes, and response header fields, along with the payload of that are shared by all versions. In this definition are core
messages (metadata and body content) and mechanisms for content protocol elements, extensibility mechanisms, and the "http" and
negotiation. "https" Uniform Resource Identifier (URI) schemes.
This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC
7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions
of RFC 7230.
Editorial Note Editorial Note
This note is not in the original RFC. This note is to be removed before publishing as an RFC.
The purpose of this document is to produce diffs that show just the Discussion of this draft takes place on the HTTP working group
changes from text in the original RFCs that were input for http-core. mailing list (ietf-http-wg@w3.org), which is archived at
Hence, the frankenRFC documents show all of the original text (including <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
stuff that has been deleted) plus some new text [in brackets] or new
headings to anchor the context, rearranged to minimize the resulting
diffs when compared to the most recently published version of
draft-ietf-httpbis-semantics.
After this document is updated to match any reorg changes in the latest Working Group information can be found at <https://httpwg.org/>;
version, the franken diffs are saved and published in this directory as source code and issues list for this draft can be found at
diff_semantics_frfc_to_NN.html (where NN is the I-D draft revision) <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix C.17.
Status of This Memo Status of This Memo
This is an Internet Standards Track document. This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
This document is a product of the Internet Engineering Task Force Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 28 November 2021.
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Table of Contents Table of Contents
1. Introduction ....................................................6 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1. Conformance and Error Handling .............................6 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2. Syntax Notation ............................................6 1.2. History and Evolution . . . . . . . . . . . . . . . . . . 10
2. Resources .......................................................7 1.3. Core Semantics . . . . . . . . . . . . . . . . . . . . . 11
3. Representations .................................................7 1.4. Specifications Obsoleted by this Document . . . . . . . . 11
3.1. Representation Metadata ....................................8 2. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.1. Processing Representation Data ......................8 2.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 12
3.1.2. Encoding for Compression or Integrity ..............11 2.2. Requirements Notation . . . . . . . . . . . . . . . . . . 13
3.1.3. Audience Language ..................................13 2.3. Length Requirements . . . . . . . . . . . . . . . . . . . 14
3.1.4. Identification .....................................14 2.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 15
3.2. Representation Data .......................................17 2.5. Protocol Version . . . . . . . . . . . . . . . . . . . . 15
3.3. Payload Semantics .........................................17 3. Terminology and Core Concepts . . . . . . . . . . . . . . . . 16
3.4. Content Negotiation .......................................18 3.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.1. Proactive Negotiation ..............................19 3.2. Representations . . . . . . . . . . . . . . . . . . . . . 17
3.4.2. Reactive Negotiation ...............................20 3.3. Connections, Clients and Servers . . . . . . . . . . . . 17
4. Request Methods ................................................21 3.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Overview ..................................................21 3.5. User Agents . . . . . . . . . . . . . . . . . . . . . . . 18
4.2. Common Method Properties ..................................22 3.6. Origin Server . . . . . . . . . . . . . . . . . . . . . . 19
4.2.1. Safe Methods .......................................22 3.7. Intermediaries . . . . . . . . . . . . . . . . . . . . . 20
4.2.2. Idempotent Methods .................................23 3.8. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.3. Cacheable Methods ..................................24 3.9. Example Message Exchange . . . . . . . . . . . . . . . . 22
4.3. Method Definitions ........................................24 4. Identifiers in HTTP . . . . . . . . . . . . . . . . . . . . . 23
4.3.1. GET ................................................24 4.1. URI References . . . . . . . . . . . . . . . . . . . . . 23
4.3.2. HEAD ...............................................25 4.2. HTTP-Related URI Schemes . . . . . . . . . . . . . . . . 24
4.3.3. POST ...............................................25 4.2.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 25
4.3.4. PUT ................................................26 4.2.2. https URI Scheme . . . . . . . . . . . . . . . . . . 25
4.3.5. DELETE .............................................29 4.2.3. http(s) Normalization and Comparison . . . . . . . . 26
4.3.6. CONNECT ............................................30 4.2.4. Deprecation of userinfo in http(s) URIs . . . . . . . 27
4.3.7. OPTIONS ............................................31 4.2.5. http(s) References with Fragment Identifiers . . . . 27
4.3.8. TRACE ..............................................32 4.3. Authoritative Access . . . . . . . . . . . . . . . . . . 28
5. Request Header Fields ..........................................33 4.3.1. URI Origin . . . . . . . . . . . . . . . . . . . . . 28
5.1. Controls ..................................................33 4.3.2. http origins . . . . . . . . . . . . . . . . . . . . 29
5.1.1. Expect .............................................34 4.3.3. https origins . . . . . . . . . . . . . . . . . . . . 30
5.1.2. Max-Forwards .......................................36 4.3.4. https certificate verification . . . . . . . . . . . 31
5.2. Conditionals ..............................................36 4.3.5. IP-ID reference identity . . . . . . . . . . . . . . 32
5.3. Content Negotiation .......................................37 5. Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.3.1. Quality Values .....................................37 5.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 32
5.3.2. Accept .............................................38 5.2. Field Lines and Combined Field Value . . . . . . . . . . 33
5.3.3. Accept-Charset .....................................40 5.3. Field Order . . . . . . . . . . . . . . . . . . . . . . . 34
5.3.4. Accept-Encoding ....................................41 5.4. Field Limits . . . . . . . . . . . . . . . . . . . . . . 35
5.3.5. Accept-Language ....................................42 5.5. Field Values . . . . . . . . . . . . . . . . . . . . . . 35
5.4. Authentication Credentials ................................44 5.6. Common Rules for Defining Field Values . . . . . . . . . 37
5.5. Request Context ...........................................44 5.6.1. Lists (#rule ABNF Extension) . . . . . . . . . . . . 37
5.5.1. From ...............................................44 5.6.1.1. Sender Requirements . . . . . . . . . . . . . . . 37
5.5.2. Referer ............................................45 5.6.1.2. Recipient Requirements . . . . . . . . . . . . . 38
5.5.3. User-Agent .........................................46 5.6.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 38
6. Response Status Codes ..........................................47 5.6.3. Whitespace . . . . . . . . . . . . . . . . . . . . . 39
6.1. Overview of Status Codes ..................................48 5.6.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 39
6.2. Informational 1xx .........................................50 5.6.5. Comments . . . . . . . . . . . . . . . . . . . . . . 40
6.2.1. 100 Continue .......................................50 5.6.6. Parameters . . . . . . . . . . . . . . . . . . . . . 40
6.2.2. 101 Switching Protocols ............................50 5.6.7. Date/Time Formats . . . . . . . . . . . . . . . . . . 41
6.3. Successful 2xx ............................................51 6. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 43
6.3.1. 200 OK .............................................51 6.1. Framing and Completeness . . . . . . . . . . . . . . . . 44
6.3.2. 201 Created ........................................52 6.2. Control Data . . . . . . . . . . . . . . . . . . . . . . 45
6.3.3. 202 Accepted .......................................52 6.3. Header Fields . . . . . . . . . . . . . . . . . . . . . . 46
6.3.4. 203 Non-Authoritative Information ..................52 6.4. Content . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.3.5. 204 No Content .....................................53 6.4.1. Content Semantics . . . . . . . . . . . . . . . . . . 46
6.3.6. 205 Reset Content ..................................53 6.4.2. Identifying Content . . . . . . . . . . . . . . . . . 47
6.4. Redirection 3xx ...........................................54 6.5. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 48
6.4.1. 300 Multiple Choices ...............................55 6.5.1. Limitations on use of Trailers . . . . . . . . . . . 49
6.4.2. 301 Moved Permanently ..............................56 6.5.2. Processing Trailer Fields . . . . . . . . . . . . . . 50
6.4.3. 302 Found ..........................................56 7. Routing HTTP Messages . . . . . . . . . . . . . . . . . . . . 50
6.4.4. 303 See Other ......................................57 7.1. Determining the Target Resource . . . . . . . . . . . . . 50
6.4.5. 305 Use Proxy ......................................58 7.2. Host and :authority . . . . . . . . . . . . . . . . . . . 51
6.4.6. 306 (Unused) .......................................58 7.3. Routing Inbound Requests . . . . . . . . . . . . . . . . 52
6.4.7. 307 Temporary Redirect .............................58 7.3.1. To a Cache . . . . . . . . . . . . . . . . . . . . . 52
6.5. Client Error 4xx ..........................................58 7.3.2. To a Proxy . . . . . . . . . . . . . . . . . . . . . 52
6.5.1. 400 Bad Request ....................................58 7.3.3. To the Origin . . . . . . . . . . . . . . . . . . . . 52
6.5.2. 402 Payment Required ...............................59 7.4. Rejecting Misdirected Requests . . . . . . . . . . . . . 53
6.5.3. 403 Forbidden ......................................59 7.5. Response Correlation . . . . . . . . . . . . . . . . . . 53
6.5.4. 404 Not Found ......................................59 7.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 54
6.5.5. 405 Method Not Allowed .............................59 7.6.1. Connection . . . . . . . . . . . . . . . . . . . . . 54
6.5.6. 406 Not Acceptable .................................60 7.6.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 56
6.5.7. 408 Request Timeout ................................60 7.6.3. Via . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.5.8. 409 Conflict .......................................60 7.7. Message Transformations . . . . . . . . . . . . . . . . . 58
6.5.9. 410 Gone ...........................................60 7.8. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.5.10. 411 Length Required ...............................61 8. Representation Data and Metadata . . . . . . . . . . . . . . 62
6.5.11. 413 Payload Too Large .............................61 8.1. Representation Data . . . . . . . . . . . . . . . . . . . 62
6.5.12. 414 URI Too Long ..................................61 8.2. Representation Metadata . . . . . . . . . . . . . . . . . 62
6.5.13. 415 Unsupported Media Type ........................62 8.3. Content-Type . . . . . . . . . . . . . . . . . . . . . . 62
6.5.14. 417 Expectation Failed ............................62 8.3.1. Media Type . . . . . . . . . . . . . . . . . . . . . 63
6.5.15. 426 Upgrade Required ..............................62 8.3.2. Charset . . . . . . . . . . . . . . . . . . . . . . . 64
6.6. Server Error 5xx ..........................................62 8.3.3. Multipart Types . . . . . . . . . . . . . . . . . . . 64
6.6.1. 500 Internal Server Error ..........................63 8.4. Content-Encoding . . . . . . . . . . . . . . . . . . . . 65
6.6.2. 501 Not Implemented ................................63 8.4.1. Content Codings . . . . . . . . . . . . . . . . . . . 66
6.6.3. 502 Bad Gateway ....................................63 8.4.1.1. Compress Coding . . . . . . . . . . . . . . . . . 66
6.6.4. 503 Service Unavailable ............................63 8.4.1.2. Deflate Coding . . . . . . . . . . . . . . . . . 66
6.6.5. 504 Gateway Timeout ................................63 8.4.1.3. Gzip Coding . . . . . . . . . . . . . . . . . . . 67
6.6.6. 505 HTTP Version Not Supported .....................64 8.5. Content-Language . . . . . . . . . . . . . . . . . . . . 67
7. Response Header Fields .........................................64 8.5.1. Language Tags . . . . . . . . . . . . . . . . . . . . 68
7.1. Control Data ..............................................64 8.6. Content-Length . . . . . . . . . . . . . . . . . . . . . 68
7.1.1. Origination Date ...................................65 8.7. Content-Location . . . . . . . . . . . . . . . . . . . . 70
7.1.2. Location ...........................................68 8.8. Validator Fields . . . . . . . . . . . . . . . . . . . . 72
7.1.3. Retry-After ........................................69 8.8.1. Weak versus Strong . . . . . . . . . . . . . . . . . 72
7.1.4. Vary ...............................................70 8.8.2. Last-Modified . . . . . . . . . . . . . . . . . . . . 74
7.2. Validator Header Fields ...................................71 8.8.2.1. Generation . . . . . . . . . . . . . . . . . . . 74
7.3. Authentication Challenges .................................72 8.8.2.2. Comparison . . . . . . . . . . . . . . . . . . . 75
7.4. Response Context ..........................................72 8.8.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 76
7.4.1. Allow ..............................................72 8.8.3.1. Generation . . . . . . . . . . . . . . . . . . . 77
7.4.2. Server .............................................73 8.8.3.2. Comparison . . . . . . . . . . . . . . . . . . . 77
8. IANA Considerations ............................................73 8.8.3.3. Example: Entity-Tags Varying on Content-Negotiated
8.1. Method Registry ...........................................73 Resources . . . . . . . . . . . . . . . . . . . . . 78
8.1.1. Procedure ..........................................74 8.8.4. When to Use Entity-Tags and Last-Modified Dates . . . 79
8.1.2. Considerations for New Methods .....................74 9. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
8.1.3. Registrations ......................................75 9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 80
8.2. Status Code Registry ......................................75 9.2. Common Method Properties . . . . . . . . . . . . . . . . 82
8.2.1. Procedure ..........................................75 9.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 82
8.2.2. Considerations for New Status Codes ................76 9.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 83
8.2.3. Registrations ......................................76 9.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 84
8.3. Header Field Registry .....................................77 9.3. Method Definitions . . . . . . . . . . . . . . . . . . . 84
8.3.1. Considerations for New Header Fields ...............78 9.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.3.2. Registrations ......................................80 9.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 85
8.4. Content Coding Registry ...................................81 9.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 86
8.4.1. Procedure ..........................................81 9.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.4.2. Registrations ......................................81 9.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 90
9. Security Considerations ........................................81 9.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 91
9.1. Attacks Based on File and Path Names ......................82 9.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 92
9.2. Attacks Based on Command, Code, or Query Injection ........82 9.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 93
9.3. Disclosure of Personal Information ........................83 10. Message Context . . . . . . . . . . . . . . . . . . . . . . . 94
9.4. Disclosure of Sensitive Information in URIs ...............83 10.1. Request Context Fields . . . . . . . . . . . . . . . . . 94
9.5. Disclosure of Fragment after Redirects ....................84 10.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 94
9.6. Disclosure of Product Information .........................84 10.1.2. From . . . . . . . . . . . . . . . . . . . . . . . . 96
9.7. Browser Fingerprinting ....................................84 10.1.3. Referer . . . . . . . . . . . . . . . . . . . . . . 97
10. Acknowledgments ...............................................85 10.1.4. TE . . . . . . . . . . . . . . . . . . . . . . . . . 98
11. References ....................................................85 10.1.5. Trailer . . . . . . . . . . . . . . . . . . . . . . 99
11.1. Normative References .....................................85 10.1.6. User-Agent . . . . . . . . . . . . . . . . . . . . . 99
11.2. Informative References ...................................86 10.2. Response Context Fields . . . . . . . . . . . . . . . . 100
Appendix A. Differences between HTTP and MIME .....................89 10.2.1. Allow . . . . . . . . . . . . . . . . . . . . . . . 101
A.1. MIME-Version ..............................................89 10.2.2. Date . . . . . . . . . . . . . . . . . . . . . . . . 101
A.2. Conversion to Canonical Form ..............................89 10.2.3. Location . . . . . . . . . . . . . . . . . . . . . . 102
A.3. Conversion of Date Formats ................................90 10.2.4. Retry-After . . . . . . . . . . . . . . . . . . . . 104
A.4. Conversion of Content-Encoding ..........................90 10.2.5. Server . . . . . . . . . . . . . . . . . . . . . . . 104
A.5. Conversion of Content-Transfer-Encoding .................90 11. HTTP Authentication . . . . . . . . . . . . . . . . . . . . . 105
A.6. MHTML and Line Length Limitations .........................90 11.1. Authentication Scheme . . . . . . . . . . . . . . . . . 105
Appendix B. Changes from RFC 2616 .................................91 11.2. Authentication Parameters . . . . . . . . . . . . . . . 105
Appendix C. Imported ABNF .........................................93 11.3. Challenge and Response . . . . . . . . . . . . . . . . . 106
Appendix D. Collected ABNF ........................................94 11.4. Credentials . . . . . . . . . . . . . . . . . . . . . . 107
Index .............................................................97 11.5. Establishing a Protection Space (Realm) . . . . . . . . 107
11.6. Authenticating Users to Origin Servers . . . . . . . . . 108
11.6.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 108
11.6.2. Authorization . . . . . . . . . . . . . . . . . . . 109
11.6.3. Authentication-Info . . . . . . . . . . . . . . . . 110
11.7. Authenticating Clients to Proxies . . . . . . . . . . . 110
11.7.1. Proxy-Authenticate . . . . . . . . . . . . . . . . . 110
11.7.2. Proxy-Authorization . . . . . . . . . . . . . . . . 111
11.7.3. Proxy-Authentication-Info . . . . . . . . . . . . . 111
12. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 112
12.1. Proactive Negotiation . . . . . . . . . . . . . . . . . 113
12.2. Reactive Negotiation . . . . . . . . . . . . . . . . . . 114
12.3. Request Content Negotiation . . . . . . . . . . . . . . 115
12.4. Content Negotiation Field Features . . . . . . . . . . . 115
12.4.1. Absence . . . . . . . . . . . . . . . . . . . . . . 115
12.4.2. Quality Values . . . . . . . . . . . . . . . . . . . 115
12.4.3. Wildcard Values . . . . . . . . . . . . . . . . . . 116
12.5. Content Negotiation Fields . . . . . . . . . . . . . . . 116
12.5.1. Accept . . . . . . . . . . . . . . . . . . . . . . . 116
12.5.2. Accept-Charset . . . . . . . . . . . . . . . . . . . 119
12.5.3. Accept-Encoding . . . . . . . . . . . . . . . . . . 119
12.5.4. Accept-Language . . . . . . . . . . . . . . . . . . 121
12.5.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . 122
13. Conditional Requests . . . . . . . . . . . . . . . . . . . . 124
13.1. Preconditions . . . . . . . . . . . . . . . . . . . . . 124
13.1.1. If-Match . . . . . . . . . . . . . . . . . . . . . . 125
13.1.2. If-None-Match . . . . . . . . . . . . . . . . . . . 126
13.1.3. If-Modified-Since . . . . . . . . . . . . . . . . . 128
13.1.4. If-Unmodified-Since . . . . . . . . . . . . . . . . 130
13.1.5. If-Range . . . . . . . . . . . . . . . . . . . . . . 131
13.2. Evaluation of Preconditions . . . . . . . . . . . . . . 132
13.2.1. When to Evaluate . . . . . . . . . . . . . . . . . . 133
13.2.2. Precedence of Preconditions . . . . . . . . . . . . 133
14. Range Requests . . . . . . . . . . . . . . . . . . . . . . . 135
14.1. Range Units . . . . . . . . . . . . . . . . . . . . . . 135
14.1.1. Range Specifiers . . . . . . . . . . . . . . . . . . 136
14.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . 137
14.2. Range . . . . . . . . . . . . . . . . . . . . . . . . . 138
14.3. Accept-Ranges . . . . . . . . . . . . . . . . . . . . . 140
14.4. Content-Range . . . . . . . . . . . . . . . . . . . . . 140
14.5. Partial PUT . . . . . . . . . . . . . . . . . . . . . . 142
14.6. Media Type multipart/byteranges . . . . . . . . . . . . 143
15. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 145
15.1. Overview of Status Codes . . . . . . . . . . . . . . . . 146
15.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 146
15.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 147
15.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 147
15.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 147
15.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 148
15.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 148
15.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 149
15.3.4. 203 Non-Authoritative Information . . . . . . . . . 149
15.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 149
15.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 150
15.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 150
15.3.7.1. Single Part . . . . . . . . . . . . . . . . . . 151
15.3.7.2. Multiple Parts . . . . . . . . . . . . . . . . . 152
15.3.7.3. Combining Parts . . . . . . . . . . . . . . . . 153
15.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 154
15.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 156
15.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 157
15.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 157
15.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 158
15.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 159
15.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 159
15.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 159
15.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 160
15.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 160
15.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 160
15.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 161
15.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 161
15.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 161
15.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 161
15.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 162
15.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 162
15.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 162
15.5.8. 407 Proxy Authentication Required . . . . . . . . . 163
15.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 163
15.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 163
15.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 163
15.5.12. 411 Length Required . . . . . . . . . . . . . . . . 164
15.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 164
15.5.14. 413 Content Too Large . . . . . . . . . . . . . . . 164
15.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 165
15.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 165
15.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 165
15.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 166
15.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 166
15.5.20. 421 Misdirected Request . . . . . . . . . . . . . . 167
15.5.21. 422 Unprocessable Content . . . . . . . . . . . . . 167
15.5.22. 426 Upgrade Required . . . . . . . . . . . . . . . . 167
15.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 168
15.6.1. 500 Internal Server Error . . . . . . . . . . . . . 168
15.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 168
15.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 168
15.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 168
15.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 169
15.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 169
16. Extending HTTP . . . . . . . . . . . . . . . . . . . . . . . 169
16.1. Method Extensibility . . . . . . . . . . . . . . . . . . 170
16.1.1. Method Registry . . . . . . . . . . . . . . . . . . 170
16.1.2. Considerations for New Methods . . . . . . . . . . . 170
16.2. Status Code Extensibility . . . . . . . . . . . . . . . 171
16.2.1. Status Code Registry . . . . . . . . . . . . . . . . 171
16.2.2. Considerations for New Status Codes . . . . . . . . 171
16.3. Field Extensibility . . . . . . . . . . . . . . . . . . 172
16.3.1. Field Name Registry . . . . . . . . . . . . . . . . 173
16.3.2. Considerations for New Fields . . . . . . . . . . . 174
16.3.2.1. Considerations for New Field Names . . . . . . . 175
16.3.2.2. Considerations for New Field Values . . . . . . 176
16.4. Authentication Scheme Extensibility . . . . . . . . . . 176
16.4.1. Authentication Scheme Registry . . . . . . . . . . . 177
16.4.2. Considerations for New Authentication Schemes . . . 177
16.5. Range Unit Extensibility . . . . . . . . . . . . . . . . 178
16.5.1. Range Unit Registry . . . . . . . . . . . . . . . . 179
16.5.2. Considerations for New Range Units . . . . . . . . . 179
16.6. Content Coding Extensibility . . . . . . . . . . . . . . 179
16.6.1. Content Coding Registry . . . . . . . . . . . . . . 179
16.6.2. Considerations for New Content Codings . . . . . . . 180
16.7. Upgrade Token Registry . . . . . . . . . . . . . . . . . 180
17. Security Considerations . . . . . . . . . . . . . . . . . . . 181
17.1. Establishing Authority . . . . . . . . . . . . . . . . . 181
17.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 182
17.3. Attacks Based on File and Path Names . . . . . . . . . . 183
17.4. Attacks Based on Command, Code, or Query Injection . . . 183
17.5. Attacks via Protocol Element Length . . . . . . . . . . 184
17.6. Attacks using Shared-dictionary Compression . . . . . . 184
17.7. Disclosure of Personal Information . . . . . . . . . . . 185
17.8. Privacy of Server Log Information . . . . . . . . . . . 185
17.9. Disclosure of Sensitive Information in URIs . . . . . . 186
17.10. Application Handling of Field Names . . . . . . . . . . 186
17.11. Disclosure of Fragment after Redirects . . . . . . . . . 187
17.12. Disclosure of Product Information . . . . . . . . . . . 188
17.13. Browser Fingerprinting . . . . . . . . . . . . . . . . . 188
17.14. Validator Retention . . . . . . . . . . . . . . . . . . 189
17.15. Denial-of-Service Attacks Using Range . . . . . . . . . 189
17.16. Authentication Considerations . . . . . . . . . . . . . 190
17.16.1. Confidentiality of Credentials . . . . . . . . . . 190
17.16.2. Credentials and Idle Clients . . . . . . . . . . . 190
17.16.3. Protection Spaces . . . . . . . . . . . . . . . . . 191
17.16.4. Additional Response Fields . . . . . . . . . . . . 191
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 191
18.1. URI Scheme Registration . . . . . . . . . . . . . . . . 192
18.2. Method Registration . . . . . . . . . . . . . . . . . . 192
18.3. Status Code Registration . . . . . . . . . . . . . . . . 192
18.4. Field Name Registration . . . . . . . . . . . . . . . . 195
18.5. Authentication Scheme Registration . . . . . . . . . . . 197
18.6. Content Coding Registration . . . . . . . . . . . . . . 198
18.7. Range Unit Registration . . . . . . . . . . . . . . . . 198
18.8. Media Type Registration . . . . . . . . . . . . . . . . 199
18.9. Port Registration . . . . . . . . . . . . . . . . . . . 199
18.10. Upgrade Token Registration . . . . . . . . . . . . . . . 199
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 199
19.1. Normative References . . . . . . . . . . . . . . . . . . 199
19.2. Informative References . . . . . . . . . . . . . . . . . 201
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 208
Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 212
B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 212
B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 213
B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 213
B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 215
B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 216
B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 216
B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 216
B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 216
B.9. Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 216
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 216
C.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 216
C.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 217
C.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 217
C.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 219
C.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 220
C.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 220
C.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 221
C.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 222
C.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 224
C.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 225
C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . . . . . 226
C.12. Since draft-ietf-httpbis-semantics-10 . . . . . . . . . . 226
C.13. Since draft-ietf-httpbis-semantics-11 . . . . . . . . . . 228
C.14. Since draft-ietf-httpbis-semantics-12 . . . . . . . . . . 228
C.15. Since draft-ietf-httpbis-semantics-13 . . . . . . . . . . 230
C.16. Since draft-ietf-httpbis-semantics-14 . . . . . . . . . . 231
C.17. Since draft-ietf-httpbis-semantics-15 . . . . . . . . . . 233
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 234
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 246
1. Introduction 1. Introduction
1.1. Purpose 1.1. Purpose
[new] The Hypertext Transfer Protocol (HTTP) is a family of stateless,
application-level, request/response protocols that share a generic
interface, extensible semantics, and self-descriptive messages to
enable flexible interaction with network-based hypertext information
systems.
HTTP is a generic interface protocol for information systems. It is HTTP hides the details of how a service is implemented by presenting
designed
to hide the details of how a service is implemented by presenting
a uniform interface to clients that is independent of the types of a uniform interface to clients that is independent of the types of
resources provided. Likewise, servers do not need to be aware of resources provided. Likewise, servers do not need to be aware of
each client's purpose: an HTTP request can be considered in isolation each client's purpose: a request can be considered in isolation
rather than being associated with a specific type of client or a rather than being associated with a specific type of client or a
predetermined sequence of application steps. predetermined sequence of application steps. This allows general-
The result is a protocol that can be used effectively in many different purpose implementations to be used effectively in many different
contexts and for which implementations can evolve independently over time. contexts, reduces interaction complexity, and enables independent
evolution over time.
HTTP is also designed for use as an intermediation protocol for HTTP is also designed for use as an intermediation protocol, wherein
translating communication to and from non-HTTP information systems. proxies and gateways can translate non-HTTP information systems into
HTTP proxies and gateways can provide access to alternative a more generic interface.
information services by translating their diverse protocols into a
hypertext format that can be viewed and manipulated by clients in the
same way as HTTP services.
One consequence of this flexibility is that the protocol cannot be One consequence of this flexibility is that the protocol cannot be
defined in terms of what occurs behind the interface. Instead, we defined in terms of what occurs behind the interface. Instead, we
are limited to defining the syntax of communication, the intent of are limited to defining the syntax of communication, the intent of
received communication, and the expected behavior of recipients. If received communication, and the expected behavior of recipients. If
the communication is considered in isolation, then successful actions the communication is considered in isolation, then successful actions
ought to be reflected in corresponding changes to the observable ought to be reflected in corresponding changes to the observable
interface provided by servers. However, since multiple clients might interface provided by servers. However, since multiple clients might
act in parallel and perhaps at cross-purposes, we cannot require that act in parallel and perhaps at cross-purposes, we cannot require that
such changes be observable beyond the scope of a single response. such changes be observable beyond the scope of a single response.
1.2. History and Evolution 1.2. History and Evolution
HTTP has been in use since 1990. The first version, later referred HTTP has been the primary information transfer protocol for the World
to as HTTP/0.9, was a simple protocol for hypertext data transfer Wide Web since its introduction in 1990. It began as a trivial
across the Internet, using only a single request method (GET) and no mechanism for low-latency requests, with a single method (GET) to
metadata. request transfer of a presumed hypertext document identified by a
given pathname. As the Web grew, HTTP was extended to enclose
requests and responses within messages, transfer arbitrary data
formats using MIME-like media types, and route requests through
intermediaries. These protocols were eventually defined as HTTP/0.9
and HTTP/1.0 (see [RFC1945]).
HTTP/1.0, as defined by [RFC1945], added a range of HTTP/1.1 was designed to refine the protocol's features while
request methods and MIME-like messaging, allowing for metadata to be retaining compatibility with the existing text-based messaging
transferred and modifiers placed on the request/response semantics. syntax, improving its interoperability, scalability, and robustness
However, HTTP/1.0 did not sufficiently take into consideration the across the Internet. This included length-based data delimiters for
effects of hierarchical proxies, caching, the need for persistent both fixed and dynamic (chunked) content, a consistent framework for
connections, or name-based virtual hosts. The proliferation of content negotiation, opaque validators for conditional requests,
incompletely implemented applications calling themselves "HTTP/1.0" cache controls for better cache consistency, range requests for
further necessitated a protocol version change in order for two partial updates, and default persistent connections. HTTP/1.1 was
communicating applications to determine each other's true introduced in 1995 and published on the standards track in 1997
capabilities. [RFC2068], revised in 1999 [RFC2616], and revised again in 2014
([RFC7230] - [RFC7235]).
[new] HTTP/2 ([RFC7540]) introduced a multiplexed session layer on top of
the existing TLS and TCP protocols for exchanging concurrent HTTP
messages with efficient field compression and server push. HTTP/3
([HTTP3]) provides greater independence for concurrent messages by
using QUIC as a secure multiplexed transport over UDP instead of TCP.
[new] All three major versions of HTTP rely on the semantics defined by
this document. They have not obsoleted each other because each one
has specific benefits and limitations depending on the context of
use. Implementations are expected to choose the most appropriate
transport and messaging syntax for their particular context.
[new] This revision of HTTP separates the definition of semantics (this
document) and caching ([Caching]) from the current HTTP/1.1 messaging
syntax ([Messaging]) to allow each major protocol version to progress
independently while referring to the same core semantics.
1.3. Semantics 1.3. Core Semantics
HTTP provides a uniform interface for interacting with a resource HTTP provides a uniform interface for interacting with a resource
(Section 2), regardless of its type, nature, or implementation, via (Section 3.1) - regardless of its type, nature, or implementation -
the manipulation and transfer of representations (Section 3). by sending messages that manipulate or transfer representations
(Section 3.2).
Each Hypertext Transfer Protocol (HTTP) message is either a request Each message is either a request or a response. A client constructs
or a response. A server listens on a connection for a request, request messages that communicate its intentions and routes those
parses each message received, interprets the message semantics in messages toward an identified origin server. A server listens for
relation to the identified request target, and responds to that requests, parses each message received, interprets the message
request with one or more response messages. A client constructs semantics in relation to the identified target resource, and responds
request messages to communicate specific intentions, examines to that request with one or more response messages. The client
received responses to see if the intentions were carried out, and examines received responses to see if its intentions were carried
determines how to interpret the results. This document defines out, determining what to do next based on the status codes and
HTTP/1.1 request and response semantics in terms of the architecture content received.
defined in [RFC7230].
HTTP semantics include the intentions defined by each request method HTTP semantics include the intentions defined by each request method
(Section 4), extensions to those semantics that might be described in (Section 9), extensions to those semantics that might be described in
request header fields (Section 5), the meaning of status codes to request header fields, status codes that describe the response
indicate a machine-readable response (Section 6), and the meaning of (Section 15), and other control data and resource metadata that might
other control data and resource metadata that might be given in be given in response fields.
response header fields (Section 7).
This document also defines representation metadata that describe how Semantics also include representation metadata that describe how
a payload is intended to be interpreted by a recipient, the request content is intended to be interpreted by a recipient, request header
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 3.4). negotiation_ (Section 12).
This document defines HTTP/1.1 range requests, partial responses, and 1.4. Specifications Obsoleted by this Document
the multipart/byteranges media type.
This document obsoletes the following specifications:
+============================================+===========+=========+
| Title | Reference | Changes |
+============================================+===========+=========+
| HTTP Over TLS | [RFC2818] | B.1 |
+--------------------------------------------+-----------+---------+
| HTTP/1.1 Message Syntax and Routing [*] | [RFC7230] | B.2 |
+--------------------------------------------+-----------+---------+
| HTTP/1.1 Semantics and Content | [RFC7231] | B.3 |
+--------------------------------------------+-----------+---------+
| HTTP/1.1 Conditional Requests | [RFC7232] | B.4 |
+--------------------------------------------+-----------+---------+
| HTTP/1.1 Range Requests | [RFC7233] | B.5 |
+--------------------------------------------+-----------+---------+
| HTTP/1.1 Authentication | [RFC7235] | B.6 |
+--------------------------------------------+-----------+---------+
| HTTP Status Code 308 (Permanent Redirect) | [RFC7538] | B.7 |
+--------------------------------------------+-----------+---------+
| HTTP Authentication-Info and Proxy- | [RFC7615] | B.8 |
| Authentication-Info Response Header Fields | | |
+--------------------------------------------+-----------+---------+
| HTTP Client-Initiated Content-Encoding | [RFC7694] | B.9 |
+--------------------------------------------+-----------+---------+
Table 1
[*] This document only obsoletes the portions of RFC 7230 that are
independent of the HTTP/1.1 messaging syntax and connection
management; the remaining bits of RFC 7230 are obsoleted by
"HTTP/1.1" [Messaging].
2. Conformance 2. Conformance
2.1. Syntax Notation 2.1. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234], extended with the notation for case-
[RFC7230], that allows for compact definition of comma-separated sensitivity in strings defined in [RFC7405].
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix C describes rules imported from other It also uses a list extension, defined in Section 5.6.1, that allows
documents. Appendix D shows the collected grammar with all list for compact definition of comma-separated lists using a "#" operator
operators expanded to standard ABNF notation. (similar to how the "*" operator indicates repetition). Appendix A
shows the collected grammar with all list operators expanded to
standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote As a convention, ABNF rule names prefixed with "obs-" denote
"obsolete" grammar rules that appear for historical reasons. "obsolete" grammar rules that appear for historical reasons.
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
(line feed), OCTET (any 8-bit sequence of data), SP (space), and (line feed), OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character). VCHAR (any visible US-ASCII character).
Section 5.6 defines some generic syntactic components for field
values.
This specification uses the terms "character", "character encoding This specification uses the terms "character", "character encoding
scheme", "charset", and "protocol element" as they are defined in scheme", "charset", and "protocol element" as they are defined in
[RFC6365]. [RFC6365].
2.2. Requirements Notation 2.2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
Conformance criteria and considerations regarding error handling are capitals, as shown here.
defined in Section 2.5 of [RFC7230].
This specification targets conformance criteria according to the role This specification targets conformance criteria according to the role
of a participant in HTTP communication. Hence, HTTP requirements are of a participant in HTTP communication. Hence, requirements are
placed on senders, recipients, clients, servers, user agents, placed on senders, recipients, clients, servers, user agents,
intermediaries, origin servers, proxies, gateways, or caches, intermediaries, origin servers, proxies, gateways, or caches,
depending on what behavior is being constrained by the requirement. depending on what behavior is being constrained by the requirement.
Additional (social) requirements are placed on implementations, Additional (social) requirements are placed on implementations,
resource owners, and protocol element registrations when they apply resource owners, and protocol element registrations when they apply
beyond the scope of a single communication. beyond the scope of a single communication.
The verb "generate" is used instead of "send" where a requirement The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely applies only to implementations that create the protocol element,
forwarding a received element downstream. rather than an implementation that forwards a received element
downstream.
An implementation is considered conformant if it complies with all of An implementation is considered conformant if it complies with all of
the requirements associated with the roles it partakes in HTTP. the requirements associated with the roles it partakes in HTTP.
Conformance includes both the syntax and semantics of protocol A sender MUST NOT generate protocol elements that do not match the
elements. A sender MUST NOT generate protocol elements that convey a grammar defined by the corresponding ABNF rules. Within a given
meaning that is known by that sender to be false. A sender MUST NOT message, a sender MUST NOT generate protocol elements or syntax
generate protocol elements that do not match the grammar defined by alternatives that are only allowed to be generated by participants in
the corresponding ABNF rules. Within a given message, a sender MUST other roles (i.e., a role that the sender does not have for that
NOT generate protocol elements or syntax alternatives that are only message).
allowed to be generated by participants in other roles (i.e., a role
that the sender does not have for that message). Conformance to HTTP includes both conformance to the particular
messaging syntax of the protocol version in use and conformance to
the semantics of protocol elements sent. For example, a client that
claims conformance to HTTP/1.1 but fails to recognize the features
required of HTTP/1.1 recipients will fail to interoperate with
servers that adjust their responses in accordance with those claims.
Features that reflect user choices, such as content negotiation and
user-selected extensions, can impact application behavior beyond the
protocol stream; sending protocol elements that inaccurately reflect
a user's choices will confuse the user and inhibit choice.
When an implementation fails semantic conformance, recipients of that
implementation's messages will eventually develop workarounds to
adjust their behavior accordingly. A recipient MAY employ such
workarounds while remaining conformant to this protocol if the
workarounds are limited to the implementations at fault. For
example, servers often scan portions of the User-Agent field value,
and user agents often scan the Server field value, to adjust their
own behavior with respect to known bugs or poorly chosen defaults.
2.3. Length Requirements 2.3. Length Requirements
When a received protocol element is parsed, the recipient MUST be A recipient SHOULD parse a received protocol element defensively,
able to parse any value of reasonable length that is applicable to with only marginal expectations that the element will conform to its
the recipient's role and that matches the grammar defined by the ABNF grammar and fit within a reasonable buffer size.
corresponding ABNF rules.
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 two decades of HTTP
use and is expected to continue changing in the future. 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 request target. references when received as a target URI.
Note, however, that some received protocol elements might not be parsed. For Many received protocol elements are only parsed to the extent
example, an intermediary forwarding a message might parse a header-field necessary to identify and forward that element downstream. For
into generic field-name and field-value components, but then forward the example, an intermediary might parse a received field into its field
header field without further parsing inside the field-value. name and field value components, but then forward the field without
further parsing inside the field value.
2.4. Error Handling 2.4. Error Handling
A recipient MUST interpret a received protocol element according to A recipient MUST interpret a received protocol element according to
the semantics defined for it by this specification, including the semantics defined for it by this specification, including
extensions to this specification, unless the recipient has determined extensions to this specification, unless the recipient has determined
(through experience or configuration) that the sender incorrectly (through experience or configuration) that the sender incorrectly
implements what is implied by those semantics. For example, an implements what is implied by those semantics. For example, an
origin server might disregard the contents of a received origin server might disregard the contents of a received
Accept-Encoding header field if inspection of the User-Agent header Accept-Encoding header field if inspection of the User-Agent header
skipping to change at line 422 skipping to change at page 15, line 27
Unless noted otherwise, a recipient MAY attempt to recover a usable Unless noted otherwise, a recipient MAY attempt to recover a usable
protocol element from an invalid construct. HTTP does not define protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct specific error handling mechanisms except when they have a direct
impact on security, since different applications of the protocol impact on security, since different applications of the protocol
require different error handling strategies. For example, a Web require different error handling strategies. For example, a Web
browser might wish to transparently recover from a response where the browser might wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF, whereas a Location header field doesn't parse according to the ABNF, whereas a
systems control client might consider any form of error recovery to systems control client might consider any form of error recovery to
be dangerous. be dangerous.
[new] Some requests can be automatically retried by a client in the event
of an underlying connection failure, as described in Section 9.2.2.
2.5. Protocol Version 2.5. Protocol Version
The HTTP 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 HTTP 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 and able to understand for version to which the sender is conformant (able to understand for
future communication. future communication).
[new] While HTTP's core semantics don't change between protocol versions,
the expression of them "on the wire" can change, and so the HTTP
version number changes when incompatible changes are made to the wire
format. Additionally, HTTP allows incremental, backwards-compatible
changes to be made to the protocol without changing its version
through the use of defined extension points (Section 16).
The protocol version as a whole indicates the sender's conformance with The protocol version as a whole indicates the sender's conformance
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. specification of HTTP. For example, the version "HTTP/1.1" is
defined by the combined specifications of this document, "HTTP
Caching" [Caching], and "HTTP/1.1" [Messaging].
The intention of HTTP's versioning design is that the major number HTTP's major version number is incremented when an incompatible
will only be incremented if an incompatible message syntax is message syntax is introduced. The minor number is incremented when
introduced, and that the minor number will only be 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.
However, the minor version was not incremented for the changes
introduced between [RFC2068] and [RFC2616], and this revision has
specifically avoided any such changes to the protocol.
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
(by clients). (by clients).
[new] When a major version of HTTP does not define any minor versions, the
minor version "0" is implied and is used when referring to that
protocol within a protocol element that requires sending a minor
version.
3. Architecture 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 and syntax productions used to define HTTP. terminology and syntax productions 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. Each resource is might be used to interact with resources. Most resources are
identified by a Uniform Resource Identifier (URI), as described in identified by a Uniform Resource Identifier (URI), as described in
Section 2.7 of [RFC7230]. 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 4) and a few semantics in the request method (Section 9) and a few request-
request-modifying header fields (Section 5). If there is a conflict modifying header fields. A resource cannot treat a request in a
between the method semantics and any semantic implied by the URI manner inconsistent with the semantics of the method of the request.
itself, as described in Section 4.2.1, the method semantics take For example, though the URI of a resource might imply semantics that
precedence. are not safe, a client can expect the resource to avoid actions that
are unsafe when processing a request with a safe method (see
Section 9.2.1).
HTTP relies upon the Uniform Resource Identifier (URI) standard HTTP relies upon the Uniform Resource Identifier (URI) standard
[RFC3986] to indicate the target resource (Section 5.1) and [RFC3986] to indicate the target resource (Section 7.1) and
relationships between resources. relationships between resources.
3.2. Representations 3.2. Representations
For the purposes of HTTP, a "representation" is information that is A _representation_ is information that is intended to reflect a past,
intended to reflect a past, current, or desired state of a given current, or desired state of a given resource, in a format that can
resource, in a format that can be readily communicated via the be readily communicated via the protocol. A representation consists
protocol, and that consists of a set of representation metadata and a of a set of representation metadata and a potentially unbounded
potentially unbounded stream of representation data. stream of representation data (Section 8).
[new] HTTP allows "information hiding" behind its uniform interface by
defining communication with respect to a transferable representation
of the resource state, rather than transferring the resource itself.
This allows the resource identified by a URI to be anything,
including temporal functions like "the current weather in Laguna
Beach", while potentially providing information that represents that
resource at the time a message is generated [REST].
Considering that a resource could be anything, and that the uniform The uniform interface is similar to a window through which one can
interface provided by HTTP is similar to a window through which one observe and act upon a thing only through the communication of
can observe and act upon such a thing only through the communication messages to an independent actor on the other side. A shared
of messages to some independent actor on the other side, an
abstraction is needed to represent ("take the place of") the current abstraction is needed to represent ("take the place of") the current
or desired state of that thing in our communications. That or desired state of that thing in our communications. When a
abstraction is called a representation [REST]. representation is hypertext, it can provide both a representation of
the resource state and processing instructions that help guide the
recipient's future interactions.
An origin server might be provided with, or be capable of generating, A target resource might be provided with, or be capable of
multiple representations that are each intended to reflect the generating, multiple representations that are each intended to
current state of a target resource. In such cases, some algorithm is reflect the resource's current state. An algorithm, usually based on
used by the origin server to select one of those representations as content negotiation (Section 12), would be used to select one of
most applicable to a given request, usually based on content those representations as being most applicable to a given request.
negotiation. This "selected representation" is used to provide the This _selected representation_ provides the data and metadata for
data and metadata for evaluating conditional requests [RFC7232] and evaluating conditional requests (Section 13) and constructing the
constructing the payload for 200 (OK) and 304 (Not Modified) content for 200 (OK), 206 (Partial Content), and 304 (Not Modified)
responses to GET (Section 4.3.1). responses to GET (Section 9.3.1).
3.3. Connections 3.3. Connections, Clients and Servers
HTTP is a stateless request/response protocol that operates by HTTP is a client/server protocol that operates over a reliable
exchanging messages (Section 3) across a reliable transport- or transport- or session-layer _connection_.
session-layer "connection" (Section 6).
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 can be understood in isolation. Many implementations depend message's semantics can be understood in isolation, and that the
on HTTP's stateless design in order to reuse proxied connections or relationship between connections and messages on them has no impact
dynamically load balance requests across multiple servers. Hence, a on the interpretation of those messages. For example, a CONNECT
server MUST NOT assume that two requests on the same connection are request (Section 9.3.6) or a request with the Upgrade header field
from the same user agent unless the connection is secured and (Section 7.8) can occur at any time, not just in the first message on
specific to that agent. Some non-standard HTTP extensions (e.g., a connection. Many implementations depend on HTTP's stateless design
[RFC4559]) have been known to violate this requirement, resulting in in order to reuse proxied connections or dynamically load balance
security and interoperability problems. requests across multiple servers.
A connection might be used for multiple request/response exchanges, As a result, a server MUST NOT assume that two requests on the same
as defined in Section 6.3. connection are from the same user agent unless the connection is
secured and specific to that agent. Some non-standard HTTP
extensions (e.g., [RFC4559]) have been known to violate this
requirement, resulting in security and interoperability problems.
3.4. Messages 3.4. Messages
The terms "sender" and "recipient" HTTP is a stateless request/response protocol for exchanging
_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 an HTTP request to a server in the form of a request A client sends requests to a server in the form of a _request_
message, beginning with a request-line that includes a method, URI, message with a method (Section 9) and request target (Section 7.1).
and protocol version (Section 3.1.1), followed by header fields The request might also contain header fields (Section 6.3) for
containing request modifiers, client information, and representation request modifiers, client information, and representation metadata,
metadata (Section 3.2), an empty line to indicate the end of the content (Section 6.4) intended for processing in accordance with the
header section, and finally a message body containing the payload method, and trailer fields (Section 6.5) to communicate information
body (if any, Section 3.3). collected while sending the content.
When a client constructs an HTTP/1.1 request message, it sends the
target URI in one of various forms, as defined in (Section 5.3 of
[RFC7230]). When a request is received, the server reconstructs an
effective request URI for the target resource (Section 5.5 of
[RFC7230]).
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
HTTP response messages, each beginning with a status line that includes _response_ messages, each including a status code (Section 15). The
the protocol version, a success or error code, and textual reason response might also contain header fields for server information,
phrase (Section 3.1.2), possibly followed by header fields containing resource metadata, and representation metadata, content to be
server information, resource metadata, and representation metadata interpreted in accordance with the status code, and trailer fields to
(Section 3.2), an empty line to indicate the end of the header section, communicate information collected while sending the content.
and finally a message body containing the payload body (if any, Section
3.3).
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, including (but not limited to) browsers, spiders that initiate a request.
(web-based robots), command-line tools, custom applications, and
mobile apps.
When considering the design of HTTP, it is easy to fall into a trap The most familiar form of user agent is the general-purpose Web
of thinking that all user agents are general-purpose browsers and all browser, but that's only a small percentage of implementations.
origin servers are large public websites. That is not the case in Other common user agents include spiders (web-traversing robots),
practice. Common HTTP user agents include household appliances, command-line tools, billboard screens, household appliances, scales,
stereos, scales, firmware update scripts, command-line programs, light bulbs, firmware update scripts, mobile apps, and communication
mobile apps, and communication devices in a multitude of shapes and devices in a multitude of shapes and sizes.
sizes.
The term "user agent" does not imply that there is a human user Being a user agent does not imply that there is a human user directly
directly interacting with the software agent at the time of a interacting with the software agent at the time of a request. In
request. In many cases, a user agent is installed or configured to many cases, a user agent is installed or configured to run in the
run in the background and save its results for later inspection (or background and save its results for later inspection (or save only a
save only a subset of those results that might be interesting or subset of those results that might be interesting or erroneous).
erroneous). Spiders, for example, are typically given a start URI Spiders, for example, are typically given a start URI and configured
and configured to follow certain behavior while crawling the Web as a to follow certain behavior while crawling the Web as a hypertext
hypertext graph. graph.
The implementation diversity of HTTP means that not all user agents Many user agents cannot, or choose not to, make interactive
can make interactive suggestions to their user or provide adequate suggestions to their user or provide adequate warning for security or
warning for security or privacy concerns. In the few cases where privacy concerns. In the few cases where this specification requires
this specification requires reporting of errors to the user, it is reporting of errors to the user, it is acceptable for such reporting
acceptable for such reporting to only be observable in an error to only be observable in an error console or log file. Likewise,
console or log file. Likewise, requirements that an automated action requirements that an automated action be confirmed by the user before
be confirmed by the user before proceeding might be met via advance proceeding might be met via advance configuration choices, run-time
configuration choices, run-time options, or simple avoidance of the options, or simple avoidance of the unsafe action; confirmation does
unsafe action; confirmation does not imply any specific user not imply any specific user interface or interruption of normal
interface or interruption of normal processing if the user has processing if the user has already made that choice.
already made that choice.
3.6. Origin Server 3.6. Origin Server
The term "origin server" refers to the 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.
Likewise, common HTTP The most familiar form of origin server are large public websites.
origin servers include home automation units, configurable However, like user agents being equated with browsers, it is easy to
be misled into thinking that all origin servers are alike. Common
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, ad selectors, and video-delivery feeds, traffic cameras, real-time ad selectors, and video-on-demand
platforms. platforms.
Most HTTP communication consists of a retrieval request (GET) for a Most HTTP communication consists of a retrieval request (GET) for a
representation of some resource identified by a URI. In the simplest representation of some resource identified by a URI. In the simplest
case, this might be accomplished via a single bidirectional case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin connection (===) between the user agent (UA) and the origin server
server (O). (O).
request > request >
UA ======================================= O UA ======================================= O
< response < response
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
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 and
"outbound" means toward the user agent. _outbound_ means toward the user agent.
A "proxy" is a message-forwarding agent that is selected by the A _proxy_ is a message-forwarding agent that is chosen by the client,
client, usually via local configuration rules, to receive requests usually via local configuration rules, to receive requests for some
for some type(s) of absolute URI and attempt to satisfy those type(s) of absolute URI and attempt to satisfy those requests via
requests via translation through the HTTP interface. Some translation through the HTTP interface. Some translations are
translations are minimal, such as for proxy requests for "http" URIs, minimal, such as for proxy requests for "http" URIs, whereas other
whereas other requests might require translation to and from entirely requests might require translation to and from entirely different
different application-level protocols. Proxies are often used to application-level protocols. Proxies are often used to group an
group an organization's HTTP requests through a common intermediary organization's HTTP requests through a common intermediary for the
for the sake of security, annotation services, or shared caching. sake of security, annotation services, or shared caching. Some
Some proxies are designed to apply transformations to selected proxies are designed to apply transformations to selected messages or
messages or payloads while they are being forwarded, as described in content while they are being forwarded, as described in Section 7.7.
Section 5.7.2.
A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as A _gateway_ (a.k.a. _reverse proxy_) is an intermediary that acts as
an origin server for the outbound connection but translates received an origin server for the outbound connection but translates received
requests and forwards them inbound to another server or servers. requests and forwards them inbound to another server or servers.
Gateways are often used to encapsulate legacy or untrusted Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through information services, to improve server performance through
"accelerator" caching, and to enable partitioning or load balancing _accelerator_ caching, and to enable partitioning or load balancing
of HTTP services across multiple machines. of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server also apply to All HTTP requirements applicable to an origin server also apply to
the outbound communication of a gateway. A gateway communicates with the outbound communication of a gateway. A gateway communicates with
inbound servers using any protocol that it desires, including private inbound servers using any protocol that it desires, including private
extensions to HTTP that are outside the scope of this specification. extensions to HTTP that are outside the scope of this specification.
However, an HTTP-to-HTTP gateway that wishes to interoperate with However, an HTTP-to-HTTP gateway that wishes to interoperate with
third-party HTTP servers ought to conform to user agent requirements third-party HTTP servers needs to conform to user agent requirements
on the gateway's inbound connection. on the gateway's inbound connection.
A "tunnel" acts as a blind relay between two connections without A _tunnel_ acts as a blind relay between two connections without
changing the messages. Once active, a tunnel is not considered a changing the messages. Once active, a tunnel is not considered a
party to the HTTP communication, though the tunnel might have been party to the HTTP communication, though the tunnel might have been
initiated by an HTTP request. A tunnel ceases to exist when both initiated by an HTTP request. A tunnel ceases to exist when both
ends of the relayed connection are closed. Tunnels are used to ends of the relayed connection are closed. Tunnels are used to
extend a virtual connection through an intermediary, such as when extend a virtual connection through an intermediary, such as when
Transport Layer Security (TLS, [RFC5246]) is used to establish Transport Layer Security (TLS, [RFC8446]) is used to establish
confidential communication through a shared firewall proxy. confidential communication through a shared firewall proxy.
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 a man-in-the-middle indistinguishable (at a protocol level) from an on-path attacker,
attack, often introducing security flaws or interoperability problems often introducing security flaws or interoperability problems due to
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] or "captive portal") differs from as a _transparent proxy_ [RFC1919]) differs from an HTTP proxy
an HTTP proxy because it is not selected by the client. Instead, an because it is not chosen by the client. Instead, an interception
interception proxy filters or redirects outgoing TCP port 80 packets proxy filters or redirects outgoing TCP port 80 packets (and
(and occasionally other common port traffic). Interception proxies occasionally other common port traffic). Interception proxies are
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 by a server while it is 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
< < < <
A response is "cacheable" if a cache is allowed to store a copy of Figure 3
A response is _cacheable_ if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. Even the response message for use in answering subsequent requests. Even
when a response is cacheable, there might be additional constraints when a response is cacheable, there might be additional constraints
placed by the client or by the origin server on when that cached placed by the client or by the origin server on when that cached
response can be used for a particular request. HTTP requirements for response can be used for a particular request. HTTP requirements for
cache behavior and cacheable responses are defined in Section 2 of cache behavior and cacheable responses are defined in [Caching].
[RFC7234].
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 These include national hierarchies of proxy caches to save bandwidth
transoceanic bandwidth, collaborative systems that broadcast or and reduce latency, Content Delivery Networks that use gateway
multicast cache entries, archives of pre-fetched cache entries for caching to optimise regional and global distribution of popular
use in off-line or high-latency environments, and so on. sites, collaborative systems that broadcast or multicast cache
entries, archives of pre-fetched cache entries for use in off-line or
high-latency environments, and so on.
3.9. Example Request and Response 3.9. Example Message Exchange
The following example illustrates a typical message exchange for a The following example illustrates a typical HTTP/1.1 message exchange
GET request (Section 4.3.1 of [RFC7231]) on the URI for a GET request (Section 9.3.1) on the URI "http://www.example.com/
"http://www.example.com/hello.txt": hello.txt":
Client request: Client request:
GET /hello.txt HTTP/1.1 GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3 User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com Host: www.example.com
Accept-Language: en, mi Accept-Language: en, mi
Server response: Server response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00" ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes Accept-Ranges: bytes
Content-Length: 51 Content-Length: 51
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Hello World! My payload includes a trailing CRLF. Hello World! My content includes a trailing CRLF.
4. Identifiers in HTTP 4. Identifiers in HTTP
Uniform Resource Identifiers (URIs) [RFC3986] are used throughout Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
HTTP as the means for identifying resources (Section 2 of [RFC7231]). HTTP as the means for identifying resources (Section 3.1).
4.1. URI References 4.1. URI References
URI references are used to target requests, indicate redirects, and URI references are used to target requests, indicate redirects, and
define relationships. define relationships.
The definitions of "URI-reference", "absolute-URI", "relative-part", The definitions of "URI-reference", "absolute-URI", "relative-part",
"scheme", "authority", "port", "host", "path-abempty", "segment", "authority", "port", "host", "path-abempty", "segment", and "query"
"query", and "fragment" are adopted from the URI generic syntax. An are adopted from the URI generic syntax. An "absolute-path" rule is
"absolute-path" rule is defined for protocol elements that can defined for protocol elements that can contain a non-empty path
contain a non-empty path component. (This rule differs slightly from component. (This rule differs slightly from the path-abempty rule of
the path-abempty rule of RFC 3986, which allows for an empty path to RFC 3986, which allows for an empty path to be used in references,
be used in references, and path-absolute rule, which does not allow and path-absolute rule, which does not allow paths that begin with
paths that begin with "//".) A "partial-URI" rule is defined for "//".) A "partial-URI" rule is defined for protocol elements that
protocol elements that can contain a relative URI but not a fragment can contain a relative URI but not a fragment component.
component.
URI-reference = <URI-reference, see [RFC3986], Section 4.1> URI-reference = <URI-reference, see [RFC3986], Section 4.1>
absolute-URI = <absolute-URI, see [RFC3986], Section 4.3> absolute-URI = <absolute-URI, see [RFC3986], Section 4.3>
relative-part = <relative-part, see [RFC3986], Section 4.2> relative-part = <relative-part, see [RFC3986], Section 4.2>
scheme = <scheme, see [RFC3986], Section 3.1>
authority = <authority, see [RFC3986], Section 3.2> authority = <authority, see [RFC3986], Section 3.2>
uri-host = <host, see [RFC3986], Section 3.2.2> uri-host = <host, see [RFC3986], Section 3.2.2>
port = <port, see [RFC3986], Section 3.2.3> port = <port, see [RFC3986], Section 3.2.3>
path-abempty = <path-abempty, see [RFC3986], Section 3.3> path-abempty = <path-abempty, see [RFC3986], Section 3.3>
segment = <segment, see [RFC3986], Section 3.3> segment = <segment, see [RFC3986], Section 3.3>
query = <query, see [RFC3986], Section 3.4> query = <query, see [RFC3986], Section 3.4>
fragment = <fragment, see [RFC3986], Section 3.5>
absolute-path = 1*( "/" segment ) absolute-path = 1*( "/" segment )
partial-URI = relative-part [ "?" query ] partial-URI = relative-part [ "?" query ]
Each protocol element in HTTP that allows a URI reference will Each protocol element in HTTP that allows a URI reference will
indicate in its ABNF production whether the element allows any form indicate in its ABNF production whether the element allows any form
of reference (URI-reference), only a URI in absolute form of reference (URI-reference), only a URI in absolute form (absolute-
(absolute-URI), only the path and optional query components, or some URI), only the path and optional query components, or some
combination of the above. Unless otherwise indicated, URI references combination of the above. Unless otherwise indicated, URI references
are parsed relative to the effective request URI (Section 5.5). are parsed relative to the target URI (Section 7.1).
[new] It is RECOMMENDED that all senders and recipients support, at a
minimum, URIs with lengths of 8000 octets in protocol elements. Note
that this implies some structures and on-wire representations (for
example, the request line in HTTP/1.1) will necessarily be larger in
some cases.
4.2. URI Schemes 4.2. HTTP-Related URI Schemes
IANA maintains the registry of URI Schemes [BCP115] at IANA maintains the registry of URI Schemes [BCP35] at
<http://www.iana.org/assignments/uri-schemes/>. <https://www.iana.org/assignments/uri-schemes/>. Although requests
might target any URI scheme, the following schemes are inherent to
HTTP servers:
This document defines the following URI schemes. +============+====================================+=======+
| URI Scheme | Description | Ref. |
+============+====================================+=======+
| http | Hypertext Transfer Protocol | 4.2.1 |
+------------+------------------------------------+-------+
| https | Hypertext Transfer Protocol Secure | 4.2.2 |
+------------+------------------------------------+-------+
+------------+------------------------------------+---------------+ Table 2
| URI Scheme | Description | Reference |
+------------+------------------------------------+---------------+
| http | Hypertext Transfer Protocol | Section 2.7.1 |
| https | Hypertext Transfer Protocol Secure | Section 2.7.2 |
+------------+------------------------------------+---------------+
Note that the presence of a URI with a given authority component does Note that the presence of an "http" or "https" URI does not imply
not imply that there is always an HTTP server listening for that there is always an HTTP server at the identified origin
connections on that host and port. Anyone can mint a URI. What the listening for connections. Anyone can mint a URI, whether or not a
authority component determines is who has the right to respond server exists and whether or not that server currently maps that
authoritatively to requests that target the identified resource. The identifier to a resource. The delegated nature of registered names
delegated nature of registered names and IP addresses creates a and IP addresses creates a federated namespace whether or not an HTTP
federated namespace, based on control over the indicated host and server is present.
port, whether or not an HTTP server is present.
4.2.1. http URI Scheme 4.2.1. http URI Scheme
The "http" URI scheme is hereby defined for the purpose of minting The "http" URI scheme is hereby defined for minting identifiers
identifiers according to their association with the hierarchical within the hierarchical namespace governed by a potential HTTP origin
namespace governed by a potential HTTP origin server listening for server listening for TCP ([RFC0793]) connections on a given port.
TCP ([RFC0793]) connections on a given port.
http-URI = "http:" "//" authority path-abempty [ "?" query ] http-URI = "http" "://" authority path-abempty [ "?" query ]
[ "#" fragment ]
The origin server for an "http" URI is identified by the authority The origin server for an "http" URI is identified by the authority
component, which includes a host identifier and optional TCP port component, which includes a host identifier and optional port number
([RFC3986], Section 3.2.2). If the port subcomponent is empty or not ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not
given, TCP port 80 (the reserved port for WWW services) is the given, TCP port 80 (the reserved port for WWW services) is the
default. default. The origin determines who has the right to respond
authoritatively to requests that target the 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 serve The hierarchical path component and optional query component identify
as an identifier for a potential target resource within that the target resource within that origin server's name space.
origin server's name space.
4.2.2. https URI Scheme 4.2.2. https URI Scheme
The "https" URI scheme is hereby defined for the purpose of minting The "https" URI scheme is hereby defined for minting identifiers
identifiers according to their association with the hierarchical within the hierarchical namespace governed by a potential origin
namespace governed by a potential HTTP origin server listening to a server listening for TCP connections on a given port and capable of
given TCP port for TLS-secured connections ([RFC5246]). establishing a TLS ([RFC8446]) connection that has been secured for
HTTP communication. In this context, _secured_ specifically means
that the server has been authenticated as acting on behalf of the
identified authority and all HTTP communication with that server has
confidentiality and integrity protection that is acceptable to both
client and server.
https-URI = "https:" "//" authority path-abempty [ "?" query ] https-URI = "https" "://" authority path-abempty [ "?" query ]
[ "#" fragment ]
All of the requirements listed above for the "http" scheme are also The origin server for an "https" URI is identified by the authority
requirements for the "https" scheme, except that TCP port 443 is the component, which includes a host identifier and optional port number
default if the port subcomponent is empty or not given, and the user ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not
agent MUST ensure that its connection to the origin server is secured given, TCP port 443 (the reserved port for HTTP over TLS) is the
through the use of strong encryption, end-to-end, prior to sending default. The origin determines who has the right to respond
the first HTTP request. authoritatively to requests that target the identified resource, as
defined in Section 4.3.3.
Note that the "https" URI scheme depends on both TLS and TCP for A sender MUST NOT generate an "https" URI with an empty host
establishing authority. identifier. A recipient that processes such a URI reference MUST
reject it as invalid.
The process for authoritative access to an "https" identified The hierarchical path component and optional query component identify
resource is defined in [RFC2818]. the target resource within that origin server's name space.
A client MUST ensure that its HTTP requests for an "https" resource
are secured, prior to being communicated, and that it only accepts
secured responses to those requests. Note that the definition of
what cryptographic mechanisms are acceptable to client and server are
usually negotiated and can change over time.
Resources made available via the "https" scheme have no shared Resources made available via the "https" scheme have no shared
identity with the "http" scheme even if their resource identifiers identity with the "http" scheme. They are distinct origins with
indicate the same authority (the same host listening to the same TCP separate namespaces. However, an extension to HTTP that is defined
port). They are distinct namespaces and are considered to be distinct to apply to all origins with the same host, such as the Cookie
origin servers. However, an extension to HTTP that is defined
to apply to entire host domains, such as the Cookie
protocol [RFC6265], can allow information set by one service to protocol [RFC6265], can allow information set by one service to
impact communication with other services within a matching group of impact communication with other services within a matching group of
host domains. host domains.
4.2.3. http and https URI Normalization and Comparison 4.2.3. http(s) Normalization and Comparison
Since the "http" and "https" schemes conform to the URI generic The "http" and "https" URI are normalized and compared according to
syntax, such URIs are normalized and compared according to the the methods defined in Section 6 of [RFC3986], using the defaults
algorithm defined in Section 6 of [RFC3986], using the defaults
described above for each scheme. described above for each scheme.
If the port is equal to the default port for a scheme, the normal HTTP does not require use of a specific method for determining
form is to omit the port subcomponent. When not being used in equivalence. For example, a cache key might be compared as a simple
absolute form as the request target of an OPTIONS request, an empty string, after syntax-based normalization, or after scheme-based
path component is equivalent to an absolute path of "/", so the normalization.
normal form is to provide a path of "/" instead. The scheme and host
are case-insensitive and normally provided in lowercase; all other Two HTTP URIs that are equivalent after normalization (using any
components are compared in a case-sensitive manner. Characters other method) can be assumed to identify the same resource, and any HTTP
than those in the "reserved" set are equivalent to their component MAY perform normalization. As a result, distinct resources
percent-encoded octets: the normal form is to not encode them (see SHOULD NOT be identified by HTTP URIs that are equivalent after
Sections 2.1 and 2.2 of [RFC3986]). normalization (using any method defined in Section 6.2 of [RFC3986]).
Scheme-based normalization (Section 6.2.3 of [RFC3986]) of "http" and
"https" URIs involves the following additional rules:
* If the port is equal to the default port for a scheme, the normal
form is to omit the port subcomponent.
* When not being used as the target of an OPTIONS request, an empty
path component is equivalent to an absolute path of "/", so the
normal form is to provide a path of "/" instead.
* The scheme and host are case-insensitive and normally provided in
lowercase; all other components are compared in a case-sensitive
manner.
* Characters other than those in the "reserved" set are equivalent
to their percent-encoded octets: the normal form is to not encode
them (see Sections 2.1 and 2.2 of [RFC3986]).
For example, the following three URIs are equivalent: For example, the following three URIs are equivalent:
http://example.com:80/~smith/home.html http://example.com:80/~smith/home.html
http://EXAMPLE.com/%7Esmith/home.html http://EXAMPLE.com/%7Esmith/home.html
http://EXAMPLE.com:/%7esmith/home.html http://EXAMPLE.com:/%7esmith/home.html
4.2.4. Deprecated userinfo 4.2.4. Deprecation of userinfo in http(s) URIs
The URI generic syntax for authority also includes a deprecated The URI generic syntax for authority also includes a userinfo
userinfo subcomponent ([RFC3986], Section 3.2.1) for including user subcomponent ([RFC3986], Section 3.2.1) for including user
authentication information in the URI. authentication information in the URI. In that subcomponent, the use
of the format "user:password" is deprecated.
Some implementations make use of the userinfo component for internal Some implementations make use of the userinfo component for internal
configuration of authentication information, such as within command configuration of authentication information, such as within command
invocation options, configuration files, or bookmark lists, even invocation options, configuration files, or bookmark lists, even
though such usage might expose a user identifier or password. though such usage might expose a user identifier or password.
A sender MUST NOT generate the userinfo subcomponent (and its "@" A sender MUST NOT generate the userinfo subcomponent (and its "@"
delimiter) when an "http" URI reference is generated delimiter) when an "http" or "https" URI reference is generated
within a message as a request target or header field value. within a message as a target URI or field value.
Before making use of an "http" URI reference received from Before making use of an "http" or "https" URI reference received from
an untrusted source, a recipient SHOULD parse for userinfo and treat an untrusted source, a recipient SHOULD parse for userinfo and treat
its presence as an error; it is likely being used to obscure the its presence as an error; it is likely being used to obscure the
authority for the sake of phishing attacks. authority for the sake of phishing attacks.
4.2.5. Fragment Identifiers on http(s) URI References 4.2.5. http(s) References with Fragment Identifiers
The optional fragment component allows for indirect identification of a Fragment identifiers allow for indirect identification of a secondary
secondary resource, independent of the URI scheme, as defined in Section resource, independent of the URI scheme, as defined in Section 3.5 of
3.5 of [RFC3986]. [RFC3986]. Some protocol elements that refer to a URI allow
inclusion of a fragment, while others do not. They are distinguished
by use of the ABNF rule for elements where fragment is allowed;
otherwise, a specific rule that excludes fragments is used.
[new] | *Note:* The fragment identifier component is not part of the
| scheme definition for a URI scheme (see Section 4.3 of
| [RFC3986]), thus does not appear in the ABNF definitions for
| the "http" and "https" URI schemes above.
4.3. Authoritative Access 4.3. Authoritative Access
[new] Authoritative access refers to dereferencing a given identifier, for
the sake of access to the identified resource, in a way that the
client believes is authoritative (controlled by the resource owner).
The process for determining that access is defined by the URI scheme
and often uses data within the URI components, such as the authority
component when the generic syntax is used. However, authoritative
access is not limited to the identified mechanism.
[new] Section 4.3.1 defines the concept of an origin as an aid to such
uses, and the subsequent subsections explain how to establish a
peer's association with an authority to represent an origin.
See Section 9.1 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
[new] The _origin_ for a given URI is the triple of scheme, host, and port
after normalizing the scheme and host to lowercase and normalizing
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
[new] https://Example.Com/happy.js
[new] would have the origin
[new] { "https", "example.com", "443" }
[new] which can also be described as the normalized URI prefix with port
always present:
[new] https://example.com:443
[new] Each origin defines its own namespace and controls how identifiers
within that namespace are mapped to resources. In turn, how the
origin responds to valid requests, consistently over time, determines
the semantics that users will associate with a URI, and the
usefulness of those semantics is what ultimately transforms these
mechanisms into a "resource" for users to reference and access in the
future.
[new] Two origins are distinct if they differ in scheme, host, or port.
Even when it can be verified that the same entity controls two
distinct origins, the two namespaces under those origins are distinct
unless explicitly aliased by a server authoritative for that origin.
Origin is also used within HTML and related Web protocols, beyond the
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 is specific to TCP-based services because the name delegation scheme (Section 4.2.1) is specific to associating authority with
process depends on TCP for establishing authority. An HTTP service whomever controls the origin server listening for TCP connections on
based on some other underlying connection protocol would presumably the indicated port of whatever host is identified within the
be identified using a different URI scheme, just as the "https" authority component. This is a very weak sense of authority because
scheme (below) is used for resources that require an end-to-end it depends on both client-specific name resolution mechanisms and
secured connection. Other protocols might also be used to provide communication that might not be secured from an on-path attacker.
access to "http" identified resources -- it is only the authoritative Nevertheless, it is a sufficient minimum for binding "http"
interface that is specific to TCP. identifiers to an origin server for consistent resolution within a
trusted environment.
If the host identifier is provided as an IP address, the origin If the host identifier is provided as an IP address, the origin
server is the listener (if any) on the indicated TCP port at that IP server is the listener (if any) on the indicated TCP port at that IP
address. If host is a registered name, the registered name is an address. If host is a registered name, the registered name is an
indirect identifier for use with a name resolution service, such as indirect identifier for use with a name resolution service, such as
DNS, to find an address for that origin server. DNS, to find an address for an appropriate origin server.
When an "http" URI is used within a context that calls for access to When an "http" URI is used within a context that calls for access to
the indicated resource, a client MAY attempt access by resolving the the indicated resource, a client MAY attempt access by resolving the
host to an IP address, establishing a TCP connection to that address host identifier to an IP address, establishing a TCP connection to
on the indicated port, and sending an HTTP request message that address on the indicated port, and sending an HTTP request
(Section 3) containing the URI's identifying data (Section 5) to the message to the server containing the URI's identifying data.
server.
If the server responds to that 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 6 of [RFC7231], then that response response message, as described in Section 15, then that response is
is
considered an authoritative answer to the client's request. considered an authoritative answer to the client's request.
[new] Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative
response is always necessary (see [Caching]). For example, the Alt-
Svc header field [RFC7838] allows an origin server to identify other
services that are also authoritative for that origin. Access to
"http" identified resources might also be provided by protocols
outside the scope of this document.
4.3.3. https origins 4.3.3. https origins
[new] The "https" scheme (Section 4.2.2) associates authority based on the
ability of a server to use the private key corresponding to a
certificate that the client considers to be trustworthy for the
identified origin server. The client usually relies upon a chain of
trust, conveyed from some prearranged or configured trust anchor, to
deem a certificate trustworthy (Section 4.3.4).
[new] In HTTP/1.1 and earlier, a client will only attribute authority to a
server when they are communicating over a successfully established
and secured connection specifically to that URI origin's host. The
connection establishment and certificate verification are used as
proof of authority.
[new] In HTTP/2 and HTTP/3, a client will attribute authority to a server
when they are communicating over a successfully established and
secured connection if the URI origin's host matches any of the hosts
present in the server's certificate and the client believes that it
could open a connection to that host for that URI. In practice, a
client will make a DNS query to check that the origin's host contains
the same server IP address as the established connection. This
restriction can be removed by the origin server sending an equivalent
ORIGIN frame [RFC8336].
[new] The request target's host and port value are passed within each HTTP
request, identifying the origin and distinguishing it from other
namespaces that might be controlled by the same server (Section 7.2).
It is the origin's responsibility to ensure that any services
provided with control over its certificate's private key are equally
responsible for managing the corresponding "https" namespaces, or at
least prepared to reject requests that appear to have been
misdirected. A server might be unwilling to serve as the origin for
some hosts even when they have the authority to do so.
[new] For example, if a network attacker causes connections for port N to
be received at port Q, checking the target URI is necessary to ensure
that the attacker can't cause "https://example.com:N/foo" to be
replaced by "https://example.com:Q/foo" without consent.
[new] Note that the "https" scheme does not rely on TCP and the connected
port number for associating authority, since both are outside the
secured communication and thus cannot be trusted as definitive.
Hence, the HTTP communication might take place over any channel that
has been secured, as defined in Section 4.2.2, including protocols
that don't use TCP.
[new] When an "https" URI is used within a context that calls for access to
the indicated resource, a client MAY attempt access by resolving the
host identifier to an IP address, establishing a TCP connection to
that address on the indicated port, securing the connection end-to-
end by successfully initiating TLS over TCP with confidentiality and
integrity protection, and sending an HTTP request message over that
connection containing the URI's identifying data.
[new] If the server responds to such a request with a non-interim HTTP
response message, as described in Section 15, then that response is
considered an authoritative answer to the client's request.
[new] Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative
response is always necessary (see [Caching]).
4.3.4. https certificate verification 4.3.4. https certificate verification
In general, HTTP/TLS requests are generated by dereferencing a URI. To establish a secured connection to dereference a URI, a client MUST
As a consequence, the hostname for the server is known to the client. verify that the service's identity is an acceptable match for the
If the hostname is available, the client MUST check it against the URI's origin server. Certificate verification is used to prevent
server's identity as presented in the server's Certificate message, server impersonation by an on-path attacker or by an attacker that
in order to prevent man-in-the-middle attacks. controls name resolution. This process requires that a client be
If a subjectAltName extension of type dNSName is present, that MUST configured with a set of trust anchors.
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by In general, a client MUST verify the service identity using the
[RFC2459]. If more than one identity of a given type is present in verification process defined in Section 6 of [RFC6125]. The client
the certificate (e.g., more than one dNSName name, a match in any one MUST construct a reference identity from the service's host: if the
of the set is considered acceptable.) Names may contain the wildcard host is a literal IP address (Section 4.3.5), the reference identity
character * which is considered to match any single domain name is an IP-ID, otherwise the host is a name and the reference identity
component or component fragment. E.g., *.a.com matches foo.a.com but is a DNS-ID.
not bar.foo.a.com. f*.com matches foo.com but not bar.com.
In some cases, the URI is specified as an IP address rather than a
hostname. In this case, the iPAddress subjectAltName must be present
in the certificate and must exactly match the IP in the URI.
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted.
(For instance, a client may be connecting to a machine whose address
and hostname are dynamic but the client knows the certificate that
the server will present.) In such cases, it is important to narrow
the scope of acceptable certificates as much as possible in order
to prevent man in the middle attacks.
In special cases, it may be appropriate for the client to simply 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-
ID might be used by older clients.
A client might be specially configured to accept an alternative form
of server identity verification. For example, a client might be
connecting to a server whose address and hostname are dynamic, with
an expectation that the service will present a specific certificate
(or a certificate matching some externally defined reference
identity) rather than one matching the dynamic URI's origin server
identifier.
In special cases, it might be appropriate for a client to simply
ignore the server's identity, but it must be understood that this ignore the server's identity, but it must be understood that this
leaves the connection open to active attack. leaves a connection open to active attack.
If the hostname does not match the identity in the certificate, user If the certificate is not valid for the URI's origin server, a user
oriented clients MUST either notify the user (clients MAY give the agent MUST either notify the user (user agents MAY give the user an
user the opportunity to continue with the connection in any case) or option to continue with the connection in any case) or terminate the
terminate the connection with a bad certificate error. Automated connection with a bad certificate error. Automated clients MUST log
clients MUST log the error to an appropriate audit log (if available) the error to an appropriate audit log (if available) and SHOULD
and SHOULD terminate the connection (with a bad certificate error). terminate the connection (with a bad certificate error). Automated
Automated clients MAY provide a configuration setting that disables clients MAY provide a configuration setting that disables this check,
this check, but MUST provide a setting which enables it. but MUST provide a setting which enables it.
Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against 4.3.5. IP-ID reference identity
attacks where this source is compromised. For example, if the URI was
obtained by clicking on an HTML page which was itself obtained A server that is identified using an IP address literal in the "host"
without using HTTP/TLS, a man in the middle could have replaced the field of an "https" URI has a reference identity of type IP-ID. An
URI. In order to prevent this form of attack, users should carefully IP version 4 address uses the "IPv4address" ABNF rule and an IP
examine the certificate presented by the server to determine if it version 6 address uses the "IP-literal" production with the
meets their expectations. "IPv6address" option; see Section 3.2.2 of [RFC3986]. A reference
*3.2. Client Identity [paras squished together to anchor context]* identity of IP-ID contains the decoded bytes of the IP address.
Typically, the server has no external knowledge of what the client's
identity ought to be and so checks (other than that the client has a An IP version 4 address is 4 octets and an IP version 6 address is 16
certificate chain rooted in an appropriate CA) are not possible. If a octets. Use of IP-ID is not defined for any other IP version. The
server has such knowledge (typically from some source external to iPAddress choice in the certificate subjectAltName extension does not
HTTP or TLS) it SHOULD check the identity as described above. explicitly include the IP version and so relies on the length of the
address to distinguish versions; see Section 4.2.1.6 of [RFC5280].
A reference identity of type IP-ID matches if the address is
identical to an iPAddress value of the subjectAltName extension of
the certificate.
5. Fields 5. Fields
Header fields are key:value pairs that can be used to communicate HTTP uses _fields_ to provide data in the form of extensible key/
data about the message, its payload, the target resource, or the value pairs with a registered key namespace. Fields are sent and
connection (i.e., control data). See Section 3.2 of [RFC7230] for a received within the header and trailer sections of messages
general definition of header field syntax in HTTP messages. (Section 6).
5.1. Field Names 5.1. Field Names
The field-name token labels the corresponding field-value as having A field name labels the corresponding field value as having the
the semantics defined by that header field. For example, the Date semantics defined by that name. For example, the Date header field
header field is defined in Section 7.1.1.2 of [RFC7231] as containing is defined in Section 10.2.2 as containing the origination timestamp
the origination timestamp for the message in which it appears. for the message in which it appears.
field-name = token field-name = token
The requirements for header field names are defined in [BCP90]. Field names are case-insensitive and ought to be registered within
the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
Section 16.3.1.
The interpretation of a header field does not change between minor versions The interpretation of a field does not change between minor versions
of the same major HTTP version, though the default behavior of a of the same major HTTP version, though the default behavior of a
recipient in the absence of such a field can change. Unless recipient in the absence of such a field can change. Unless
specified otherwise, header fields defined in HTTP/1.1 are defined specified otherwise, fields are defined for all versions of HTTP. In
for all versions of HTTP/1.x. In particular, the Host and Connection fields ought to be recognized by
particular, the Host and Connection header fields ought to be implemented by all HTTP implementations whether or not they advertise conformance
all HTTP/1.x implementations whether or not they advertise conformance
with HTTP/1.1. with HTTP/1.1.
New header fields can be introduced without changing the protocol version if New fields can be introduced without changing the protocol version if
their defined semantics allow them to be safely ignored by recipients their defined semantics allow them to be safely ignored by recipients
that do not recognize them. Header field extensibility is that do not recognize them; see Section 16.3.
discussed in Section 3.2.1.
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 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 fields. fields. Other recipients SHOULD ignore unrecognized header and
These requirements allow HTTP's functionality to be enhanced without trailer fields. Adhering to these requirements allows HTTP's
requiring prior update of deployed intermediaries. functionality to be extended without updating or removing deployed
intermediaries.
5.2. Field Lines and Combined Field Value
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
line value_ that conveys data for that instance of the field.
When a field name is only present once in a section, the combined
_field value_ for that field consists of the corresponding field line
value. When a field name is repeated within a section, its combined
field value consists of the list of corresponding field line values
within that section, concatenated in order, with each field line
value separated by a comma.
For example, this section:
Example-Field: Foo, Bar
Example-Field: Baz
contains two field lines, both with the field name "Example-Field".
The first field line has a field line value of "Foo, Bar", while the
second field line value is "Baz". The field value for "Example-
Field" is a list with three members: "Foo", "Bar", and "Baz".
5.3. Field Order 5.3. Field Order
A recipient MAY combine multiple header fields with the same field A recipient MAY combine multiple field lines within a field section
name into one "field-name: field-value" pair, without changing the that have the same field name into one field line, without changing
semantics of the message, by appending each subsequent field value to the semantics of the message, by appending each subsequent field line
the combined field value in order, separated by a comma. value to the initial field line value in order, separated by a comma
(",") and optional whitespace (OWS, defined in Section 5.6.3). For
consistency, use comma SP.
The order in which header fields with the same field name are received is The order in which field lines with the same name are received is
therefore significant to the interpretation of the combined field value; a therefore significant to the interpretation of the field value; a
proxy MUST NOT change the order of these field values when proxy MUST NOT change the order of these field line values when
forwarding a message. forwarding a message.
A sender MUST NOT generate multiple header fields with the same field This means that, aside from the well-known exception noted below, a
name in a message unless either the entire field value for that sender MUST NOT generate multiple field lines with the same name in a
header field is defined as a comma-separated list [i.e., #(values)] message (whether in the headers or trailers), or append a field line
or the header field is a well-known exception (as noted below). when a field line of the same name already exists in the message,
unless that field's definition allows multiple field line values to
be recombined as a comma-separated list [i.e., at least one
alternative of the field's definition allows a comma-separated list,
such as an ABNF rule of #(values) defined in Section 5.6.1].
Note: In practice, the "Set-Cookie" header field ([RFC6265]) often | *Note:* In practice, the "Set-Cookie" header field ([RFC6265])
appears multiple times in a response message and does not use the | often appears in a response message across multiple field lines
list syntax, violating the above requirements on multiple header | and does not use the list syntax, violating the above
fields with the same name. Since it cannot be combined into a | requirements on multiple field lines with the same field name.
single field-value, recipients ought to handle "Set-Cookie" as a | Since it cannot be combined into a single field value,
special case while processing header fields. (See Appendix A.2.3 | recipients ought to handle "Set-Cookie" as a special case while
of [Kri2001] for details.) | processing fields. (See Appendix A.2.3 of [Kri2001] for
| details.)
The order in which header fields with differing field names are The order in which field lines with differing field names are
received is not significant. However, it is good practice to send received in a section is not significant. However, it is good
header fields that contain control data first, such as Host on practice to send header fields that contain additional control data
requests and Date on responses, so that implementations can decide first, such as Host on requests and Date on responses, so that
when not to handle a message as early as possible. implementations can decide when not to handle a message as early as
possible.
A server MUST NOT apply a request to the target resource until the A server MUST NOT apply a request to the target resource until it
entire request header section is received, since later header fields receives the entire request header section, since later header field
might include conditionals, authentication credentials, or lines might include conditionals, authentication credentials, or
deliberately misleading duplicate header fields that would impact deliberately misleading duplicate header fields that could impact
request processing. request processing.
5.4. Field Limits 5.4. Field Limits
HTTP does not place a predefined limit on the length of each header HTTP does not place a predefined limit on the length of each field
field or on the length of the header section as a whole, as described line, field value, or on the length of a header or trailer section as
in Section 2.5. Various ad hoc limitations on individual header a whole, as described in Section 2. Various ad hoc limitations on
field length are found in practice, often depending on the specific individual lengths are found in practice, often depending on the
field semantics. specific field's semantics.
A server that receives a request header field, or set of fields, A server that receives a request header field line, field value, or
larger than it wishes to process MUST respond with an appropriate 4xx set of fields larger than it wishes to process MUST respond with an
(Client Error) status code. Ignoring such header fields would appropriate 4xx (Client Error) status code. Ignoring such header
increase the server's vulnerability to request smuggling attacks fields would increase the server's vulnerability to request smuggling
(Section 9.5). attacks (Section 11.2 of [Messaging]).
A client MAY discard or truncate received header fields that are A client MAY discard or truncate received field lines that are larger
larger than the client wishes to process if the field semantics are than the client wishes to process if the field semantics are such
such that the dropped value(s) can be safely ignored without changing that the dropped value(s) can be safely ignored without changing the
the message framing or response semantics. message framing or response semantics.
5.5. Field Values 5.5. Field Values
New header field values typically have their syntax defined using HTTP field values consist of a sequence of characters in a format
ABNF ([RFC5234]), using the extension defined in Section 7 of [RFC7230] defined by the field's grammar. Each field's grammar is usually
as necessary, and defined using ABNF ([RFC5234]).
field-value = *( field-content / obs-fold ) field-value = *field-content
field-content = field-vchar [ 1*( SP / HTAB ) field-vchar ] field-content = field-vchar
[ 1*( SP / HTAB / field-vchar ) field-vchar ]
field-vchar = VCHAR / obs-text field-vchar = VCHAR / obs-text
Leading and trailing whitespace in raw field values is removed upon A field value does not include leading or trailing whitespace. When
field parsing (Section 3.2.4 of [RFC7230]). Field definitions where a specific version of HTTP allows such whitespace to appear in a
leading or trailing whitespace in values is significant will have to message, a field parsing implementation MUST exclude such whitespace
use a container syntax such as quoted-string (Section 3.2.6 of prior to evaluating the field value.
[RFC7230]).
are usually constrained to the range of Field values are usually constrained to the range of US-ASCII
US-ASCII characters. Header fields needing a greater range of characters [USASCII]. Fields needing a greater range of characters
characters can use an encoding such as the one defined in [RFC5987]. can use an encoding, such as the one defined in [RFC8187].
Historically, HTTP has allowed field content with text in the Historically, HTTP allowed field content with text in the ISO-8859-1
ISO-8859-1 charset [ISO-8859-1], supporting other charsets only charset [ISO-8859-1], supporting other charsets only through use of
through use of [RFC2047] encoding. In practice, most HTTP header [RFC2047] encoding. Specifications for newly defined fields SHOULD
field values use only a subset of the US-ASCII charset [USASCII]. limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB.
Newly defined header fields SHOULD limit their field values to A recipient SHOULD treat other octets in field content (obs-text) as
US-ASCII octets. A recipient SHOULD treat other octets in field opaque data.
content (obs-text) as opaque data.
[new] Field values containing CR or NUL characters are invalid and
dangerous, due to the varying ways that implementations might parse
and interpret those characters; a recipient of CR or NUL within a
field value MUST either reject the message or replace each of those
characters with SP before further processing or forwarding of that
message. Field values containing other CTL characters are also
invalid; however, recipients MAY retain such characters for the sake
of robustness if they only appear within safe field value contexts
(e.g., opaque data).
[new] Fields that only anticipate a single member as the field value are
referred to as _singleton fields_.
[new] 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
is used as a common notation for defining field values that can
contain multiple members.
Because commas (",") are used as a generic delimiter between Because commas (",") are used as the delimiter between members, they
field-values, they need to be treated with care if they are allowed need to be treated with care if they are allowed as data within a
in the field-value. Typically, components that might contain a comma member. This is true for both list-based and singleton fields, since
are protected with double-quotes using the quoted-string ABNF a singleton field might be erroneously sent with multiple members and
production. detecting such errors improves interoperability. Fields that expect
to contain a comma within a member, such as within an HTTP-date or
URI-reference element, ought to be defined with delimiters around
that element to distinguish any comma within that data from potential
list separators.
For example, a textual date and a URI (either of which might contain For example, a textual date and a URI (either of which might contain
a comma) could be safely carried in field-values like these: a comma) could be safely carried in list-based field values like
these:
Example-URI-Field: "http://example.com/a.html,foo", Example-URIs: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/" "http://without-a-comma.example.com/"
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005" Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double-quote delimiters almost always are used with the Note that double-quote delimiters are almost always used with the
quoted-string production; using a different syntax inside quoted-string production; using a different syntax inside double-
double-quotes will likely cause unnecessary confusion. quotes will likely cause unnecessary confusion.
Many header fields use a format including (case-insensitively) named Many fields (such as Content-Type, defined in Section 8.3) use a
parameters (for instance, Content-Type, defined in Section 3.1.1.5). common syntax for parameters that allows both unquoted (token) and
Allowing both unquoted (token) and quoted (quoted-string) syntax for quoted (quoted-string) syntax for a parameter value (Section 5.6.6).
the parameter value enables recipients to use existing parser Use of common syntax allows recipients to reuse existing parser
components. When allowing both forms, the meaning of a parameter components. When allowing both forms, the meaning of a parameter
value ought to be independent of the syntax used for it (for an value ought to be the same whether it was received as a token or a
example, see the notes on parameter handling for media types in quoted string.
Section 3.1.1.1).
Historically, HTTP header field values could be extended over Historically, HTTP field values could be extended over multiple lines
multiple lines by preceding each extra line with at least one space by preceding each extra line with at least one space or horizontal
or horizontal tab (obs-fold). tab (obs-fold). This document assumes that any such obsolete line
folding has been removed prior to interpreting the field value (e.g.,
as described in Section 5.2 of [Messaging]).
Consequently, this specification does not use ABNF rules | *Note:* For defining field value syntax, this specification
to define each "Field-Name: Field Value" pair, as was done in | uses an ABNF rule named after the field name to define the
previous editions. Instead, this specification uses ABNF rules that | allowed grammar for that field's value (after said value has
are named according to each registered field name, wherein the rule | been extracted from the underlying messaging syntax and
defines the valid grammar for that field's corresponding field values | multiple instances combined into a list).
(i.e., after the field-value has been extracted from the header
section by a generic field parser).
5.6. Field Value Components 5.6. Common Rules for Defining Field Values
5.6.1. ABNF List Extension: #rule 5.6.1. Lists (#rule ABNF Extension)
A #rule extension to the ABNF rules of [RFC5234] is used to improve A #rule extension to the ABNF rules of [RFC5234] is used to improve
readability in the definitions of some header field values. readability in the definitions of some list-based field values.
A construct "#" is defined, similar to "*", for defining A construct "#" is defined, similar to "*", for defining comma-
comma-delimited lists of elements. The full form is "<n>#<m>element" delimited lists of elements. The full form is "<n>#<m>element"
indicating at least <n> and at most <m> elements, each separated by a indicating at least <n> and at most <m> elements, each separated by a
single comma (",") and optional whitespace (OWS). single comma (",") and optional whitespace (OWS, defined in
Section 5.6.3).
5.6.1.1. Sender Requirements 5.6.1.1. Sender Requirements
In any production that uses the list construct, a sender MUST NOT In any production that uses the list construct, a sender MUST NOT
generate empty list elements. In other words, a sender MUST generate generate empty list elements. In other words, a sender MUST generate
lists that satisfy the following syntax: lists that satisfy the following syntax:
1#element => element *( OWS "," OWS element ) 1#element => element *( OWS "," OWS element )
and: and:
#element => [ 1#element ] #element => [ 1#element ]
and for n >= 1 and m > 1: and for n >= 1 and m > 1:
<n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element ) <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
Appendix B shows the collected ABNF for recipients after the list Appendix A shows the collected ABNF for senders after the list
constructs have been expanded. constructs have been expanded.
5.6.1.2. Recipient Requirements 5.6.1.2. Recipient Requirements
For compatibility with legacy list rules, a Empty elements do not contribute to the count of elements present. A
recipient MUST parse and ignore a reasonable number of empty list recipient MUST parse and ignore a reasonable number of empty list
elements: enough to handle common mistakes by senders that merge elements: enough to handle common mistakes by senders that merge
values, but not so much that they could be used as a denial-of- values, but not so much that they could be used as a denial-of-
service mechanism. In other words, a recipient MUST accept lists service mechanism. In other words, a recipient MUST accept lists
that satisfy the following syntax: that satisfy the following syntax:
#element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ] #element => [ element ] *( OWS "," OWS [ element ] )
1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )
Empty elements do not contribute to the count of elements present. Note that because of the potential presence of empty list elements,
the RFC 5234 ABNF cannot enforce the cardinality of list elements,
and consequently all cases are mapped as if there was no cardinality
specified.
For example, given these ABNF productions: For example, given these ABNF productions:
example-list = 1#example-list-elmt example-list = 1#example-list-elmt
example-list-elmt = token ; see Section 3.2.6 example-list-elmt = token ; see Section 5.6.2
Then the following are valid values for example-list (not including Then the following are valid values for example-list (not including
the double quotes, which are present for delimitation only): the double quotes, which are present for delimitation only):
"foo,bar" "foo,bar"
"foo ,bar," "foo ,bar,"
"foo , ,bar,charlie " "foo , ,bar,charlie"
In contrast, the following values would be invalid, since at least In contrast, the following values would be invalid, since at least
one non-empty element is required by the example-list production: one non-empty element is required by the example-list production:
"" ""
"," ","
", ," ", ,"
5.6.2. Tokens 5.6.2. Tokens
[new] Tokens are short textual identifiers that do not include whitespace
or delimiters.
token = 1*tchar token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA / DIGIT / ALPHA
; any VCHAR, except delimiters ; any VCHAR, except delimiters
Most HTTP header field values are defined using common syntax Many HTTP field values are defined using common syntax components,
components (token, quoted-string, and comment) separated by separated by whitespace or specific delimiting characters.
whitespace or specific delimiting characters. Delimiters are chosen Delimiters are chosen from the set of US-ASCII visual characters not
from the set of US-ASCII visual characters not allowed in a token allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").
(DQUOTE and "(),/:;<=>?@[\]{}").
5.6.3. Whitespace 5.6.3. Whitespace
This specification uses three rules to denote the use of linear This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace). BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets The OWS rule is used where zero or more linear whitespace octets
might appear. For protocol elements where optional whitespace is might appear. For protocol elements where optional whitespace is
preferred to improve readability, a sender SHOULD generate the preferred to improve readability, a sender SHOULD generate the
optional whitespace as a single SP; otherwise, a sender SHOULD NOT optional whitespace as a single SP; otherwise, a sender SHOULD NOT
generate optional whitespace except as needed to white out invalid or generate optional whitespace except as needed to overwrite invalid or
unwanted protocol elements during in-place message filtering. unwanted protocol elements during in-place message filtering.
The RWS rule is used when at least one linear whitespace octet is The RWS rule is used when at least one linear whitespace octet is
required to separate field tokens. A sender SHOULD generate RWS as a required to separate field tokens. A sender SHOULD generate RWS as a
single SP. single SP.
OWS and RWS have the same semantics as a single SP. Any content
known to be defined as OWS or RWS MAY be replaced with a single SP
before interpreting it or forwarding the message downstream.
The BWS rule is used where the grammar allows optional whitespace The BWS rule is used where the grammar allows optional whitespace
only for historical reasons. A sender MUST NOT generate BWS in only for historical reasons. A sender MUST NOT generate BWS in
messages. A recipient MUST parse for such bad whitespace and remove messages. A recipient MUST parse for such bad whitespace and remove
it before interpreting the protocol element. it before interpreting the protocol element.
BWS has no semantics. Any content known to be defined as BWS MAY be
removed before interpreting it or forwarding the message downstream.
OWS = *( SP / HTAB ) OWS = *( SP / HTAB )
; optional whitespace ; optional whitespace
RWS = 1*( SP / HTAB ) RWS = 1*( SP / HTAB )
; required whitespace ; required whitespace
BWS = OWS BWS = OWS
; "bad" whitespace ; "bad" whitespace
5.6.4. Quoted Strings 5.6.4. Quoted Strings
A string of text is parsed as a single value if it is quoted using A string of text is parsed as a single value if it is quoted using
double-quote marks. double-quote marks.
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP /%x21 / %x23-5B / %x5D-7E / obs-text qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
obs-text = %x80-FF obs-text = %x80-FF
The backslash octet ("\") can be used as a single-octet quoting The backslash octet ("\") can be used as a single-octet quoting
mechanism within quoted-string and comment constructs. Recipients mechanism within quoted-string and comment constructs. Recipients
that process the value of a quoted-string MUST handle a quoted-pair that process the value of a quoted-string MUST handle a quoted-pair
as if it were replaced by the octet following the backslash. as if it were replaced by the octet following the backslash.
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text ) quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
A sender SHOULD NOT generate a quoted-pair in a quoted-string except A sender SHOULD NOT generate a quoted-pair in a quoted-string except
where necessary to quote DQUOTE and backslash octets occurring within where necessary to quote DQUOTE and backslash octets occurring within
that string. A sender SHOULD NOT generate a quoted-pair in a comment that string. A sender SHOULD NOT generate a quoted-pair in a comment
except where necessary to quote parentheses ["(" and ")"] and except where necessary to quote parentheses ["(" and ")"] and
backslash octets occurring within that comment. backslash octets occurring within that comment.
5.6.5. Comments 5.6.5. Comments
Comments can be included in some HTTP header fields by surrounding Comments can be included in some HTTP fields by surrounding the
the comment text with parentheses. Comments are only allowed in comment text with parentheses. Comments are only allowed in fields
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
[new] Parameters are instances of name=value pairs; they are often used in
field values as a common syntax for appending auxiliary information
to an item. Each parameter is usually delimited by an immediately
preceding semicolon.
parameter = token "=" ( token / quoted-string ) parameters = *( OWS ";" OWS [ parameter ] )
parameter = parameter-name "=" parameter-value
parameter-name = token
parameter-value = ( token / quoted-string )
The parameter name tokens are case-insensitive. Parameter names are case-insensitive. Parameter values might or
Parameter values might or might not be case-sensitive, depending on might not be case-sensitive, depending on the semantics of the
the semantics of the parameter name. parameter name. Examples of parameters and some equivalent forms can
be seen in media types (Section 8.3.1) and the Accept header field
(Section 12.5.1).
A parameter value that matches the token production can be A parameter value that matches the token production can be
transmitted either as a token or within a quoted-string. The quoted transmitted either as a token or within a quoted-string. The quoted
and unquoted values are equivalent. and unquoted values are equivalent.
Note: Unlike some similar constructs in other header fields, media | *Note:* Parameters do not allow whitespace (not even "bad"
type parameters do not allow whitespace (even "bad" whitespace) | whitespace) around the "=" character.
around the "=" character.
5.6.7. Date/Time Formats 5.6.7. Date/Time Formats
Prior to 1995, there were three different formats commonly used by Prior to 1995, there were three different formats commonly used by
servers to communicate timestamps. For compatibility with old servers to communicate timestamps. For compatibility with old
implementations, all three are defined here. The preferred format is implementations, all three are defined here. The preferred format is
a fixed-length and single-zone subset of the date and time a fixed-length and single-zone subset of the date and time
specification used by the Internet Message Format [RFC5322]. specification used by the Internet Message Format [RFC5322].
HTTP-date = IMF-fixdate / obs-date HTTP-date = IMF-fixdate / obs-date
An example of the preferred format is An example of the preferred format is
Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate
Examples of the two obsolete formats are Examples of the two obsolete formats are
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
A recipient that parses a timestamp value in an HTTP header field A recipient that parses a timestamp value in an HTTP field MUST
MUST accept all three HTTP-date formats. When a sender generates a accept all three HTTP-date formats. When a sender generates a field
header field that contains one or more timestamps defined as that contains one or more timestamps defined as HTTP-date, the sender
HTTP-date, the sender MUST generate those timestamps in the MUST generate those timestamps in the IMF-fixdate format.
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. A sender that generates HTTP-date values from a local to be in UTC. A sender that generates HTTP-date values from a local
clock ought to use NTP ([RFC5905]) or some similar protocol to clock ought to use NTP ([RFC5905]) or some similar protocol to
synchronize its clock to UTC. synchronize its clock to 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]
day-name = %x4D.6F.6E ; "Mon", case-sensitive day-name = %s"Mon" / %s"Tue" / %s"Wed"
/ %x54.75.65 ; "Tue", case-sensitive / %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"
/ %x57.65.64 ; "Wed", case-sensitive
/ %x54.68.75 ; "Thu", case-sensitive
/ %x46.72.69 ; "Fri", case-sensitive
/ %x53.61.74 ; "Sat", case-sensitive
/ %x53.75.6E ; "Sun", case-sensitive
date1 = day SP month SP year date1 = day SP month SP year
; e.g., 02 Jun 1982 ; e.g., 02 Jun 1982
day = 2DIGIT day = 2DIGIT
month = %x4A.61.6E ; "Jan", case-sensitive month = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
/ %x46.65.62 ; "Feb", case-sensitive / %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
/ %x4D.61.72 ; "Mar", case-sensitive / %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
/ %x41.70.72 ; "Apr", case-sensitive
/ %x4D.61.79 ; "May", case-sensitive
/ %x4A.75.6E ; "Jun", case-sensitive
/ %x4A.75.6C ; "Jul", case-sensitive
/ %x41.75.67 ; "Aug", case-sensitive
/ %x53.65.70 ; "Sep", case-sensitive
/ %x4F.63.74 ; "Oct", case-sensitive
/ %x4E.6F.76 ; "Nov", case-sensitive
/ %x44.65.63 ; "Dec", case-sensitive
year = 4DIGIT year = 4DIGIT
GMT = %x47.4D.54 ; "GMT", case-sensitive GMT = %s"GMT"
time-of-day = hour ":" minute ":" second time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:60 (leap second) ; 00:00:00 - 23:59:60 (leap second)
hour = 2DIGIT hour = 2DIGIT
minute = 2DIGIT minute = 2DIGIT
second = 2DIGIT second = 2DIGIT
Obsolete formats: Obsolete formats:
obs-date = rfc850-date / asctime-date obs-date = rfc850-date / asctime-date
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
date2 = day "-" month "-" 2DIGIT date2 = day "-" month "-" 2DIGIT
; e.g., 02-Jun-82 ; e.g., 02-Jun-82
day-name-l = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive day-name-l = %s"Monday" / %s"Tuesday" / %s"Wednesday"
/ %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive / %s"Thursday" / %s"Friday" / %s"Saturday"
/ %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive / %s"Sunday"
/ %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
/ %x46.72.69.64.61.79 ; "Friday", case-sensitive
/ %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
/ %x53.75.6E.64.61.79 ; "Sunday", case-sensitive
asctime-date = day-name SP date3 SP time-of-day SP year asctime-date = day-name SP date3 SP time-of-day SP year
date3 = month SP ( 2DIGIT / ( SP 1DIGIT )) date3 = month SP ( 2DIGIT / ( SP 1DIGIT ))
; e.g., Jun 2 ; e.g., Jun 2
HTTP-date is case sensitive. A sender MUST NOT generate additional HTTP-date is case sensitive. Note that Section 4.2 of [Caching]
whitespace in an HTTP-date beyond that specifically included as SP in relaxes this for cache recipients.
the grammar. The semantics of day-name, day, month, year, and
time-of-day are the same as those defined for the Internet Message A sender MUST NOT generate additional whitespace in an HTTP-date
Format constructs with the corresponding name ([RFC5322], Section beyond that specifically included as SP in the grammar. The
3.3). semantics of day-name, day, month, year, and time-of-day are the same
as those defined for the Internet Message Format constructs with the
corresponding name ([RFC5322], Section 3.3).
Recipients of a timestamp value in rfc850-date format, which uses a Recipients of a timestamp value in rfc850-date format, which uses a
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 example, messages are occasionally forwarded over HTTP from a non-
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 only | *Note:* HTTP requirements for the date/time stamp format apply
to their usage within the protocol stream. Implementations are | only to their usage within the protocol stream.
not required to use these formats for user presentation, request | Implementations are not required to use these formats for user
logging, etc. | presentation, request logging, etc.
6. Message Abstraction 6. Message Abstraction
[new] Each major version of HTTP defines its own syntax for communicating
messages. This section defines an abstract data type for HTTP
messages based on a generalization of those message characteristics,
common structure, and capacity for conveying semantics. This
abstraction is used to define requirements on senders and recipients
that are independent of the HTTP version, such that a message in one
version can be relayed through other versions without changing its
meaning.
A _message_ consists of control data to describe and route the
message, a headers lookup table of key/value pairs for extending that
control data and conveying additional information about the sender,
message, content, or context, a potentially unbounded stream of
content, and a trailers lookup table of key/value pairs for
communicating information obtained while sending the content.
Framing and control data is sent first, followed by a header section
containing fields for the headers table. When a message includes
content, the content is sent after the header section, potentially
followed by a trailer section that might contain fields for the
trailers table.
Messages are expected to be processed as a stream, wherein the
purpose of that stream and its continued processing is revealed while
being read. Hence, control data describes what the recipient needs
to know immediately, header fields describe what needs to be known
before receiving content, the content (when present) presumably
contains what the recipient wants or needs to fulfill the message
semantics, and trailer fields provide optional metadata that was
unknown prior to sending the content.
Messages are intended to be _self-descriptive_: everything a
recipient needs to know about the message can be determined by
looking at the message itself, after decoding or reconstituting parts
that have been compressed or elided in transit, without requiring an
understanding of the sender's current application state (established
via prior messages). However, a client MUST retain knowledge of the
request when parsing, interpreting, or caching a corresponding
response. For example, responses to the HEAD method look just like
the beginning of a response to GET, but cannot be parsed in the same
manner.
Note that this message abstraction is a generalization across many
versions of HTTP, including features that might not be found in some
versions. For example, trailers were introduced within the HTTP/1.1
chunked transfer coding as a trailer section after the content. An
equivalent feature is present in HTTP/2 and HTTP/3 within the header
block that terminates each stream.
6.1. Framing and Completeness 6.1. Framing and Completeness
[new] Message framing indicates how each message begins and ends, such that
each message can be distinguished from other messages or noise on the
same connection. Each major version of HTTP defines its own framing
mechanism.
[new] HTTP/0.9 and early deployments of HTTP/1.0 used closure of the
underlying connection to end a response. For backwards
compatibility, this implicit framing is also allowed in HTTP/1.1.
However, implicit framing can fail to distinguish an incomplete
response if the connection closes early. For that reason, almost all
modern implementations use explicit framing in the form of length-
delimited sequences of message data.
A message is considered _complete_ when all of the octets indicated
by its framing are available. Note that, when no explicit framing is
used, a response message that is ended by the underlying connection's
close is considered complete even though it might be
indistinguishable from an incomplete response, unless a transport-
level error indicates that it is not complete.
6.2. Control Data 6.2. Control Data
HTTP communication is initiated by a user agent for some purpose. Messages start with control data that describe its primary purpose.
The purpose is a combination of request semantics, which are defined Request message control data includes a request method (Section 9),
in [RFC7231], and a target resource upon which to apply those request target (Section 7.1), and protocol version (Section 2.5).
semantics. Response message control data includes a status code (Section 15),
optional reason phrase, and protocol version.
In HTTP/1.1 ([Messaging]) and earlier, control data is sent as the
first line of a message. In HTTP/2 ([RFC7540]) and HTTP/3 ([HTTP3]),
control data is sent as pseudo-header fields with a reserved name
prefix (e.g., ":authority").
Every HTTP message has a protocol version. Depending on the version
in use, it might be identified within the message explicitly or
inferred by the connection over which the message is received.
Recipients use that version information to determine limitations or
potential for later communication with that sender.
When a message is forwarded by an intermediary, the protocol version
is updated to reflect the version used by that intermediary. The Via
header field (Section 7.6.3) is used to communicate upstream protocol
information within a forwarded message.
A client SHOULD send a request version equal to the highest version A client SHOULD send a request version equal to the highest version
to which the client is conformant and whose major version is no to which the client is conformant and whose major version is no
higher than the highest version supported by the server, if this is higher than the highest version supported by the server, if this is
known. A client MUST NOT send a version to which it is not known. A client MUST NOT send a version to which it is not
conformant. conformant.
A client MAY send a lower request version if it is known that the A client MAY send a lower request version if it is known that the
server incorrectly implements the HTTP specification, but only after server incorrectly implements the HTTP specification, but only after
the client has attempted at least one normal request and determined the client has attempted at least one normal request and determined
from the response status code or header fields (e.g., Server) that from the response status code or header fields (e.g., Server) that
the server improperly handles higher request versions. the server improperly handles higher request versions.
A server SHOULD send a response version equal to the highest version A server SHOULD send a response version equal to the highest version
to which the server is conformant that has a major version less than to which the server is conformant that has a major version less than
or equal to the one received in the request. A server MUST NOT send or equal to the one received in the request. A server MUST NOT send
a version to which it is not conformant. A server can send a 505 a version to which it is not conformant. A server can send a 505
(HTTP Version Not Supported) response if it wishes, for any reason, (HTTP Version Not Supported) response if it wishes, for any reason,
to refuse service of the client's major protocol version. to refuse service of the client's major protocol version.
When an HTTP message is received with a major version number that the A recipient that receives a message with a major version number that
recipient implements, but a higher minor version number than what the it implements and a minor version number higher than what it
recipient implements, the recipient SHOULD process the message as if implements SHOULD process the message as if it were in the highest
it were in the highest minor version within that major version to minor version within that major version to which the recipient is
which the recipient is conformant. A recipient can assume that a conformant. A recipient can assume that a message with a higher
message with a higher minor version, when sent to a recipient that minor version, when sent to a recipient that has not yet indicated
has not yet indicated support for that higher version, is support for that higher version, is sufficiently backwards-compatible
sufficiently backwards-compatible to be safely processed by any to be safely processed by any implementation of the same major
implementation of the same major version. version.
6.3. Header Fields 6.3. Header Fields
[new] Fields (Section 5) that are sent/received before the content are
referred to as "header fields" (or just "headers", colloquially).
The _header section_ of a message consists of a sequence of header
field lines. Each header field might modify or extend message
semantics, describe the sender, define the content, or provide
additional context.
| *Note:* We refer to named fields specifically as a "header
| field" when they are only allowed to be sent in the header
| section.
6.4. Content 6.4. Content
Some HTTP messages transfer a complete or partial representation as HTTP messages often transfer a complete or partial representation as
the message "payload". In some cases, a payload might contain only the message _content_: a stream of octets sent after the header
the associated representation's header fields (e.g., responses to section, as delineated by the message framing.
HEAD) or only some part(s) of the representation data (e.g., the 206
(Partial Content) status code).
[new] This abstract definition of content reflects the data after it has
been extracted from the message framing. For example, an HTTP/1.1
message body (Section 6 of [Messaging]) might consist of a stream of
data encoded with the chunked transfer coding - a sequence of data
chunks, one zero-length chunk, and a trailer section - whereas the
content of that same message includes only the data stream after the
transfer coding has been decoded; it does not include the chunk
lengths, chunked framing syntax, nor the trailer fields
(Section 6.5).
6.4.1. Content Semantics 6.4.1. Content Semantics
The purpose of a payload in a request is defined by the method The purpose of content in a request is defined by the method
semantics. semantics (Section 9).
For example, a representation in the payload of a PUT request For example, a representation in the content of a PUT request
(Section 4.3.4) represents the desired state of the target resource (Section 9.3.4) represents the desired state of the target resource
if the request is successfully applied, whereas a representation after the request is successfully applied, whereas a representation
in the payload of a POST request (Section 4.3.3) represents in the content of a POST request (Section 9.3.3) represents
information to be processed by the target resource. information to be processed by the target resource.
In a response, the payload's purpose is defined by both the request In a response, the content's purpose is defined by both the request
method and the response status code. For example, the payload of a method and the response status code (Section 15). For example, the
200 (OK) response to GET (Section 4.3.1) represents the current state content of a 200 (OK) response to GET (Section 9.3.1) represents the
of the target resource, as observed at the time of the message current state of the target resource, as observed at the time of the
origination date (Section 7.1.1.2), whereas the payload of the same message origination date (Section 10.2.2), whereas the content of the
status code in a response to POST might represent either the same status code in a response to POST might represent either the
processing result or the new state of the target resource after processing result or the new state of the target resource after
applying the processing. applying the processing.
[new] The content of a 206 (Partial Content) response to GET contains
either a single part of the selected representation or a multipart
message body containing multiple parts of that representation, as
described in Section 15.3.7.
Response messages with an error status code usually contain a payload Response messages with an error status code usually contain content
that represents the error condition, such that it describes that represents the error condition, such that the content describes
the error state and what next steps are suggested for resolving it. the error state and what steps are suggested for resolving it.
Responses to the HEAD request method (Section 4.3.2 Responses to the HEAD request method (Section 9.3.2) never include
of [RFC7231]) never include a message body because the associated content; the associated response header fields indicate only what
response header fields (e.g., Transfer-Encoding, Content-Length, their values would have been if the request method had been GET
etc.), if present, indicate only what their values would have been if (Section 9.3.1).
the request method had been GET (Section 4.3.1 of [RFC7231]).
2xx (Successful) responses to a CONNECT request method 2xx (Successful) responses to a CONNECT request method
(Section 4.3.6 of [RFC7231]) switch to tunnel mode instead of (Section 9.3.6) switch the connection to tunnel mode instead of
having a message body. having content.
All 1xx (Informational), 204 (No Content), and 304 (Not Modified) All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
responses do not include a message body. responses do not include content.
All other responses do include a message body, although the body All other responses do include content, although that content might
might be of zero length. be of zero length.
6.4.2. Identifying Content 6.4.2. Identifying Content
When a complete or partial representation is transferred in a message When a complete or partial representation is transferred as message
payload, it is often desirable for the sender to supply, or the content, it is often desirable for the sender to supply, or the
recipient to determine, an identifier for a resource corresponding to recipient to determine, an identifier for a resource corresponding to
that representation. that representation.
For a request message: For a request message:
o If the request has a Content-Location header field, then the * If the request has a Content-Location header field, then the
sender asserts that the payload is a representation of the sender asserts that the content is a representation of the
resource identified by the Content-Location field-value. However, resource identified by the Content-Location field value. However,
such an assertion cannot be trusted unless it can be verified by such an assertion cannot be trusted unless it can be verified by
other means (not defined by this specification). The information other means (not defined by this specification). The information
might still be useful for revision history links. might still be useful for revision history links.
o Otherwise, the payload is unidentified. * Otherwise, the content is unidentified.
For a response message, the following rules are applied in order For a response message, the following rules are applied in order
until a match is found: until a match is found:
1. If the request method is GET or HEAD and the response status code 1. If the request method is HEAD or the response status code is 204
is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not (No Content) or 304 (Not Modified), there is no content in the
Modified), the payload is a representation of the resource response.
identified by the effective request URI (Section 5.5 of
[RFC7230]).
2. If the request method is GET or HEAD and the response status code 2. If the request method is GET and the response status code is 200
is 203 (Non-Authoritative Information), the payload is a potentially (OK), the content is a representation of the resource identified
by the target URI (Section 7.1).
3. If the request method is GET and the response status code is 203
(Non-Authoritative Information), the content is a potentially
modified or enhanced representation of the target resource as modified or enhanced representation of the target resource as
provided by an intermediary. provided by an intermediary.
3. If the response has a Content-Location header field and its 4. If the request method is GET and the response status code is 206
field-value is a reference to the same URI as the effective (Partial Content), the content is one or more parts of a
request URI, the payload is a representation of the resource representation of the resource identified by the target URI
identified by the effective request URI. (Section 7.1).
4. If the response has a Content-Location header field and its 5. If the response has a Content-Location header field and its field
field-value is a reference to a URI different from the effective value is a reference to the same URI as the target URI, the
request URI, then the sender asserts that the content is a representation of the target resource.
payload is a representation of the resource identified by the
Content-Location field-value. 6. If the response has a Content-Location header field and its field
value is a reference to a URI different from the target URI, then
the sender asserts that the content is a representation of the
resource identified by the Content-Location field value.
However, such an assertion cannot be trusted unless it can be However, such an assertion cannot be trusted unless it can be
verified by other means (not defined by this specification). verified by other means (not defined by this specification).
5. Otherwise, the payload is unidentified. 7. Otherwise, the content is unidentified.
6.4.3. Payload Metadata
Header fields that specifically describe the payload, rather than the
associated representation, are referred to as "payload header
fields". Payload header fields are defined in other parts of this
specification, due to their impact on message parsing.
6.5. Trailer Fields 6.5. Trailer Fields
A trailer allows the sender to include additional fields at the end Fields (Section 5) that are located within a _trailer section_ are
of a chunked message in order to supply metadata that might be are referred to as "trailer fields" (or just "trailers",
dynamically generated while the message body is sent, such as a colloquially). Trailer fields can be useful for supplying message
message integrity check, digital signature, or post-processing integrity checks, digital signatures, delivery metrics, or post-
status. The trailer fields are identical to header fields, except processing status information.
they are sent in a chunked trailer instead of the message's header
section. Trailer fields ought to be processed and stored separately from the
fields in the header section to avoid contradicting message semantics
known at the time the header section was complete. The presence or
absence of certain header fields might impact choices made for the
routing or processing of the message as a whole before the trailers
are received; those choices cannot be unmade by the later discovery
of trailer fields.
6.5.1. Limitations on use of Trailers 6.5.1. Limitations on use of Trailers
A sender MUST NOT generate a trailer that contains a field necessary A trailer section is only possible when supported by the version of
for message framing (e.g., Transfer-Encoding and Content-Length), HTTP in use and enabled by an explicit framing mechanism. For
routing (e.g., Host), request modifiers (e.g., controls and example, the chunked coding in HTTP/1.1 allows a trailer section to
conditionals in Section 5 of [RFC7231]), authentication (e.g., see be sent after the content (Section 7.1.2 of [Messaging]).
[RFC7235] and [RFC6265]), response control data (e.g., see Section
7.1 of [RFC7231]), or determining how to process the payload (e.g.,
Content-Encoding, Content-Type, Content-Range, and Trailer).
When a chunked message containing a non-empty trailer is received, Many fields cannot be processed outside the header section because
the recipient MAY process the fields (aside from those forbidden their evaluation is necessary prior to receiving the content, such as
above) as if they were appended to the message's header section. A those that describe message framing, routing, authentication, request
recipient MUST ignore (or consider as an error) any fields that are modifiers, response controls, or content format. A sender MUST NOT
forbidden to be sent in a trailer, since processing them as if they generate a trailer field unless the sender knows the corresponding
were present in the header section might bypass external security header field name's definition permits the field to be sent in
filters. trailers.
[new] Trailer fields can be difficult to process by intermediaries that
forward messages from one protocol version to another. If the entire
message can be buffered in transit, some intermediaries could merge
trailer fields into the header section (as appropriate) before it is
forwarded. However, in most cases, the trailers are simply
discarded. A recipient MUST NOT merge a trailer field into a header
section unless the recipient understands the corresponding header
field definition and that definition explicitly permits and defines
how trailer field values can be safely merged.
[new] The presence of the keyword "trailers" in the TE header field
(Section 10.1.4) of a request indicates that the client is willing to
accept trailer fields, on behalf of itself and any downstream
clients. For requests from an intermediary, this implies that all
downstream clients are willing to accept trailer fields in the
forwarded response. Note that the presence of "trailers" does not
mean that the client(s) will process any particular trailer field in
the response; only that the trailer section(s) will not be dropped by
any of the clients.
Unless the request includes a TE header field indicating "trailers" Because of the potential for trailer fields to be discarded in
is acceptable, as described in Section 4.3, a server SHOULD NOT transit, a server SHOULD NOT generate trailer fields that it believes
generate trailer fields that it believes are necessary for the user are necessary for the user agent to receive.
agent to receive. Without a TE containing "trailers", the server
ought to assume that the trailer fields might be silently discarded
along the path to the user agent. This requirement allows
intermediaries to forward a de-chunked message to an HTTP/1.0
recipient without buffering the entire response.
6.5.2. Processing Trailer Fields 6.5.2. Processing Trailer Fields
[new] The "Trailer" header field (Section 10.1.5) can be sent to indicate
fields likely to be sent in the trailer section, which allows
[new] recipients to prepare for their receipt before processing the
content. For example, this could be useful if a field name indicates
that a dynamic checksum should be calculated as the content is
received and then immediately checked upon receipt of the trailer
field value.
[new] Like header fields, trailer fields with the same name are processed
in the order received; multiple trailer field lines with the same
name have the equivalent semantics as appending the multiple values
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
if each member value is only processed once per field line received.
[new] 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
(but separate from) the header fields. Additional processing
expectations, if any, can be defined within the field specification
for a field intended for use in trailers.
7. Routing HTTP Messages 7. Routing HTTP Messages
HTTP request message routing is determined by each client based on HTTP request message routing is determined by each client based on
the target resource, the client's proxy configuration, and the target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the response routing follows the same connection chain back to the
client. client.
7.1. Determining the Target Resource 7.1. Determining the Target Resource
HTTP is used in a wide variety of applications, ranging from Although HTTP is used in a wide variety of applications, most clients
general-purpose computers to home appliances. In some cases, rely on the same resource identification mechanism and configuration
communication options are hard-coded in a client's configuration. techniques as general-purpose Web browsers. Even when communication
However, most HTTP clients rely on the same resource identification options are hard-coded in a client's configuration, we can think of
mechanism and configuration techniques as general-purpose Web their combined effect as a URI reference (Section 4.1).
browsers.
A URI reference (Section 2.7) is typically used as an A URI reference is resolved to its absolute form in order to obtain
identifier for the "target resource", which a user agent would the _target URI_. The target URI excludes the reference's fragment
resolve to its absolute form in order to obtain the "target URI". component, if any, since fragment identifiers are reserved for
The target URI excludes the reference's fragment component, if any, client-side processing ([RFC3986], Section 3.5).
since fragment identifiers are reserved for client-side processing
([RFC3986], Section 3.5).
[new] To perform an action on a _target resource_, the client sends a
request message containing enough components of its parsed target URI
to enable recipients to identify that same resource. For historical
reasons, the parsed target URI components, collectively referred to
as the _request target_, are sent within the message control data and
the Host header field (Section 7.2).
6.1.3. Reconstructing the Target URI There are two unusual cases for which the request target components
are in a method-specific form:
Once an inbound connection is obtained, the client sends an HTTP * For CONNECT (Section 9.3.6), the request target is the host name
request message (Section 3) with a request-target derived from the and port number of the tunnel destination, separated by a colon.
target URI.
Since the request-target often contains only part of the user agent's * For OPTIONS (Section 9.3.7), the request target can be a single
target URI, a server reconstructs the intended target as an asterisk ("*").
"effective request URI" to properly service the request. This
reconstruction involves both the server's local configuration and
information communicated in the request-target, Host header field,
and connection context.
For a user agent, the effective request URI is the target URI. See the respective method definitions for details. These forms MUST
NOT be used with other methods.
Once the effective request URI has been constructed, an origin server Upon receipt of a client's request, a server reconstructs the target
needs to decide whether or not to provide service for that URI via URI from the received components in accordance with their local
the connection in which the request was received. For example, the configuration and incoming connection context. This reconstruction
request might have been misdirected, deliberately or accidentally, is specific to each major protocol version. For example, Appendix of
such that the information within a received request-target or Host [Messaging] defines how a server determines the target URI of an
header field differs from the host or port upon which the connection HTTP/1.1 request.
has been made. If the connection is from a trusted gateway, that
inconsistency might be expected; otherwise, it might indicate an | *Note:* Previous specifications defined the recomposed target
attempt to bypass security filters, trick the server into delivering | URI as a distinct concept, the _effective request URI_.
non-public content, or poison a cache. See Section 9 for security
considerations regarding message routing.
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 on a single IP address. host names.
[new] In HTTP/2 [RFC7540] and HTTP/3 [HTTP3], the Host header field is, in
some cases, supplanted by the ":authority" pseudo-header field of a
request's control data.
Host = uri-host [ ":" port ] ; Section 2.7.1 Host = uri-host [ ":" port ] ; Section 4
Since the Host field-value is critical information for handling a The target URI's authority information is critical for handling a
request, a user agent SHOULD generate Host as the first header field request. A user agent MUST generate a Host header field in a request
following the request-line. unless it sends that information as an ":authority" pseudo-header
field. A user agent that sends Host SHOULD send it as the first
field in the header section of a request.
For example, a GET request to the origin server for For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with: <http://www.example.org/pub/WWW/> would begin with:
GET /pub/WWW/ HTTP/1.1 GET /pub/WWW/ HTTP/1.1
Host: www.example.org Host: www.example.org
Since the Host header field acts as an application-level routing Since the host and port information acts as an application-level
mechanism, it is a frequent target for malware seeking to poison a routing mechanism, it is a frequent target for malware seeking to
shared cache or redirect a request to an unintended server. An poison a shared cache or redirect a request to an unintended server.
interception proxy is particularly vulnerable if it relies on the An interception proxy is particularly vulnerable if it relies on the
Host field-value for redirecting requests to internal servers, or for host and port information for redirecting requests to internal
use as a cache key in a shared cache, without first verifying that servers, or for use as a cache key in a shared cache, without first
the intercepted connection is targeting a valid IP address for that verifying that the intercepted connection is targeting a valid IP
host. address for that host.
7.3. Routing Inbound 7.3. Routing Inbound Requests
Once the target URI is determined, a client needs to decide whether a Once the target URI and its origin are determined, a client decides
network request is necessary to accomplish the desired semantics and, whether a network request is necessary to accomplish the desired
if so, where that request is to be directed. semantics and, if so, where that request is to be directed.
If the client has a cache [RFC7234] and the request can be satisfied 7.3.1. To a Cache
If the client has a cache [Caching] and the request can be satisfied
by it, then the request is usually directed there first. by it, then the request is usually directed there first.
7.3.2. To a Proxy
If the request is not satisfied by a cache, then a typical client If the request is not satisfied by a cache, then a typical client
will check its configuration to determine whether a proxy is to be will check its configuration to determine whether a proxy is to be
used to satisfy the request. Proxy configuration is implementation- used to satisfy the request. Proxy configuration is implementation-
dependent, but is often based on URI prefix matching, selective dependent, but is often based on URI prefix matching, selective
authority matching, or both, and the proxy itself is usually authority matching, or both, and the proxy itself is usually
identified by an "http" or "https" URI. If a proxy is applicable, identified by an "http" or "https" URI. If a proxy is applicable,
the client connects inbound by establishing (or reusing) a connection the client connects inbound by establishing (or reusing) a connection
to that proxy. to that proxy.
7.3.3. To the Origin
If no proxy is applicable, a typical client will invoke a handler If no proxy is applicable, a typical client will invoke a handler
routine, usually specific to the target URI's scheme, to connect routine, usually specific to the target URI's scheme, to connect
directly to an authority for the target resource. How that is directly to an origin for the target resource. How that is
accomplished is dependent on the target URI scheme and defined by its accomplished is dependent on the target URI scheme and defined by its
associated specification, similar to how this specification defines associated specification.
origin server access for resolution of the "http" (Section 2.7.1) and
"https" (Section 2.7.2) schemes.
HTTP requirements regarding connection management are defined in
Section 6.
7.4. Rejecting Misdirected Requests 7.4. Rejecting Misdirected Requests
[new] Before performing a request, a server decides whether or not to
provide service for the target URI via the connection in which the
request is received. For example, a request might have been
misdirected, deliberately or accidentally, such that the information
within a received Host header field differs from the connection's
host or port.
[new] If the connection is from a trusted gateway, such inconsistency might
be expected; otherwise, it might indicate an attempt to bypass
security filters, trick the server into delivering non-public
content, or poison a cache. See Section 17 for security
considerations regarding message routing.
[new] The 421 (Misdirected Request) status code in a response indicates
that the origin server has rejected the request because it appears to
have been misdirected (Section 15.5.20).
7.5. Response Correlation 7.5. Response Correlation
[new] A connection might be used for multiple request/response exchanges.
The mechanism used to correlate between request and response messages
is version dependent; some versions of HTTP use implicit ordering of
messages, while others use an explicit identifier.
[new] All responses, regardless of the status code (including interim
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
its corresponding request is complete. Likewise, clients are not
expected to wait any specific amount of time for a response. Clients
(including intermediaries) might abandon a request if the response is
not forthcoming within a reasonable period of time.
A client that receives a response while it is still sending the
associated request SHOULD continue sending that request, unless it
receives an explicit indication to the contrary (see, e.g.,
Section 9.5 of [Messaging] and Section 6.4 of [RFC7540]).
7.6. Message Forwarding 7.6. Message Forwarding
As described in Section 2.3, intermediaries can serve a variety of As described in Section 3.7, intermediaries can serve a variety of
roles in the processing of HTTP requests and responses. Some roles in the processing of HTTP requests and responses. Some
intermediaries are used to improve performance or availability. intermediaries are used to improve performance or availability.
Others are used for access control or to filter content. Since an Others are used for access control or to filter content. Since an
HTTP stream has characteristics similar to a pipe-and-filter HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an architecture, there are no inherent limits to the extent an
intermediary can enhance (or interfere) with either direction of the intermediary can enhance (or interfere) with either direction of the
stream. stream.
Intermediaries are expected to forward messages even when protocol
elements are not recognized (e.g., new methods, status codes, or
field names), since that preserves extensibility for downstream
recipients.
An intermediary not acting as a tunnel MUST implement the Connection An intermediary not acting as a tunnel MUST implement the Connection
header field, as specified in Section 6.1, and exclude fields from header field, as specified in Section 7.6.1, and exclude fields from
being forwarded that are only intended for the incoming connection. being forwarded that are only intended for the incoming connection.
An intermediary MUST NOT forward a message to itself unless it is An intermediary MUST NOT forward a message to itself unless it is
protected from an infinite request loop. In general, an intermediary protected from an infinite request loop. In general, an intermediary
ought to recognize its own server names, including any aliases, local ought to recognize its own server names, including any aliases, local
variations, or literal IP addresses, and respond to such requests variations, or literal IP addresses, and respond to such requests
directly. directly.
An HTTP message can be parsed as a stream for incremental processing An HTTP message can be parsed as a stream for incremental processing
or forwarding downstream. However, recipients cannot rely on or forwarding downstream. However, recipients cannot rely on
incremental delivery of partial messages, since some implementations incremental delivery of partial messages, since some implementations
will buffer or delay message forwarding for the sake of network will buffer or delay message forwarding for the sake of network
efficiency, security checks, or payload transformations. efficiency, security checks, or content transformations.
7.6.1. Connection 7.6.1. Connection
The "Connection" header field allows the sender to indicate desired The "Connection" header field allows the sender to list desired
control options for the current connection. In order to avoid control options for the current connection.
confusing downstream recipients, a proxy or gateway MUST remove or
replace any received connection options before forwarding the
message.
When a header 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
information, and therefore do not allow the Connection field.
A proxy or gateway 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 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 connection
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 header 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.
[new] Furthermore, intermediaries SHOULD remove or replace field(s) whose
semantics are known to require removal before forwarding, whether or
not they appear as a Connection option, after applying those fields'
semantics. This includes but is not limited to:
[new] * Proxy-Connection (Appendix C.2.2 of [Messaging])
[new] * Keep-Alive (Section 19.7.1 of [RFC2068])
[new] * TE (Section 10.1.4)
[new] * Transfer-Encoding (Section 6.1 of [Messaging])
* Upgrade (Section 7.8)
The Connection header field's value has the following grammar: The Connection header field's value has the following grammar:
Connection = 1#connection-option Connection = #connection-option
connection-option = token connection-option = token
Connection options are case-insensitive. Connection options are case-insensitive.
A sender MUST NOT send a connection option corresponding to a header A sender MUST NOT send a connection option corresponding to a field
field that is intended for all recipients of the payload. For that is intended for all recipients of the content. For example,
example, Cache-Control is never appropriate as a connection option Cache-Control is never appropriate as a connection option
(Section 5.2 of [RFC7234]). (Section 5.2 of [Caching]).
The connection options do not always correspond to a header field Connection options do not always correspond to a field present in the
present in the message, since a connection-specific field might not be needed if
message, since a connection-specific header 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 header field that is received without a
corresponding connection option usually indicates that the field has corresponding connection option usually indicates that the field has
been improperly forwarded by an intermediary and ought to be ignored been improperly forwarded by an intermediary and ought to be ignored
by the recipient. by the recipient.
When defining new connection options, specification authors ought to When defining a new connection option that does not correspond to a
survey existing header field names and ensure that the new connection field, specification authors ought to reserve the corresponding field
option does not share the same name as an already deployed header name anyway in order to avoid later collisions. Such reserved field
field. Defining a new connection option essentially reserves that names are registered in the Hypertext Transfer Protocol (HTTP) Field
potential field-name for carrying additional information related to Name Registry (Section 16.3.1).
the connection option, since it would be unwise for senders to use
that field-name for anything else.
The "close" connection option is defined for a sender to signal that
this connection will be closed after completion of the response. For
example,
Connection: close
in either the request or the response header fields indicates that
the sender is going to close the connection after the current
request/response is complete (Section 6.6).
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 4.3.8) and OPTIONS (Section 4.3.7) 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
The Max-Forwards value is a decimal integer indicating the remaining The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded. number of times this request message can be forwarded.
Each intermediary that receives a TRACE or OPTIONS request containing Each intermediary that receives a TRACE or OPTIONS request containing
a Max-Forwards header field MUST check and update its value prior to a Max-Forwards header field MUST check and update its value prior to
forwarding the request. If the received value is zero (0), the forwarding the request. If the received value is zero (0), the
intermediary MUST NOT forward the request; instead, the intermediary intermediary MUST NOT forward the request; instead, the intermediary
MUST respond as the final recipient. If the received Max-Forwards MUST respond as the final recipient. If the received Max-Forwards
value is greater than zero, the intermediary MUST generate an updated value is greater than zero, the intermediary MUST generate an updated
Max-Forwards field in the forwarded message with a field-value that Max-Forwards field in the forwarded message with a field value that
is the lesser of a) the received value decremented by one (1) or b) is the lesser of a) the received value decremented by one (1) or b)
the recipient's maximum supported value for Max-Forwards. the recipient's maximum supported value for Max-Forwards.
A recipient MAY ignore a Max-Forwards header field received with any A recipient MAY ignore a Max-Forwards header field received with any
other request methods. other request methods.
7.6.3. Via 7.6.3. Via
The "Via" header field indicates the presence of intermediate The "Via" header field indicates the presence of intermediate
protocols and recipients between the user agent and the server (on protocols and recipients between the user agent and the server (on
requests) or between the origin server and the client (on responses), requests) or between the origin server and the client (on responses),
similar to the "Received" header field in email (Section 3.6.7 of similar to the "Received" header field in email (Section 3.6.7 of
[RFC5322]). Via can be used for tracking message forwards, avoiding [RFC5322]). Via can be used for tracking message forwards, avoiding
request loops, and identifying the protocol capabilities of senders request loops, and identifying the protocol capabilities of senders
along the request/response chain. along the request/response chain.
Via = 1#( received-protocol RWS received-by [ RWS comment ] ) Via = #( received-protocol RWS received-by [ RWS comment ] )
received-protocol = [ protocol-name "/" ] protocol-version received-protocol = [ protocol-name "/" ] protocol-version
; see Section 6.7 ; see Section 7.8
received-by = ( uri-host [ ":" port ] ) / pseudonym received-by = pseudonym [ ":" port ]
pseudonym = token pseudonym = token
Multiple Via field values represent each proxy or gateway that has Each member of the Via field value represents a proxy or gateway that
forwarded the message. Each intermediary appends its own information has forwarded the message. Each intermediary appends its own
about how the message was received, such that the end result is information about how the message was received, such that the end
ordered according to the sequence of forwarding recipients. result is ordered according to the sequence of forwarding recipients.
A proxy MUST send an appropriate Via header field, as described A proxy MUST send an appropriate Via header field, as described
below, in each message that it forwards. An HTTP-to-HTTP gateway below, in each message that it forwards. An HTTP-to-HTTP gateway
MUST send an appropriate Via header field in each inbound request MUST send an appropriate Via header field in each inbound request
message and MAY send a Via header field in forwarded response message and MAY send a Via header field in forwarded response
messages. messages.
For each intermediary, the received-protocol indicates the protocol For each intermediary, the received-protocol indicates the protocol
and protocol version used by the upstream sender of the message. and protocol version used by the upstream sender of the message.
Hence, the Via field value records the advertised protocol Hence, the Via field value records the advertised protocol
capabilities of the request/response chain such that they remain capabilities of the request/response chain such that they remain
visible to downstream recipients; this can be useful for determining visible to downstream recipients; this can be useful for determining
what backwards-incompatible features might be safe to use in what backwards-incompatible features might be safe to use in
response, or within a later request, as described in Section 2.6. response, or within a later request, as described in Section 2.5.
For brevity, the protocol-name is omitted when the received protocol For brevity, the protocol-name is omitted when the received protocol
is HTTP. is HTTP.
The received-by portion of the field value is normally the host and The received-by portion is normally the host and optional port number
optional port number of a recipient server or client that of a recipient server or client that subsequently forwarded the
subsequently forwarded the message. However, if the real host is message. However, if the real host is considered to be sensitive
considered to be sensitive information, a sender MAY replace it with information, a sender MAY replace it with a pseudonym. If a port is
a pseudonym. If a port is not provided, a recipient MAY interpret not provided, a recipient MAY interpret that as meaning it was
that as meaning it was received on the default TCP port, if any, for received on the default TCP port, if any, for the received-protocol.
the received-protocol.
A sender MAY generate comments in the Via header field to identify A sender MAY generate comments to identify the software of each
the software of each recipient, analogous to the User-Agent and recipient, analogous to the User-Agent and Server header fields.
Server header fields. However, all comments in the Via field are However, comments in Via are optional, and a recipient MAY remove
optional, and a recipient MAY remove them prior to forwarding the them prior to forwarding the message.
message.
For example, a request message could be sent from an HTTP/1.0 user For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at p.example.net, which forward the request to a public proxy at p.example.net, which
completes the request by forwarding it to the origin server at completes the request by forwarding it to the origin server at
www.example.com. The request received by www.example.com would then www.example.com. The request received by www.example.com would then
have the following Via header field: have the following Via header field:
Via: 1.0 fred, 1.1 p.example.net Via: 1.0 fred, 1.1 p.example.net
An intermediary used as a portal through a network firewall SHOULD An intermediary used as a portal through a network firewall SHOULD
NOT forward the names and ports of hosts within the firewall region NOT forward the names and ports of hosts within the firewall region
unless it is explicitly enabled to do so. If not enabled, such an unless it is explicitly enabled to do so. If not enabled, such an
intermediary SHOULD replace each received-by host of any host behind intermediary SHOULD replace each received-by host of any host behind
the firewall by an appropriate pseudonym for that host. the firewall by an appropriate pseudonym for that host.
An intermediary MAY combine an ordered subsequence of Via header An intermediary MAY combine an ordered subsequence of Via header
field entries into a single such entry 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 entries 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 entries 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 payloads. 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 payloads 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 payload 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)
selected the proxy. chose the proxy.
If a proxy receives a request-target with a host name that is not a If a proxy receives a target URI with a host name that is not a fully
fully qualified domain name, it MAY add its own domain to the host qualified domain name, it MAY add its own domain to the host name it
name it received when forwarding the request. A proxy MUST NOT received when forwarding the request. A proxy MUST NOT change the
change the host name if the request-target contains a fully qualified host name if the target URI contains a fully qualified domain name.
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 request-target when forwarding it to the next inbound received target URI when forwarding it to the next inbound server,
server, except as noted above to replace an empty path with "/" or except as noted above to replace an empty path with "/" or "*".
"*".
A proxy MAY modify the message body through application or removal of
a transfer coding (Section 4).
A proxy MUST NOT transform the payload (Section 3.3 of [RFC7231]) of A proxy MUST NOT transform the content (Section 6.4) of a message
a message that contains a no-transform cache-control directive that contains a no-transform cache-control response directive
(Section 5.2 of [RFC7234]). (Section 5.2 of [Caching]). Note that this does not include changes
to the message body that do not affect the content, such as transfer
codings (Section 7 of [Messaging]).
A proxy MAY transform the payload 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 a a no-transform cache-control directive. A proxy that transforms the
payload MUST add a Warning header field with the warn-code of 214 content of a 200 (OK) response can inform downstream recipients that
("Transformation Applied") if one is not already in the message (see a transformation has been applied by changing the response status
Section 5.5 of [RFC7234]). A proxy that transforms the payload of a code to 203 (Non-Authoritative Information) (Section 15.3.4).
200 (OK) response can further inform downstream recipients that a
transformation has been applied by changing the response status code
to 203 (Non-Authoritative Information) (Section 6.3.4 of [RFC7231]).
A proxy SHOULD NOT modify header fields that provide information A proxy SHOULD NOT modify header fields that provide information
about the endpoints of the communication chain, the resource state, about the endpoints of the communication chain, the resource state,
or the selected representation (other than the payload) unless the or the selected representation (other than the 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
for transitioning from HTTP/1.1 to some other protocol on the same for transitioning from HTTP/1.1 to some other protocol on the same
connection. A client MAY send a list of protocols in the Upgrade connection.
header field of a request to invite the server to switch to one or
more of those protocols, in order of descending preference, before A client MAY send a list of protocol names in the Upgrade header
field of a request to invite the server to switch to one or more of
the named protocols, in order of descending preference, before
sending the final response. A server MAY ignore a received Upgrade sending the final response. A server MAY ignore a received Upgrade
header field if it wishes to continue using the current protocol on header field if it wishes to continue using the current protocol on
that connection. Upgrade cannot be used to insist on a protocol that connection. Upgrade cannot be used to insist on a protocol
change. change.
Upgrade = 1#protocol Upgrade = #protocol
protocol = protocol-name ["/" protocol-version] protocol = protocol-name ["/" protocol-version]
protocol-name = token protocol-name = token
protocol-version = token protocol-version = token
Although protocol names are registered with a preferred case,
recipients SHOULD use case-insensitive comparison when matching each
protocol-name to supported protocols.
A server that sends a 101 (Switching Protocols) response MUST send an A server that sends a 101 (Switching Protocols) response MUST send an
Upgrade header field to indicate the new protocol(s) to which the Upgrade header field to indicate the new protocol(s) to which the
connection is being switched; if multiple protocol layers are being connection is being switched; if multiple protocol layers are being
switched, the sender MUST list the protocols in layer-ascending switched, the sender MUST list the protocols in layer-ascending
order. A server MUST NOT switch to a protocol that was not indicated order. A server MUST NOT switch to a protocol that was not indicated
by the client in the corresponding request's Upgrade header field. A by the client in the corresponding request's Upgrade header field. A
server MAY choose to ignore the order of preference indicated by the server MAY choose to ignore the order of preference indicated by the
client and select the new protocol(s) based on other factors, such as client and select the new protocol(s) based on other factors, such as
the nature of the request or the current load on the server. the nature of the request or the current load on the server.
skipping to change at line 2172 skipping to change at page 60, line 40
Upgrade header field to indicate the acceptable protocols, in order Upgrade header field to indicate the acceptable protocols, in order
of descending preference. of descending preference.
A server MAY send an Upgrade header field in any other response to A server MAY send an Upgrade header field in any other response to
advertise that it implements support for upgrading to the listed advertise that it implements support for upgrading to the listed
protocols, in order of descending preference, when appropriate for a protocols, in order of descending preference, when appropriate for a
future request. future request.
The following is a hypothetical example sent by a client: The following is a hypothetical example sent by a client:
GET /hello.txt HTTP/1.1 GET /hello HTTP/1.1
Host: www.example.com Host: www.example.com
Connection: upgrade Connection: upgrade
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 Upgrade: websocket, IRC/6.9, RTA/x11
The capabilities and nature of the application-level communication The capabilities and nature of the application-level communication
after the protocol change is entirely dependent upon the new after the protocol change is entirely dependent upon the new
protocol(s) chosen. However, immediately after sending the 101 protocol(s) chosen. However, immediately after sending the 101
(Switching Protocols) response, the server is expected to continue (Switching Protocols) response, the server is expected to continue
responding to the original request as if it had received its responding to the original request as if it had received its
equivalent within the new protocol (i.e., the server still has an equivalent within the new protocol (i.e., the server still has an
outstanding request to satisfy after the protocol has been changed, outstanding request to satisfy after the protocol has been changed,
and is expected to do so without requiring the request to be and is expected to do so without requiring the request to be
repeated). repeated).
skipping to change at line 2200 skipping to change at page 61, line 27
follows that with the new protocol's equivalent of a response to a follows that with the new protocol's equivalent of a response to a
GET on the target resource. This allows a connection to be upgraded GET on the target resource. This allows a connection to be upgraded
to protocols with the same semantics as HTTP without the latency cost to protocols with the same semantics as HTTP without the latency cost
of an additional round trip. A server MUST NOT switch protocols of an additional round trip. A server MUST NOT switch protocols
unless the received message semantics can be honored by the new unless the received message semantics can be honored by the new
protocol; an OPTIONS request can be honored by any protocol. protocol; an OPTIONS request can be honored by any protocol.
The following is an example response to the above hypothetical The following is an example response to the above hypothetical
request: request:
HTTP/1.1 101 Switching Protocols HTTP/1.1 101 Switching Protocols
Connection: upgrade Connection: upgrade
Upgrade: HTTP/2.0 Upgrade: websocket
[... data stream switches to HTTP/2.0 with an appropriate response [... data stream switches to websocket with an appropriate response
(as defined by new protocol) to the "GET /hello.txt" request ...] (as defined by new protocol) to the "GET /hello" request ...]
When Upgrade is sent, the sender MUST also send a Connection header A sender of Upgrade MUST also send an "Upgrade" connection option in
field (Section 6.1) that contains an "upgrade" connection option, in the Connection header field (Section 7.6.1) to inform intermediaries
order to prevent Upgrade from being accidentally forwarded by not to forward this field. A server that receives an Upgrade header
intermediaries that might not implement the listed protocols. A field in an HTTP/1.0 request MUST ignore that Upgrade field.
server MUST ignore an Upgrade header field that is received in an
HTTP/1.0 request.
A client cannot begin using an upgraded protocol on the connection A client cannot begin using an upgraded protocol on the connection
until it has completely sent the request message (i.e., the client until it has completely sent the request message (i.e., the client
can't change the protocol it is sending in the middle of a message). can't change the protocol it is sending in the middle of a message).
If a server receives both an Upgrade and an Expect header field with If a server receives both an Upgrade and an Expect header field with
the "100-continue" expectation (Section 5.1.1 of [RFC7231]), the the "100-continue" expectation (Section 10.1.1), the server MUST send
server MUST send a 100 (Continue) response before sending a 101 a 100 (Continue) response before sending a 101 (Switching Protocols)
(Switching Protocols) response. response.
The Upgrade header field only applies to switching protocols on top The Upgrade header field only applies to switching protocols on top
of the existing connection; it cannot be used to switch the of the existing connection; it cannot be used to switch the
underlying connection (transport) protocol, nor to switch the underlying connection (transport) protocol, nor to switch the
existing communication to a different connection. For those existing communication to a different connection. For those
purposes, it is more appropriate to use a 3xx (Redirection) response purposes, it is more appropriate to use a 3xx (Redirection) response
(Section 6.4 of [RFC7231]). (Section 15.4).
This specification only defines the protocol name "HTTP" for use by This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of Section 2.6 and future updates to this version rules of Section 2.5 and future updates to this
specification. Additional tokens ought to be registered with IANA specification. Additional protocol names ought to be registered
using the registration procedure defined in Section 8.6. using the registration procedure defined in Section 16.7.
8. Representation Data and Metadata 8. Representation Data and Metadata
8.1. Representation Data 8.1. Representation Data
The representation data associated with an HTTP message is either The representation data associated with an HTTP message is either
provided as the payload body of the message or referred to by the provided as the content of the message or referred to by the message
message semantics and the effective request URI. The representation semantics and the target URI. The representation data is in a format
data is in a format and encoding defined by the representation and encoding defined by the representation metadata header fields.
metadata header fields.
The data type of the representation data is determined via the header The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer, fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model: ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) ) representation-data := Content-Encoding( Content-Type( data ) )
8.2. Representation Metadata 8.2. Representation Metadata
Representation header fields provide metadata about the Representation header fields provide metadata about the
representation. When a message includes a payload body, the representation. When a message includes content, the representation
representation header fields describe how to interpret the header fields describe how to interpret that data. In a response to
representation data enclosed in the payload body. In a response to a a HEAD request, the representation header fields describe the
HEAD request, the representation header fields describe the representation data that would have been enclosed in the content if
representation data that would have been enclosed in the payload body the same request had been a GET.
if the same request had been a GET.
The following header fields convey representation metadata:
8.3. Content-Type 8.3. Content-Type
The "Content-Type" header field indicates the media type of the The "Content-Type" header field indicates the media type of the
associated representation: either the representation enclosed in the associated representation: either the representation enclosed in the
message payload or the selected representation, as determined by the message content or the selected representation, as determined by the
message semantics. The indicated media type defines both the data message semantics. The indicated media type defines both the data
format and how that data is intended to be processed by a recipient, format and how that data is intended to be processed by a recipient,
within the scope of the received message semantics, after any content within the scope of the received message semantics, after any content
codings indicated by Content-Encoding are decoded. codings indicated by Content-Encoding are decoded.
Content-Type = media-type Content-Type = media-type
Media types are defined in Section 3.1.1.1. An example of the field Media types are defined in Section 8.3.1. An example of the field is
is
Content-Type: text/html; charset=ISO-8859-4 Content-Type: text/html; charset=ISO-8859-4
A sender that generates a message containing a payload body SHOULD A sender that generates a message containing content SHOULD generate
generate a Content-Type header field in that message unless the a Content-Type header field in that message unless the intended media
intended media type of the enclosed representation is unknown to the type of the enclosed representation is unknown to the sender. If a
sender. If a Content-Type header field is not present, the recipient Content-Type header field is not present, the recipient MAY either
MAY either assume a media type of "application/octet-stream" assume a media type of "application/octet-stream" ([RFC2046],
([RFC2046], Section 4.5.1) or examine the data to determine its type. Section 4.5.1) or examine the data to determine its type.
In practice, resource owners do not always properly configure their In practice, resource owners do not always properly configure their
origin server to provide the correct Content-Type for a given origin server to provide the correct Content-Type for a given
representation, with the result that some clients will examine a representation. Some user agents examine the content and, in certain
payload's content and override the specified type. Clients that do cases, override the received type (for example, see [Sniffing]).
so risk drawing incorrect conclusions, which might expose additional This "MIME sniffing" risks drawing incorrect conclusions about the
security risks (e.g., "privilege escalation"). Furthermore, it is data, which might expose the user to additional security risks (e.g.,
impossible to determine the sender's intent by examining the data "privilege escalation"). Furthermore, it is impossible to determine
format: many data formats match multiple media types that differ only the sender's intended processing model by examining the data format:
in processing semantics. Implementers are encouraged to provide a many data formats match multiple media types that differ only in
means of disabling such "content sniffing" when it is used. processing semantics. Implementers are encouraged to provide a means
to disable such sniffing.
Furthermore, although Content-Type is defined as a singleton field,
it is sometimes incorrectly generated multiple times, resulting in a
combined field value that appears to be a list. Recipients often
attempt to handle this error by using the last syntactically valid
member of the list, but note that some implementations might have
different error handling behaviors, leading to interoperability and/
or security issues.
8.3.1. Media Type 8.3.1. Media Type
HTTP uses Internet media types [RFC2046] in the Content-Type HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and
(Section 3.1.1.5) and Accept (Section 5.3.2) header fields in order Accept (Section 12.5.1) header fields in order to provide open and
to provide open and extensible data typing and type negotiation. extensible data typing and type negotiation. Media types define both
Media types define both a data format and various processing models: a data format and various processing models: how to process that data
how to process that data in accordance with each context in which it in accordance with the message context.
is received.
media-type = type "/" subtype *( OWS ";" OWS parameter ) media-type = type "/" subtype parameters
type = token type = token
subtype = token subtype = token
The type/subtype MAY be followed by parameters in the form of The type and subtype tokens are case-insensitive.
name=value pairs.
The type, subtype, and parameter name tokens are case-insensitive. The type/subtype MAY be followed by semicolon-delimited parameters
Parameter values might or might not be case-sensitive, depending on (Section 5.6.6) in the form of name=value pairs. The presence or
the semantics of the parameter name. The presence or absence of a absence of a parameter might be significant to the processing of a
parameter might be significant to the processing of a media-type, media type, depending on its definition within the media type
depending on its definition within the media type registry. registry. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name.
For example, the following For example, the following media types are equivalent in describing
examples are all equivalent, but the first is preferred for HTML text data encoded in the UTF-8 character encoding scheme, but
consistency: the first is preferred for consistency (the "charset" parameter value
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
Internet media types ought to be registered with IANA according to Media types ought to be registered with IANA according to the
the procedures defined in [BCP13]. procedures defined in [BCP13].
8.3.2. Charset 8.3.2. Charset
HTTP uses charset names to indicate or negotiate the character HTTP uses _charset_ names to indicate or negotiate the character
encoding scheme of a textual representation [RFC6365]. A charset is encoding scheme ([RFC6365], Section 1.3) of a textual representation.
identified by a case-insensitive token. In the fields defined by this document, charset names appear either
in parameters (Content-Type), or, for Accept-Encoding, in the form of
charset = token a plain token. In both cases, charset names are matched case-
insensitively.
Charset names ought to be registered in the IANA "Character Sets" Charset names ought to be registered in the IANA "Character Sets"
registry (<http://www.iana.org/assignments/character-sets>) according registry (<https://www.iana.org/assignments/character-sets>)
to the procedures defined in [RFC2978]. according to the procedures defined in Section 2 of [RFC2978].
8.3.3. Canonicalization and Text Defaults
Internet media types are registered with a canonical form in order to
be interoperable among systems with varying native encoding formats.
Representations selected or transferred via HTTP ought to be in
canonical form, for many of the same reasons described by the
Multipurpose Internet Mail Extensions (MIME) [RFC2045]. However, the
performance characteristics of email deployments (i.e., store and
forward messages to peers) are significantly different from those
common to HTTP and the Web (server-based information services).
Furthermore, MIME's constraints for the sake of compatibility with
older mail transfer protocols do not apply to HTTP (see Appendix A).
MIME's canonical form requires that media subtypes of the "text" type
use CRLF as the text line break. HTTP allows the transfer of text
media with plain CR or LF alone representing a line break, when such
line breaks are consistent for an entire representation. An HTTP
sender MAY generate, and a recipient MUST be able to parse, line
breaks in text media that consist of CRLF, bare CR, or bare LF. In
addition, text media in HTTP is not limited to charsets that use
octets 13 and 10 for CR and LF, respectively. This flexibility
regarding line breaks applies only to text within a representation
that has been assigned a "text" media type; it does not apply to
"multipart" types or HTTP elements outside the payload body (e.g.,
header fields).
If a representation is encoded with a content-coding, the underlying | *Note:* In theory, charset names are defined by the "mime-
data ought to be in a form defined above prior to being encoded. | charset" ABNF rule defined in Section 2.3 of [RFC2978] (as
| corrected in [Err1912]). That rule allows two characters that
| are not included in "token" ("{" and "}"), but no charset name
| registered at the time of this writing includes braces (see
| [Err5433]).
8.3.4. Multipart Types 8.3.3. Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of MIME provides for a number of "multipart" types - encapsulations of
one or more representations within a single message body. All one or more representations within a single message body. All
multipart types share a common syntax, as defined in Section 5.1.1 of multipart types share a common syntax, as defined in Section 5.1.1 of
[RFC2046], and include a boundary parameter as part of the media type [RFC2046], and include a boundary parameter as part of the media type
value. The message body is itself a protocol element; a sender MUST value. The message body is itself a protocol element; a sender MUST
generate only CRLF to represent line breaks between body parts. generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an HTTP message framing does not use the multipart boundary as an
indicator of message body length, though it might be used by indicator of message body length, though it might be used by
implementations that generate or process the payload. For example, implementations that generate or process the content. For example,
the "multipart/form-data" type is often used for carrying form data the "multipart/form-data" type is often used for carrying form data
in a request, as described in [RFC2388], and the "multipart/ in a request, as described in [RFC7578], and the "multipart/
byteranges" type is defined by this specification for use in some 206 byteranges" type is defined by this specification for use in some 206
(Partial Content) responses [RFC7233]. (Partial Content) responses (see Section 15.3.7).
8.4. Content-Encoding 8.4. Content-Encoding
The "Content-Encoding" header field indicates what content codings The "Content-Encoding" header field indicates what content codings
have been applied to the representation, beyond those inherent in the have been applied to the representation, beyond those inherent in the
media type, and thus what decoding mechanisms have to be applied in media type, and thus what decoding mechanisms have to be applied in
order to obtain data in the media type referenced by the Content-Type order to obtain data in the media type referenced by the Content-Type
header field. Content-Encoding is primarily used to allow a header field. Content-Encoding is primarily used to allow a
representation's data to be compressed without losing the identity of representation's data to be compressed without losing the identity of
its underlying media type. its underlying media type.
Content-Encoding = 1#content-coding Content-Encoding = #content-coding
An example of its use is An example of its use is
Content-Encoding: gzip Content-Encoding: gzip
If one or more encodings have been applied to a representation, the If one or more encodings have been applied to a representation, the
sender that applied the encodings MUST generate a Content-Encoding sender that applied the encodings MUST generate a Content-Encoding
header field that lists the content codings in the order in which header field that lists the content codings in the order in which
they were applied. Additional information about the encoding they were applied. Note that the coding named "identity" is reserved
parameters can be provided by other header fields not defined by this for its special role in Accept-Encoding, and thus SHOULD NOT be
specification. included.
[new] Additional information about the encoding parameters can be provided
by other header fields not defined by this specification.
Unlike Transfer-Encoding (Section 3.3.1 of [RFC7230]), the codings Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings
listed in Content-Encoding are a characteristic of the listed in Content-Encoding are a characteristic of the
representation; the representation is defined in terms of the coded representation; the representation is defined in terms of the coded
form, and all other metadata about the representation is about the form, and all other metadata about the representation is about the
coded form unless otherwise noted in the metadata definition. coded form unless otherwise noted in the metadata definition.
Typically, the representation is only decoded just prior to rendering Typically, the representation is only decoded just prior to rendering
or analogous usage. or analogous usage.
If the media type includes an inherent encoding, such as a data If the media type includes an inherent encoding, such as a data
format that is always compressed, then that encoding would not be format that is always compressed, then that encoding would not be
restated in Content-Encoding even if it happens to be the same restated in Content-Encoding even if it happens to be the same
skipping to change at line 2451 skipping to change at page 66, line 26
Content coding values indicate an encoding transformation that has Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are been or can be applied to a representation. Content codings are
primarily used to allow a representation to be compressed or primarily used to allow a representation to be compressed or
otherwise usefully transformed without losing the identity of its otherwise usefully transformed without losing the identity of its
underlying media type and without loss of information. Frequently, underlying media type and without loss of information. Frequently,
the representation is stored in coded form, transmitted directly, and the representation is stored in coded form, transmitted directly, and
only decoded by the final recipient. only decoded by the final recipient.
content-coding = token content-coding = token
All content-coding values are case-insensitive and ought to be All content codings are case-insensitive and ought to be registered
registered within the "HTTP Content Coding Registry", as defined in within the "HTTP Content Coding Registry", as described in
Section 8.4. Section 16.6
The following content-coding values are defined by this
specification:
compress (and x-compress): See Section 4.2.1 of [RFC7230].
deflate: See Section 4.2.2 of [RFC7230].
gzip (and x-gzip): See Section 4.2.3 of [RFC7230].
The codings defined below can be used to compress the payload of a Content-coding values are used in the Accept-Encoding
message. They are used in the Accept-Encoding (Section 12.5.3) and Content-Encoding (Section 8.4) header fields.
(Section 5.3.4) and Content-Encoding (Section 3.1.2.2) header fields.
8.4.1.1. Compress Coding 8.4.1.1. Compress Coding
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
[Welch] that is commonly produced by the UNIX file compression [Welch] that is commonly produced by the UNIX file compression
program "compress". A recipient SHOULD consider "x-compress" to be program "compress". A recipient SHOULD consider "x-compress" to be
equivalent to "compress". equivalent to "compress".
8.4.1.2. Deflate Coding 8.4.1.2. Deflate Coding
The "deflate" coding is a "zlib" data format [RFC1950] containing a The "deflate" coding is a "zlib" data format [RFC1950] containing a
"deflate" compressed data stream [RFC1951] that uses a combination of "deflate" compressed data stream [RFC1951] that uses a combination of
the Lempel-Ziv (LZ77) compression algorithm and Huffman coding. the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.
Note: Some non-conformant implementations send the "deflate" | *Note:* Some non-conformant implementations send the "deflate"
compressed data without the zlib wrapper. | compressed data without the zlib wrapper.
8.4.1.3. Gzip Coding 8.4.1.3. Gzip Coding
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
Check (CRC) that is commonly produced by the gzip file compression Check (CRC) that is commonly produced by the gzip file compression
program [RFC1952]. A recipient SHOULD consider "x-gzip" to be program [RFC1952]. A recipient SHOULD consider "x-gzip" to be
equivalent to "gzip". equivalent to "gzip".
8.5. Content-Language 8.5. Content-Language
The "Content-Language" header field describes the natural language(s) The "Content-Language" header field describes the natural language(s)
of the intended audience for the representation. Note that this of the intended audience for the representation. Note that this
might not be equivalent to all the languages used within the might not be equivalent to all the languages used within the
representation. representation.
Content-Language = 1#language-tag Content-Language = #language-tag
Language tags are defined in Section 3.1.3.1. The primary purpose of Language tags are defined in Section 8.5.1. The primary purpose of
Content-Language is to allow a user to identify and differentiate Content-Language is to allow a user to identify and differentiate
representations according to the users' own preferred language. representations according to the users' own preferred language.
Thus, if the content is intended only for a Danish-literate audience, Thus, if the content is intended only for a Danish-literate audience,
the appropriate field is the appropriate field is
Content-Language: da Content-Language: da
If no Content-Language is specified, the default is that the content If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language, sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended. or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English Waitangi", presented simultaneously in the original Maori and English
versions, would call for versions, would call for
Content-Language: mi, en Content-Language: mi, en
However, just because multiple languages are present within a However, just because multiple languages are present within a
representation does not mean that it is intended for multiple representation does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin", which is clearly intended primer, such as "A First Lesson in Latin", which is clearly intended
to be used by an English-literate audience. In this case, the to be used by an English-literate audience. In this case, the
Content-Language would properly only include "en". Content-Language would properly only include "en".
Content-Language MAY be applied to any media type -- it is not Content-Language MAY be applied to any media type - it is not limited
limited to textual documents. to textual documents.
8.5.1. Language Tags 8.5.1. Language Tags
A language tag, as defined in [RFC5646], identifies a natural A language tag, as defined in [RFC5646], identifies a natural
language spoken, written, or otherwise conveyed by human beings for language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer communication of information to other human beings. Computer
languages are explicitly excluded. languages are explicitly excluded.
HTTP uses language tags within the Accept-Language and HTTP uses language tags within the Accept-Language and
Content-Language header fields. Accept-Language uses the broader Content-Language header fields. Accept-Language uses the broader
language-range production defined in Section 5.3.5, whereas language-range production defined in Section 12.5.4, whereas
Content-Language uses the language-tag production defined below. Content-Language uses the language-tag production defined below.
language-tag = <Language-Tag, see [RFC5646], Section 2.1> language-tag = <Language-Tag, see [RFC5646], Section 2.1>
A language tag is a sequence of one or more case-insensitive subtags, A language tag is a sequence of one or more case-insensitive subtags,
each separated by a hyphen character ("-", %x2D). In most cases, a each separated by a hyphen character ("-", %x2D). In most cases, a
language tag consists of a primary language subtag that identifies a language tag consists of a primary language subtag that identifies a
broad family of related languages (e.g., "en" = English), which is broad family of related languages (e.g., "en" = English), which is
optionally followed by a series of subtags that refine or narrow that optionally followed by a series of subtags that refine or narrow that
language's range (e.g., "en-CA" = the variety of English as language's range (e.g., "en-CA" = the variety of English as
communicated in Canada). Whitespace is not allowed within a language communicated in Canada). Whitespace is not allowed within a language
tag. Example tags include: tag. Example tags include:
fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information. See [RFC5646] for further information.
8.6. Content-Length 8.6. Content-Length
The "Content-Length" header field indicates the associated
representation's data length as a decimal non-negative integer number
of octets. When transferring a representation as content, Content-
Length refers specifically to the amount of data enclosed so that it
can be used to delimit framing (e.g., Section 6.2 of [Messaging]).
In other cases, Content-Length indicates the selected
representation's current length, which can be used by recipients to
estimate transfer time or compare to previously stored
representations.
Content-Length = 1*DIGIT Content-Length = 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
A sender MUST NOT send a Content-Length header field in any message
that contains a Transfer-Encoding header field.
A user agent SHOULD send a Content-Length in a request message when A user agent SHOULD send Content-Length in a request when the method
no Transfer-Encoding is sent and the request method defines a meaning defines a meaning for enclosed content and it is not sending
for an enclosed payload body. For example, a Content-Length header Transfer-Encoding. For example, a user agent normally sends Content-
field is normally sent in a POST request even when the value is 0 Length in a POST request even when the value is 0 (indicating empty
(indicating an empty payload body). A user agent SHOULD NOT send a content). A user agent SHOULD NOT send a Content-Length header field
Content-Length header field when the request message does not contain when the request message does not contain content and the method
a payload body and the method semantics do not anticipate such a semantics do not anticipate such data.
body.
A server MAY send a Content-Length header field in a response to a A server MAY send a Content-Length header field in a response to a
HEAD request (Section 4.3.2 of [RFC7231]); a server MUST NOT send HEAD request (Section 9.3.2); a server MUST NOT send Content-Length
Content-Length in such a response unless its field-value equals the in such a response unless its field value equals the decimal number
decimal number of octets that would have been sent in the payload of octets that would have been sent in the content of a response if
body of a response if the same request had used the GET method. the same request had used the GET method.
A server MAY send a Content-Length header field in a 304 (Not A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 4.1 of Modified) response to a conditional GET request (Section 15.4.5); a
[RFC7232]); a server MUST NOT send Content-Length in such a response server MUST NOT send Content-Length in such a response unless its
unless its field-value equals the decimal number of octets that would field value equals the decimal number of octets that would have been
have been sent in the payload body of a 200 (OK) response to the same sent in the content of a 200 (OK) response to the same request.
request.
A server MUST NOT send a Content-Length header field in any response A server MUST NOT send a Content-Length header field in any response
with a status code of 1xx (Informational) or 204 (No Content). A with a status code of 1xx (Informational) or 204 (No Content). A
server MUST NOT send a Content-Length header field in any 2xx server MUST NOT send a Content-Length header field in any 2xx
(Successful) response to a CONNECT request (Section 4.3.6 of (Successful) response to a CONNECT request (Section 9.3.6).
[RFC7231]).
Aside from the cases defined above, in the absence of Aside from the cases defined above, in the absence of Transfer-
Transfer-Encoding, an origin server SHOULD send a Content-Length Encoding, an origin server SHOULD send a Content-Length header field
header field when the payload body size is known prior to sending the when the content size is known prior to sending the complete header
complete header section. This will allow downstream recipients to section. This will allow downstream recipients to measure transfer
measure transfer progress, know when a received message is complete, progress, know when a received message is complete, and potentially
and potentially reuse the connection for additional requests. reuse the connection for additional requests.
Any Content-Length field value greater than or equal to zero is Any Content-Length field value greater than or equal to zero is
valid. Since there is no predefined limit to the length of a valid. Since there is no predefined limit to the length of content,
payload, a recipient MUST anticipate potentially large decimal a recipient MUST anticipate potentially large decimal numerals and
numerals and prevent parsing errors due to integer conversion prevent parsing errors due to integer conversion overflows or
overflows (Section 9.3). precision loss due to integer conversion (Section 17.5).
If a message is received that has multiple Content-Length header Because Content-Length is used for message delimitation in HTTP/1.1,
fields with field-values consisting of the same decimal value, or a its field value can impact how the message is parsed by downstream
single Content-Length header field with a field value containing a recipients even when the immediate connection is not using HTTP/1.1.
list of identical decimal values (e.g., "Content-Length: 42, 42"), If the message is forwarded by a downstream intermediary, a Content-
indicating Length field value that is inconsistent with the received message
that duplicate Content-Length header fields have been generated or framing might cause a security failure due to request smuggling or
combined by an upstream message processor, then the recipient MUST response splitting.
either reject the message as invalid or replace the duplicated
field-values with a single valid Content-Length field containing that As a result, a sender MUST NOT forward a message with a Content-
decimal value prior to determining the message body length or Length header field value that is known to be incorrect.
forwarding the message.
Likewise, a sender MUST NOT forward a message with a Content-Length
header field value that does not match the ABNF above, with one
exception: A recipient of a Content-Length header field value
consisting of the same decimal value repeated as a comma-separated
list (e.g, "Content-Length: 42, 42"), MAY either reject the message
as invalid or replace that invalid field value with a single instance
of the decimal value, since this likely indicates that a duplicate
was generated or combined by an upstream message processor.
8.7. Content-Location 8.7. Content-Location
The "Content-Location" header field references a URI that can be used The "Content-Location" header field references a URI that can be used
as an identifier for a specific resource corresponding to the as an identifier for a specific resource corresponding to the
representation in this message's payload. 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 payload in this message. representation that is enclosed as content in this message.
Content-Location = absolute-URI / partial-URI Content-Location = absolute-URI / partial-URI
[new] The field value is either an absolute-URI or a partial-URI. In the
latter case (Section 4), the referenced URI is relative to the target
URI ([RFC3986], Section 5).
The Content-Location value is not a replacement for the effective The Content-Location value is not a replacement for the target URI
Request URI (Section 5.5 of [RFC7230]). It is representation (Section 7.1). It is representation metadata. It has the same
metadata. It has the same syntax and semantics as the header field syntax and semantics as the header field of the same name defined for
of the same name defined for MIME body parts in Section 4 of MIME body parts in Section 4 of [RFC2557]. However, its appearance
[RFC2557]. However, its appearance in an HTTP message has some in an HTTP message has some special implications for HTTP recipients.
special implications for HTTP recipients.
If Content-Location is included in a 2xx (Successful) response If Content-Location is included in a 2xx (Successful) response
message and its value refers (after conversion to absolute form) to a message and its value refers (after conversion to absolute form) to a
URI that is the same as the effective request URI, then the recipient URI that is the same as the target URI, then the recipient MAY
MAY consider the payload to be a current representation of that consider the content to be a current representation of that resource
resource at the time indicated by the message origination date. For at the time indicated by the message origination date. For a GET
a GET (Section 4.3.1) or HEAD (Section 4.3.2) request, this is the (Section 9.3.1) or HEAD (Section 9.3.2) request, this is the same as
same as the default semantics when no Content-Location is provided by the default semantics when no Content-Location is provided by the
the server. For a state-changing request like PUT (Section 4.3.4) or server. For a state-changing request like PUT (Section 9.3.4) or
POST (Section 4.3.3), it implies that the server's response contains POST (Section 9.3.3), it implies that the server's response contains
the new representation of that resource, thereby distinguishing it the new representation of that resource, thereby distinguishing it
from representations that might only report about the action (e.g., from representations that might only report about the action (e.g.,
"It worked!"). This allows authoring applications to update their "It worked!"). This allows authoring applications to update their
local copies without the need for a subsequent GET request. local copies without the need for a subsequent GET request.
If Content-Location is included in a 2xx (Successful) response If Content-Location is included in a 2xx (Successful) response
message and its field-value refers to a URI that differs from the message and its field value refers to a URI that differs from the
effective request URI, then the origin server claims that the URI is target URI, then the origin server claims that the URI is an
an identifier for a different resource corresponding to the enclosed identifier for a different resource corresponding to the enclosed
representation. Such a claim can only be trusted if both identifiers representation. Such a claim can only be trusted if both identifiers
share the same resource owner, which cannot be programmatically share the same resource owner, which cannot be programmatically
determined via HTTP. determined via HTTP.
o For a response to a GET or HEAD request, this is an indication * For a response to a GET or HEAD request, this is an indication
that the effective request URI refers to a resource that is that the target URI refers to a resource that is subject to
subject to content negotiation and the Content-Location content negotiation and the Content-Location field value is a more
field-value is a more specific identifier for the selected specific identifier for the selected representation.
representation.
o For a 201 (Created) response to a state-changing method, a * For a 201 (Created) response to a state-changing method, a
Content-Location field-value that is identical to the Location Content-Location field value that is identical to the Location
field-value indicates that this payload is a current field value indicates that this content is a current
representation of the newly created resource. representation of the newly created resource.
o Otherwise, such a Content-Location indicates that this payload is * Otherwise, such a Content-Location indicates that this content is
a representation reporting on the requested action's status and a representation reporting on the requested action's status and
that the same report is available (for future access with GET) at that the same report is available (for future access with GET) at
the given URI. For example, a purchase transaction made via a the given URI. For example, a purchase transaction made via a
POST request might include a receipt document as the payload of POST request might include a receipt document as the content of
the 200 (OK) response; the Content-Location field-value provides the 200 (OK) response; the Content-Location field value provides
an identifier for retrieving a copy of that same receipt in the an identifier for retrieving a copy of that same receipt in the
future. future.
A user agent that sends Content-Location in a request message is A user agent that sends Content-Location in a request message is
stating that its value refers to where the user agent originally stating that its value refers to where the user agent originally
obtained the content of the enclosed representation (prior to any obtained the content of the enclosed representation (prior to any
modifications made by that user agent). In other words, the user modifications made by that user agent). In other words, the user
agent is providing a back link to the source of the original agent is providing a back link to the source of the original
representation. representation.
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For example, if a client makes a PUT request on a negotiated resource For example, if a client makes a PUT request on a negotiated resource
and the origin server accepts that PUT (without redirection), then and the origin server accepts that PUT (without redirection), then
the new state of that resource is expected to be consistent with the the new state of that resource is expected to be consistent with the
one representation supplied in that PUT; the Content-Location cannot one representation supplied in that PUT; the Content-Location cannot
be used as a form of reverse content selection identifier to update be used as a form of reverse content selection identifier to update
only one of the negotiated representations. If the user agent had only one of the negotiated representations. If the user agent had
wanted the latter semantics, it would have applied the PUT directly wanted the latter semantics, it would have applied the PUT directly
to the Content-Location URI. to the Content-Location URI.
8.8. Validator Header Fields 8.8. Validator Fields
Validator header fields convey metadata about the selected Validator fields convey metadata about the selected representation
representation (Section 3). In responses to safe requests, validator (Section 3.2). In responses to safe requests, validator fields
fields describe the selected representation chosen by the origin describe the selected representation chosen by the origin server
server while handling the response. Note that, depending on the while handling the response. Note that, depending on the status code
status code semantics, the selected representation for a given semantics, the selected representation for a given response is not
response is not necessarily the same as the representation enclosed necessarily the same as the representation enclosed as response
as response payload. 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 header field in a 201 (Created) response For example, an ETag field in a 201 (Created) response communicates
communicates the entity-tag of the newly created resource's the entity-tag of the newly created resource's representation, so
representation, so that it can be used in later conditional requests that it can be used in later conditional requests to prevent the
to prevent the "lost update" problem [RFC7232]. "lost update" problem (Section 13.1).
This specification defines two forms of metadata that are commonly This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions: used to observe resource state and test for preconditions:
modification dates (Section 2.2) and opaque entity tags modification dates (Section 8.8.2) and opaque entity tags
(Section 2.3). Additional metadata that reflects resource state has (Section 8.8.3). Additional metadata that reflects resource state
been defined by various extensions of HTTP, such as Web Distributed has been defined by various extensions of HTTP, such as Web
Authoring and Versioning (WebDAV, [RFC4918]), that are beyond the Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are
scope of this specification. A resource metadata value is referred beyond the scope of this specification. A resource metadata value is
to as a "validator" when it is used within a precondition. referred to as a _validator_ when it is used within a precondition.
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 payload body 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.
Cache entries might persist for arbitrarily long periods, regardless Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an of expiration times. Thus, a cache might attempt to validate an
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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,
such as clock resolution, an inability to ensure uniqueness for all such as 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. An origin server equivalency rather than unique sequences of data. An origin server
SHOULD change a weak entity-tag whenever it considers prior SHOULD change a weak entity-tag whenever it considers prior
representations to be unacceptable as a substitute for the current representations to be unacceptable as a substitute for the current
representation. In other words, a weak entity-tag ought to change representation. In other words, a weak entity-tag ought to change
whenever the origin server wants caches to invalidate old responses. whenever the origin server wants caches to invalidate old responses.
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The "Last-Modified" header field in a response provides a timestamp The "Last-Modified" header field in a response provides a timestamp
indicating the date and time at which the origin server believes the indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the selected representation was last modified, as determined at the
conclusion of handling the request. conclusion of handling the request.
Last-Modified = HTTP-date Last-Modified = HTTP-date
An example of its use is An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
8.8.2.1. Generation 8.8.2.1. Generation
An origin server SHOULD send Last-Modified for any selected An origin server SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests and consistently determined, since its use in conditional requests
and evaluating cache freshness ([RFC7234]) results in a substantial and evaluating cache freshness ([Caching]) can substantially reduce
reduction of HTTP traffic on the Internet and can be a significant unnecessary transfers and significantly improve service availability
factor in improving service scalability and reliability. and scalability.
A representation is typically the sum of many parts behind the A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most resource interface. The last-modified time would usually be the most
recent time that any of those parts were changed. How that value is recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how the scope of this specification.
recipients of the Last-Modified header field can use its value to
make conditional requests and test the validity of locally cached
responses.
An origin server SHOULD obtain the Last-Modified value of the An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to make Date field value for its response. This allows a recipient to make
an accurate assessment of the representation's modification time, an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the especially if the representation changes near the time that the
response is generated. response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that An origin server with a clock MUST NOT send a Last-Modified date that
is later than the server's time of message origination (Date). If is later than the server's time of message origination (Date). If
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An origin server without a clock MUST NOT assign Last-Modified values An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource to a response unless these values were associated with the resource
by some other system or user with a reliable clock. by some other system or user with a reliable clock.
8.8.2.2. Comparison 8.8.2.2. Comparison
A Last-Modified time, when used as a validator in a request, is A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong, implicitly weak unless it is possible to deduce that it is strong,
using the following rules: using the following rules:
o The validator is being compared by an origin server to the actual * The validator is being compared by an origin server to the actual
current validator for the representation and, current validator for the representation and,
o That origin server reliably knows that the associated * That origin server reliably knows that the associated
representation did not change twice during the second covered by representation did not change twice during the second covered by
the presented validator. the presented validator;
or or
o The validator is about to be used by a client in an * The validator is about to be used by a client in an
If-Modified-Since, If-Unmodified-Since, or If-Range header field, If-Modified-Since, If-Unmodified-Since, or If-Range header field,
because the client has a cache entry for the associated because the client has a cache entry for the associated
representation, and representation, and
o That cache entry includes a Date value, which gives the time when * That cache entry includes a Date value which is at least one
the origin server sent the original response, and second after the Last-Modified value and the client has reason to
believe that they were generated by the same clock or that there
o The presented Last-Modified time is at least 60 seconds before the is enough difference between the Last-Modified and Date values to
Date value. make clock synchronization issues unlikely;
or or
o The validator is being compared by an intermediate cache to the * The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, and validator stored in its cache entry for the representation, and
o That cache entry includes a Date value, which gives the time when * That cache entry includes a Date value which is at least one
the origin server sent the original response, and second after the Last-Modified value and the cache has reason to
believe that they were generated by the same clock or that there
o The presented Last-Modified time is at least 60 seconds before the is enough difference between the Last-Modified and Date values to
Date value. 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. The arbitrary have a Date value equal to its Last-Modified time.
60-second limit guards against the possibility that the Date and
Last-Modified values are generated from different clocks or at
somewhat different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
8.8.3. ETag 8.8.3. ETag
The "ETag" header field in a response provides the current entity-tag The "ETag" field in a response provides the current entity-tag for
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 = %x57.2F ; "W/", case-sensitive 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-string | *Note:* Previously, opaque-tag was defined to be a quoted-
([RFC2616], Section 3.11); thus, some recipients might perform | string ([RFC2616], Section 3.11); thus, some recipients might
backslash unescaping. Servers therefore ought to avoid backslash | perform backslash unescaping. Servers therefore ought to avoid
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 2.1), then all of the characteristics of a strong validator (Section 8.8.1),
the origin server MUST mark the entity-tag as weak by prefixing its then the origin server MUST mark the entity-tag as weak by prefixing
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
Section 6.5). However, since trailers are often ignored, it is
preferable to send Etag as a header field unless the entity-tag is
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.
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applied to all changes might use an internal revision number, perhaps applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other accurately differentiate between representations. Other
implementations might use a collision-resistant hash of implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution. a modification timestamp that has sub-second resolution.
An origin server SHOULD send an ETag for any selected representation An origin server SHOULD send an ETag for any selected representation
for which detection of changes can be reasonably and consistently for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([RFC7234]) can result in a substantial evaluating cache freshness ([Caching]) can substantially reduce
reduction of HTTP network traffic and can be a significant factor in unnecessary transfers and significantly improve service availability,
improving service scalability and reliability. scalability, and reliability.
8.8.3.2. Comparison 8.8.3.2. Comparison
There are two entity-tag comparison functions, depending on whether There are two entity-tag comparison functions, depending on whether
or not the comparison context allows the use of weak validators: or not the comparison context allows the use of weak validators:
o 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.
o Weak comparison: two entity-tags are equivalent if their _Weak comparison_: two entity-tags are equivalent if their opaque-
opaque-tags match character-by-character, regardless of either or tags match character-by-character, regardless of either or both
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
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 (Section Consider a resource that is subject to content negotiation
3.4 of [RFC7231]), and where the representations sent in response to (Section 12), and where the representations sent in response to a GET
a GET request vary based on the Accept-Encoding request header field request vary based on the Accept-Encoding request header field
(Section 5.3.4 of [RFC7231]): (Section 12.5.3):
>> Request: >> Request:
GET /index HTTP/1.1 GET /index HTTP/1.1
Host: www.example.com Host: www.example.com
Accept-Encoding: gzip Accept-Encoding: gzip
In this case, the response might or might not use the gzip content In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like: coding. If it does not, the response might look like:
>> Response: >> Response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a" ETag: "123-a"
Content-Length: 70 Content-Length: 70
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would An alternative representation that does use gzip content coding would
be: be:
>> Response: >> Response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b" ETag: "123-b"
Content-Length: 43 Content-Length: 43
Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Content-Encoding: gzip Content-Encoding: gzip
...binary data... ...binary data...
Note: Content codings are a property of the representation data, | *Note:* Content codings are a property of the representation
so a strong entity-tag for a content-encoded representation has to | data, so a strong entity-tag for a content-encoded
be distinct from the entity tag of an unencoded representation to | representation has to be distinct from the entity tag of an
prevent potential conflicts during cache updates and range | unencoded representation to prevent potential conflicts during
requests. In contrast, transfer codings (Section 4 of [RFC7230]) | cache updates and range requests. In contrast, transfer
apply only during message transfer and do not result in distinct | codings (Section 7 of [Messaging]) apply only during message
entity-tags. | transfer and do not result in distinct entity-tags.
8.8.4. When to Use Entity-Tags and Last-Modified Dates 8.8.4. When to Use Entity-Tags and Last-Modified Dates
In 200 (OK) responses to GET or HEAD, an origin server: In 200 (OK) responses to GET or HEAD, an origin server:
o SHOULD send an entity-tag validator unless it is not feasible to * SHOULD send an entity-tag validator unless it is not feasible to
generate one. generate one.
o MAY send a weak entity-tag instead of a strong entity-tag, if * MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag. if it is unfeasible to send a strong entity-tag.
o SHOULD send a Last-Modified value if it is feasible to send one. * SHOULD send a Last-Modified value if it is feasible to send one.
In other words, the preferred behavior for an origin server is to In other words, the preferred behavior for an origin server is to
send both a strong entity-tag and a Last-Modified value in successful send both a strong entity-tag and a Last-Modified value in successful
responses to a retrieval request. responses to a retrieval request.
A client: A client:
o MUST send that entity-tag in any cache validation request (using * MUST send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an entity-tag has been provided by If-Match or If-None-Match) if an entity-tag has been provided by
the origin server. the origin server.
o SHOULD send the Last-Modified value in non-subrange cache * SHOULD send the Last-Modified value in non-subrange cache
validation requests (using If-Modified-Since) if only a validation requests (using If-Modified-Since) if only a Last-
Last-Modified value has been provided by the origin server. Modified value has been provided by the origin server.
o MAY send the Last-Modified value in subrange cache validation * MAY send the Last-Modified value in subrange cache validation
requests (using If-Unmodified-Since) if only a Last-Modified value requests (using If-Unmodified-Since) if only a Last-Modified value
has been provided by an HTTP/1.0 origin server. The user agent has been provided by an HTTP/1.0 origin server. The user agent
SHOULD provide a way to disable this, in case of difficulty. SHOULD provide a way to disable this, in case of difficulty.
o SHOULD send both validators in cache validation requests if both * SHOULD send both validators in cache validation requests if both
an entity-tag and a Last-Modified value have been provided by the an entity-tag and a Last-Modified value have been provided by the
origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately. respond appropriately.
9. Methods 9. Methods
9.1. Overview 9.1. Overview
The request method token is the primary source of request semantics; The request method token is the primary source of request semantics;
it indicates the purpose for which the client has made this request it indicates the purpose for which the client has made this request
and what is expected by the client as a successful result. and what is expected by the client as a successful result.
The request method's semantics might be further specialized by the The request method's semantics might be further specialized by the
semantics of some header fields when present in a request (Section 5) semantics of some header fields when present in a request if those
if those additional semantics do not conflict with the method. For additional semantics do not conflict with the method. For example, a
example, a client can send conditional request header fields client can send conditional request header fields (Section 13.1) to
(Section 5.2) to make the requested action conditional on the current make the requested action conditional on the current state of the
state of the target resource ([RFC7232]). target resource.
HTTP was originally designed to be usable as an interface to HTTP is designed to be usable as an interface to distributed object
distributed object systems. The request method was envisioned as systems. The request method invokes an action to be applied to a
applying semantics to a target resource in much the same way as target resource in much the same way that a remote method invocation
invoking a defined method on an identified object would apply can be sent to an identified object.
semantics.
method = token method = token
The method token is case-sensitive because it might be used as a The method token is case-sensitive because it might be used as a
gateway to object-based systems with case-sensitive method names. gateway to object-based systems with case-sensitive method names. By
By convention, standardized methods are defined in all-uppercase convention, standardized methods are defined in all-uppercase US-
US-ASCII letters. ASCII letters.
Unlike distributed objects, the standardized request methods in HTTP Unlike distributed objects, the standardized request methods in HTTP
are not resource-specific, since uniform interfaces provide for are not resource-specific, since uniform interfaces provide for
better visibility and reuse in network-based systems [REST]. Once better visibility and reuse in network-based systems [REST]. Once
defined, a standardized method ought to have the same semantics when defined, a standardized method ought to have the same semantics when
applied to any resource, though each resource determines for itself applied to any resource, though each resource determines for itself
whether those semantics are implemented or allowed. whether those semantics are implemented or allowed.
This specification defines a number of standardized methods that are This specification defines a number of standardized methods that are
commonly used in HTTP, as outlined by the following table. commonly used in HTTP, as outlined by the following table.
+---------+-------------------------------------------------+-------+ +=========+============================================+=======+
| Method | Description | Sec. | | Method | Description | Ref. |
+---------+-------------------------------------------------+-------+ +=========+============================================+=======+
| GET | Transfer a current representation of the target | 4.3.1 | | GET | Transfer a current representation of the | 9.3.1 |
| | resource. | | | | target resource. | |
| HEAD | Same as GET, but only transfer the status line | 4.3.2 | +---------+--------------------------------------------+-------+
| | and header section. | | | HEAD | Same as GET, but do not transfer the | 9.3.2 |
| POST | Perform resource-specific processing on the | 4.3.3 | | | response content. | |
| | request payload. | | +---------+--------------------------------------------+-------+
| PUT | Replace all current representations of the | 4.3.4 | | POST | Perform resource-specific processing on | 9.3.3 |
| | target resource with the request payload. | | | | the request content. | |
| DELETE | Remove all current representations of the | 4.3.5 | +---------+--------------------------------------------+-------+
| | target resource. | | | PUT | Replace all current representations of the | 9.3.4 |
| CONNECT | Establish a tunnel to the server identified by | 4.3.6 | | | target resource with the request content. | |
| | the target resource. | | +---------+--------------------------------------------+-------+
| OPTIONS | Describe the communication options for the | 4.3.7 | | DELETE | Remove all current representations of the | 9.3.5 |
| | target resource. | | | | target resource. | |
| TRACE | Perform a message loop-back test along the path | 4.3.8 | +---------+--------------------------------------------+-------+
| | to the target resource. | | | CONNECT | Establish a tunnel to the server | 9.3.6 |
+---------+-------------------------------------------------+-------+ | | identified by the target resource. | |
+---------+--------------------------------------------+-------+
| OPTIONS | Describe the communication options for the | 9.3.7 |
| | target resource. | |
+---------+--------------------------------------------+-------+
| TRACE | Perform a message loop-back test along the | 9.3.8 |
| | path to the target resource. | |
+---------+--------------------------------------------+-------+
Table 4
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 7.4.1). However, the set of allowed Allow header field (Section 10.2.1). However, the set of allowed
methods can change dynamically. When a request method is received methods can change dynamically. An origin server that receives a
that is unrecognized or not implemented by an origin server, the request method that is unrecognized or not implemented SHOULD respond
origin server SHOULD respond with the 501 (Not Implemented) status with the 501 (Not Implemented) status code. An origin server that
code. When a request method is received that is known by an origin receives a request method that is recognized and implemented, but not
server but not allowed for the target resource, the origin server allowed for the target resource, SHOULD respond with the 405 (Method
SHOULD respond with the 405 (Method Not Allowed) status code. Not Allowed) status code.
Additional methods, outside the scope of this specification, have Additional methods, outside the scope of this specification, have
been standardized 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" maintained by IANA, as defined in Section 8.1. 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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allow automated retrieval processes (spiders) and cache performance allow automated retrieval processes (spiders) and cache performance
optimization (pre-fetching) to work without fear of causing harm. In optimization (pre-fetching) to work without fear of causing harm. In
addition, it allows a user agent to apply appropriate constraints on addition, it allows a user agent to apply appropriate constraints on
the automated use of unsafe methods when processing potentially the automated use of unsafe methods when processing potentially
untrusted content. untrusted content.
A user agent SHOULD distinguish between safe and unsafe methods when A user agent SHOULD distinguish between safe and unsafe methods when
presenting potential actions to a user, such that the user can be presenting potential actions to a user, such that the user can be
made aware of an unsafe action before it is requested. made aware of an unsafe action before it is requested.
When a resource is constructed such that parameters within the When a resource is constructed such that parameters within the target
effective request URI have the effect of selecting an action, it is URI have the effect of selecting an action, it is the resource
the resource owner's responsibility to ensure that the action is owner's responsibility to ensure that the action is consistent with
consistent with the request method semantics. For example, it is the request method semantics. For example, it is common for Web-
common for Web-based content editing software to use actions within based content editing software to use actions within query
query parameters, such as "page?do=delete". If the purpose of such a parameters, such as "page?do=delete". If the purpose of such a
resource is to perform an unsafe action, then the resource owner MUST resource is to perform an unsafe action, then the resource owner MUST
disable or disallow that action when it is accessed using a safe disable or disallow that action when it is accessed using a safe
request method. Failure to do so will result in unfortunate side request method. Failure to do so will result in unfortunate side
effects when automated processes perform a GET on every URI reference effects when automated processes perform a GET on every URI reference
for the sake of link maintenance, pre-fetching, building a search for the sake of link maintenance, pre-fetching, building a search
index, etc. index, etc.
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.
Idempotent methods are distinguished because the request can be Idempotent methods are distinguished because the request can be
repeated automatically if a communication failure occurs before the repeated automatically if a communication failure occurs before the
client is able to read the server's response. For example, if a client is able to read the server's response. For example, if a
client sends a PUT request and the underlying connection is closed client sends a PUT request and the underlying connection is closed
before any response is received, then the client can establish a new before any response is received, then the client can establish a new
connection and retry the idempotent request. It knows that repeating connection and retry the idempotent request. It knows that repeating
the request will have the same intended effect, even if the original the request will have the same intended effect, even if the original
request succeeded, though the response might differ. request succeeded, though the response might differ.
A user agent MUST NOT automatically retry a request with a non- A client SHOULD NOT automatically retry a request with a non-
idempotent method unless it has some means to know that the request idempotent method unless it has some means to know that the request
semantics are actually idempotent, regardless of the method, or some semantics are actually idempotent, regardless of the method, or some
means to detect that the original request was never applied. means to detect that the original request was never applied.
For example, a user agent that knows (through design or For example, a user agent can repeat a POST request automatically if
configuration) that a POST request to a given resource is safe can it knows (through design or configuration) that the request is safe
repeat that request automatically. Likewise, a user agent designed for that resource. Likewise, a user agent designed specifically to
specifically to operate on a version control repository might be able operate on a version control repository might be able to recover from
to recover from partial failure conditions by checking the target partial failure conditions by checking the target resource
resource revision(s) after a failed connection, reverting or fixing revision(s) after a failed connection, reverting or fixing any
any changes that were partially applied, and then automatically changes that were partially applied, and then automatically retrying
retrying the requests that failed. the requests that failed.
Some clients take a riskier approach and attempt to guess when an
automatic retry is possible. For example, a client might
automatically retry a POST request if the underlying transport
connection closed before any part of a response is received,
particularly if an idle persistent connection was used.
A proxy MUST NOT automatically retry non-idempotent requests. A A proxy MUST NOT automatically retry non-idempotent requests. A
client SHOULD NOT automatically retry a failed automatic retry. client SHOULD NOT automatically retry a failed automatic retry.
9.2.3. Methods and Caching 9.2.3. Methods and Caching
Request methods can be defined as "cacheable" to indicate that For a cache to store and use a response, the associated method needs
responses to them are allowed to be stored for future reuse; for to explicitly allow caching, and detail under what conditions a
specific requirements see [RFC7234]. In general, safe methods that response can be used to satisfy subsequent requests; a method
do not depend on a current or authoritative response are defined as definition which does not do so cannot be cached. For additional
cacheable; this specification defines GET, HEAD, and POST as requirements see [Caching].
cacheable, although the overwhelming majority of cache
implementations only support GET and HEAD. This specification defines caching semantics for GET, HEAD, and POST,
although the overwhelming majority of cache implementations only
support GET and HEAD.
9.3. Method Definitions 9.3. Method Definitions
9.3.1. GET 9.3.1. GET
The GET method requests transfer of a current selected representation The GET method requests transfer of a current selected representation
for the target resource. GET is the primary mechanism of information for the target resource.
retrieval and the focus of almost all performance optimizations.
Hence, when people speak of retrieving some identifiable information GET is the primary mechanism of information retrieval and the focus
via HTTP, they are generally referring to making a GET request. of almost all performance optimizations. Hence, when people speak of
retrieving some identifiable information via HTTP, they are generally
referring to making a GET request. A successful response reflects
the quality of "sameness" identified by the target URI. In turn,
constructing applications such that they produce a URI for each
important resource results in more resources being available for
other applications, producing a network effect that promotes further
expansion of the Web.
It is tempting to think of resource identifiers as remote file system It is tempting to think of resource identifiers as remote file system
pathnames and of representations as being a copy of the contents of pathnames and of representations as being a copy of the contents of
such files. In fact, that is how many resources are implemented (see such files. In fact, that is how many resources are implemented (see
Section 9.1 for related security considerations). However, there are Section 17.3 for related security considerations). However, there
no such limitations in practice. The HTTP interface for a resource are no such limitations in practice.
is just as likely to be implemented as a tree of content objects, a
programmatic view on various database records, or a gateway to other The HTTP interface for a resource is just as likely to be implemented
information systems. Even when the URI mapping mechanism is tied to as a tree of content objects, a programmatic view on various database
a file system, an origin server might be configured to execute the records, or a gateway to other information systems. Even when the
files with the request as input and send the output as the URI mapping mechanism is tied to a file system, an origin server
representation rather than transfer the files directly. Regardless, might be configured to execute the files with the request as input
only the origin server needs to know how each of its resource and send the output as the representation rather than transfer the
identifiers corresponds to an implementation and how each files directly. Regardless, only the origin server needs to know how
implementation manages to select and send a current representation of each of its resource identifiers corresponds to an implementation and
the target resource in a response to GET. how each implementation manages to select and send a current
representation of the target resource in a response to GET.
A client can alter the semantics of GET to be a "range request", A client can alter the semantics of GET to be a "range request",
requesting transfer of only some part(s) of the selected requesting transfer of only some part(s) of the selected
representation, by sending a Range header field in the request representation, by sending a Range header field in the request
([RFC7233]). (Section 14.2).
A payload within a GET request message has no defined semantics; A client SHOULD NOT generate content in a GET request. Content
sending a payload body on a GET request might cause some existing received in a GET request has no defined semantics, cannot alter the
implementations to reject the request. meaning or target of the request, and might lead some implementations
to reject the request and close the connection because of its
potential as a request smuggling attack (Section 11.2 of
[Messaging]).
The response to a GET request is cacheable; a cache MAY use it to The response to a GET request is cacheable; a cache MAY use it to
satisfy subsequent GET and HEAD requests unless otherwise indicated satisfy subsequent GET and HEAD requests unless otherwise indicated
by the Cache-Control header field (Section 5.2 of [RFC7234]). by the Cache-Control header field (Section 5.2 of [Caching]).
When information retrieval is performed with a mechanism that
constructs a target URI from user-provided information, such as the
query fields of a form using GET, potentially sensitive data might be
provided that would not be appropriate for disclosure within a URI
(see Section 17.9). In some cases, the data can be filtered or
transformed such that it would not reveal such information. In
others, particularly when there is no benefit from caching a
response, using the POST method (Section 9.3.3) instead of GET can
transmit such information in the request content rather than within
the target URI.
9.3.2. HEAD 9.3.2. HEAD
The HEAD method is identical to GET except that the server MUST NOT The HEAD method is identical to GET except that the server MUST NOT
send a message body in the response (i.e., the response terminates at send content in the response. HEAD is used to obtain metadata about
the end of the header section). the selected representation without transferring its representation
This method can be used for obtaining metadata about data, often for the sake of testing hypertext links or finding recent
the selected representation without transferring the representation modifications.
data and is often used for testing hypertext links
for validity, accessibility, and recent modification.
The server SHOULD send the same header fields in response to a HEAD The server SHOULD send the same header fields in response to a HEAD
request as it would have sent if the request had been a GET, except request as it would have sent if the request method had been GET.
that the payload header fields (Section 3.3) MAY be omitted. However, a server MAY omit header fields for which a value is
determined only while generating the content. For example, some
servers buffer a dynamic response to GET until a minimum amount of
data is generated so that they can more efficiently delimit small
responses or make late decisions with regard to content selection.
Such a response to GET might contain Content-Length and Vary fields,
for example, that are not generated within a HEAD response. These
minor inconsistencies are considered preferable to generating and
discarding the content for a HEAD request, since HEAD is usually
requested for the sake of efficiency.
A payload within a HEAD request message has no defined semantics; A client SHOULD NOT generate content in a HEAD request. Content
sending a payload body on a HEAD request might cause some existing received in a HEAD request has no defined semantics, cannot alter the
implementations to reject the request. meaning or target of the request, and might lead some implementations
to reject the request and close the connection because of its
potential as a request smuggling attack (Section 11.2 of
[Messaging]).
The response to a HEAD request is cacheable; a cache MAY use it to The response to a HEAD request is cacheable; a cache MAY use it to
satisfy subsequent HEAD requests unless otherwise indicated by the satisfy subsequent HEAD requests unless otherwise indicated by the
Cache-Control header field (Section 5.2 of [RFC7234]). A HEAD Cache-Control header field (Section 5.2 of [Caching]). A HEAD
response might also have an effect on previously cached responses to response might also affect previously cached responses to GET; see
GET; see Section 4.3.5 of [RFC7234]. Section 4.3.5 of [Caching].
9.3.3. POST 9.3.3. POST
The POST method requests that the target resource process the The POST method requests that the target resource process the
representation enclosed in the request according to the resource's representation enclosed in the request according to the resource's
own specific semantics. For example, POST is used for the following own specific semantics. For example, POST is used for the following
functions (among others): functions (among others):
o Providing a block of data, such as the fields entered into an HTML * Providing a block of data, such as the fields entered into an HTML
form, to a data-handling process; form, to a data-handling process;
o Posting a message to a bulletin board, newsgroup, mailing list, * Posting a message to a bulletin board, newsgroup, mailing list,
blog, or similar group of articles; blog, or similar group of articles;
o Creating a new resource that has yet to be identified by the * Creating a new resource that has yet to be identified by the
origin server; and origin server; and
o Appending data to a resource's existing representation(s). * Appending data to a resource's existing representation(s).
An origin server indicates response semantics by choosing an An origin server indicates response semantics by choosing an
appropriate status code depending on the result of processing the appropriate status code depending on the result of processing the
POST request; almost all of the status codes defined by this POST request; almost all of the status codes defined by this
specification might be received in a response to POST (the exceptions specification could be received in a response to POST (the exceptions
being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
Satisfiable)). Satisfiable)).
If one or more resources has been created on the origin server as a If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server result of successfully processing a POST request, the origin server
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 7.1.2) and a representation that describes the status of the (Section 10.2.3) and a representation that describes the status of
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 [RFC7234]). explicit freshness information (see Section 4.2.1 of [Caching]) and a
However, POST caching is not widely implemented. For cases where an
origin server wishes the client to be able to cache the result of a
POST in a way that can be reused by a later GET, the origin server
MAY send a 200 (OK) response containing the result and a
Content-Location header field that has the same value as the POST's Content-Location header field that has the same value as the POST's
effective request URI (Section 3.1.4.2). 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
POST is required to be written through to the origin server, because
it is unsafe; see Section 4 of [Caching].
If the result of processing a POST would be equivalent to a If the result of processing a POST would be equivalent to a
representation of an existing resource, an origin server MAY redirect representation of an existing resource, an origin server MAY redirect
the user agent to that resource by sending a 303 (See Other) response the user agent to that resource by sending a 303 (See Other) response
with the existing resource's identifier in the Location field. This with the existing resource's identifier in the Location field. This
has the benefits of providing the user agent a resource identifier has the benefits of providing the user agent a resource identifier
and transferring the representation via a method more amenable to and transferring the representation via a method more amenable to
shared caching, though at the cost of an extra request if the user shared caching, though at the cost of an extra request if the user
agent does not already have the representation cached. agent does not already have the representation cached.
9.3.4. PUT 9.3.4. PUT
The PUT method requests that the state of the target resource be The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given enclosed in the request message content. A successful PUT of a given
representation would suggest that a subsequent GET on that same representation would suggest that a subsequent GET on that same
target resource will result in an equivalent representation being target resource will result in an equivalent representation being
sent in a 200 (OK) response. However, there is no guarantee that sent in a 200 (OK) response. However, there is no guarantee that
such a state change will be observable, since the target resource such a state change will be observable, since the target resource
might be acted upon by other user agents in parallel, or might be might be acted upon by other user agents in parallel, or might be
subject to dynamic processing by the origin server, before any subject to dynamic processing by the origin server, before any
subsequent GET is received. A successful response only implies that subsequent GET is received. A successful response only implies that
the user agent's intent was achieved at the time of its processing by the user agent's intent was achieved at the time of its processing by
the origin server. the origin server.
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origin server beyond what can be expressed by the intent of the user origin server beyond what can be expressed by the intent of the user
agent request and the semantics of the origin server response. It agent request and the semantics of the origin server response. It
does not define what a resource might be, in any sense of that word, does not define what a resource might be, in any sense of that word,
beyond the interface provided via HTTP. It does not define how beyond the interface provided via HTTP. It does not define how
resource state is "stored", nor how such storage might change as a resource state is "stored", nor how such storage might change as a
result of a change in resource state, nor how the origin server result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking, translates resource state into representations. Generally speaking,
all implementation details behind the resource interface are all implementation details behind the resource interface are
intentionally hidden by the server. intentionally hidden by the server.
An origin server SHOULD ignore unrecognized header fields received in This extends to how header and trailer fields are stored; while
a PUT request (i.e., do not save them as part of the resource state). common header fields like Content-Type will typically be stored and
returned upon subsequent GET requests, header and trailer field
handling is specific to the resource that received the request. As a
result, an origin server SHOULD ignore unrecognized header and
trailer fields received in a PUT request (i.e., do not save them as
part of the resource state).
An origin server MUST NOT send a validator header field An origin server MUST NOT send a validator field (Section 8.8), such
(Section 7.2), such as an ETag or Last-Modified field, in a as an ETag or Last-Modified field, in a successful response to PUT
successful response to PUT unless the request's representation data unless the request's representation data was saved without any
was saved without any transformation applied to the body (i.e., the transformation applied to the content (i.e., the resource's new
resource's new representation data is identical to the representation representation data is identical to the content received in the PUT
data received in the PUT request) and the validator field value request) and the validator field value reflects the new
reflects the new representation. This requirement allows a user representation. This requirement allows a user agent to know when
agent to know when the representation body it has in memory remains the representation it sent (and retains in memory) is the result of
current as a result of the PUT, thus not in need of being retrieved the PUT, and thus doesn't need to be retrieved again from the origin
again from the origin server, and that the new validator(s) received server. The new validator(s) received in the response can be used
in the response can be used for future conditional requests in order for future conditional requests in order to prevent accidental
to prevent accidental overwrites (Section 5.2). 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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A PUT request applied to the target resource can have side effects on A PUT request applied to the target resource can have side effects on
other resources. For example, an article might have a URI for other resources. For example, an article might have a URI for
identifying "the current version" (a resource) that is separate from identifying "the current version" (a resource) that is separate from
the URIs identifying each particular version (different resources the URIs identifying each particular version (different resources
that at one point shared the same state as the current version that at one point shared the same state as the current version
resource). A successful PUT request on "the current version" URI resource). A successful PUT request on "the current version" URI
might therefore create a new version resource in addition to changing might therefore create a new version resource in addition to changing
the state of the target resource, and might also cause links to be the state of the target resource, and might also cause links to be
added between the related resources. added between the related resources.
An origin server that allows PUT on a given target resource MUST send Some origin servers support use of the Content-Range header field
a 400 (Bad Request) response to a PUT request that contains a (Section 14.4) as a request modifier to perform a partial PUT, as
Content-Range header field (Section 4.2 of [RFC7233]), since the described in Section 14.5.
payload is likely to be partial content that has been mistakenly PUT
as a full representation.
Responses to the PUT method are not cacheable. If a successful PUT Responses to the PUT method are not cacheable. If a successful PUT
request passes through a cache that has one or more stored responses request passes through a cache that has one or more stored responses
for the effective request URI, those stored responses will be for the target URI, those stored responses will be invalidated (see
invalidated (see Section 4.4 of [RFC7234]). Section 4.4 of [Caching]).
9.3.5. DELETE 9.3.5. DELETE
The DELETE method requests that the origin server remove the The DELETE method requests that the origin server remove the
association between the target resource and its current association between the target resource and its current
functionality. In effect, this method is similar to the rm command functionality. In effect, this method is similar to the "rm" command
in UNIX: it expresses a deletion operation on the URI mapping of the in UNIX: it expresses a deletion operation on the URI mapping of the
origin server rather than an expectation that the previously origin server rather than an expectation that the previously
associated information be deleted. associated information be deleted.
If the target resource has one or more current representations, they If the target resource has one or more current representations, they
might or might not be destroyed by the origin server, and the might or might not be destroyed by the origin server, and the
associated storage might or might not be reclaimed, depending associated storage might or might not be reclaimed, depending
entirely on the nature of the resource and its implementation by the entirely on the nature of the resource and its implementation by the
origin server (which are beyond the scope of this specification). origin server (which are beyond the scope of this specification).
Likewise, other implementation aspects of a resource might need to be Likewise, other implementation aspects of a resource might need to be
deactivated or archived as a result of a DELETE, such as database or deactivated or archived as a result of a DELETE, such as database or
gateway connections. In general, it is assumed that the origin gateway connections. In general, it is assumed that the origin
server will only allow DELETE on resources for which it has a server will only allow DELETE on resources for which it has a
prescribed mechanism for accomplishing the deletion. prescribed mechanism for accomplishing the deletion.
Relatively few resources allow the DELETE method -- its primary use Relatively few resources allow the DELETE method - its primary use is
is for remote authoring environments, where the user has some for remote authoring environments, where the user has some direction
direction regarding its effect. For example, a resource that was regarding its effect. For example, a resource that was previously
previously created using a PUT request, or identified via the created using a PUT request, or identified via the Location header
Location header field after a 201 (Created) response to a POST field after a 201 (Created) response to a POST request, might allow a
request, might allow a corresponding DELETE request to undo those corresponding DELETE request to undo those actions. Similarly,
actions. Similarly, custom user agent implementations that implement custom user agent implementations that implement an authoring
an authoring function, such as revision control clients using HTTP function, such as revision control clients using HTTP for remote
for remote operations, might use DELETE based on an assumption that operations, might use DELETE based on an assumption that the server's
the server's URI space has been crafted to correspond to a version URI space has been crafted to correspond to a version repository.
repository.
If a DELETE method is successfully applied, the origin server SHOULD If a DELETE method is successfully applied, the origin server SHOULD
send send
a 202 (Accepted) status code if the action will likely succeed but * a 202 (Accepted) status code if the action will likely succeed but
has not yet been enacted, has not yet been enacted,
a 204 (No Content) status code if the action has been enacted and * a 204 (No Content) status code if the action has been enacted and
no further information is to be supplied, or no further information is to be supplied, or
a 200 (OK) status code if the action has been enacted and the * a 200 (OK) status code if the action has been enacted and the
response message includes a representation describing the status. response message includes a representation describing the status.
A payload within a DELETE request message has no defined semantics; A client SHOULD NOT generate content in a DELETE request. Content
sending a payload body on a DELETE request might cause some existing received in a DELETE request has no defined semantics, cannot alter
the meaning or target of the request, and might lead some
implementations to reject the request. implementations to reject the request.
Responses to the DELETE method are not cacheable. If a DELETE Responses to the DELETE method are not cacheable. If a successful
request passes through a cache that has one or more stored responses DELETE request passes through a cache that has one or more stored
for the effective request URI, those stored responses will be responses for the target URI, those stored responses will be
invalidated (see Section 4.4 of [RFC7234]). invalidated (see Section 4.4 of [Caching]).
9.3.6. CONNECT 9.3.6. CONNECT
The CONNECT method requests that the recipient establish a tunnel to The CONNECT method requests that the recipient establish a tunnel to
the destination origin server identified by the request-target and, the destination origin server identified by the request target and,
if successful, thereafter restrict its behavior to blind forwarding if successful, thereafter restrict its behavior to blind forwarding
of packets, in both directions, until the tunnel is closed. Tunnels of data, in both directions, until the tunnel is closed. Tunnels are
are commonly used to create an end-to-end virtual connection, through commonly used to create an end-to-end virtual connection, through one
one or more proxies, which can then be secured using TLS (Transport or more proxies, which can then be secured using TLS (Transport Layer
Layer Security, [RFC5246]). Security, [RFC8446]).
CONNECT is intended only for use in requests to a proxy. An origin CONNECT uses a special form of request target, unique to this method,
server that receives a CONNECT request for itself MAY respond with a consisting of only the host and port number of the tunnel
2xx (Successful) status code to indicate that a connection is destination, separated by a colon. There is no default port; a
established. However, most origin servers do not implement CONNECT. client MUST send the port number even if the CONNECT request is based
on a URI reference that contains an authority component with an
elided port (Section 4.1). For example,
A client sending a CONNECT request MUST send the authority form of CONNECT server.example.com:80 HTTP/1.1
request-target (Section 5.3 of [RFC7230]); i.e., the request-target Host: server.example.com
consists of only the host name and port number of the tunnel
destination, separated by a colon. For example,
CONNECT server.example.com:80 HTTP/1.1 A server MUST reject a CONNECT request that targets an empty or
Host: server.example.com:80 invalid port number, typically by responding with a 400 (Bad Request)
status code.
The recipient proxy can establish a tunnel either by directly Because CONNECT changes the request/response nature of an HTTP
connecting to the request-target or, if configured to use another connection, specific HTTP versions might have different ways of
mapping its semantics into the protocol's wire format.
CONNECT is intended for use in requests to a proxy. The recipient
can establish a tunnel either by directly connecting to the server
identified by the request target or, if configured to use another
proxy, by forwarding the CONNECT request to the next inbound proxy. proxy, by forwarding the CONNECT request to the next inbound proxy.
An origin server MAY accept a CONNECT request, but most origin
servers do not implement CONNECT.
Any 2xx (Successful) response indicates that the sender (and all Any 2xx (Successful) response indicates that the sender (and all
inbound proxies) will switch to tunnel mode immediately after the inbound proxies) will switch to tunnel mode immediately after the
blank line that concludes the successful response's header section; response header section; data received after that header section is
data received after that blank line is from the server identified by from the server identified by the request target. Any response other
the request-target. Any response other than a successful response than a successful response indicates that the tunnel has not yet been
indicates that the tunnel has not yet been formed and that the formed.
connection remains governed by HTTP.
A tunnel is closed when a tunnel intermediary detects that either A tunnel is closed when a tunnel intermediary detects that either
side has closed its connection: the intermediary MUST attempt to send side has closed its connection: the intermediary MUST attempt to send
any outstanding data that came from the closed side to the other any outstanding data that came from the closed side to the other
side, close both connections, and then discard any remaining data side, close both connections, and then discard any remaining data
left undelivered. left undelivered.
Proxy authentication might be used to establish the authority to Proxy authentication might be used to establish the authority to
create a tunnel. For example, create a tunnel. For example,
CONNECT server.example.com:80 HTTP/1.1 CONNECT server.example.com:443 HTTP/1.1
Host: server.example.com:80 Host: server.example.com:443
Proxy-Authorization: basic aGVsbG86d29ybGQ= Proxy-Authorization: basic aGVsbG86d29ybGQ=
There are significant risks in establishing a tunnel to arbitrary There are significant risks in establishing a tunnel to arbitrary
servers, particularly when the destination is a well-known or servers, particularly when the destination is a well-known or
reserved TCP port that is not intended for Web traffic. For example, reserved TCP port that is not intended for Web traffic. For example,
a CONNECT to a request-target of "example.com:25" would suggest that a CONNECT to "example.com:25" would suggest that the proxy connect to
the proxy connect to the reserved port for SMTP traffic; if allowed, the reserved port for SMTP traffic; if allowed, that could trick the
that could trick the proxy into relaying spam email. Proxies that proxy into relaying spam email. Proxies that support CONNECT SHOULD
support CONNECT SHOULD restrict its use to a limited set of known restrict its use to a limited set of known ports or a configurable
ports or a configurable whitelist of safe request targets. list of safe request targets.
A server MUST NOT send any Transfer-Encoding or Content-Length header A server MUST NOT send any Transfer-Encoding or Content-Length header
fields in a 2xx (Successful) response to CONNECT. A client MUST fields in a 2xx (Successful) response to CONNECT. A client MUST
ignore any Content-Length or Transfer-Encoding header fields received ignore any Content-Length or Transfer-Encoding header fields received
in a successful response to CONNECT. in a successful response to CONNECT.
A payload within a CONNECT request message has no defined semantics; A CONNECT request message does not have content. The interpretation
sending a payload body on a CONNECT request might cause some existing of and allowability of data sent after the header section of the
implementations to reject the request. CONNECT request message is specific to the version of HTTP in use.
Responses to the CONNECT method are not cacheable. Responses to the CONNECT method are not cacheable.
9.3.7. OPTIONS 9.3.7. OPTIONS
The OPTIONS method requests information about the communication The OPTIONS method requests information about the communication
options available for the target resource, at either the origin options available for the target resource, at either the origin
server or an intervening intermediary. This method allows a client server or an intervening intermediary. This method allows a client
to determine the options and/or requirements associated with a to determine the options and/or requirements associated with a
resource, or the capabilities of a server, without implying a resource, or the capabilities of a server, without implying a
resource action. resource action.
An OPTIONS request with an asterisk ("*") as the request-target An OPTIONS request with an asterisk ("*") as the request target
(Section 5.3 of [RFC7230]) applies to the server in general rather (Section 7.1) applies to the server in general rather than to a
than to a specific resource. Since a server's communication options specific resource. Since a server's communication options typically
typically depend on the resource, the "*" request is only useful as a depend on the resource, the "*" request is only useful as a "ping" or
"ping" or "no-op" type of method; it does nothing beyond allowing the "no-op" type of method; it does nothing beyond allowing the client to
client to test the capabilities of the server. For example, this can test the capabilities of the server. For example, this can be used
be used to test a proxy for HTTP/1.1 conformance (or lack thereof). to test a proxy for HTTP/1.1 conformance (or lack thereof).
If the request-target is not an asterisk, the OPTIONS request applies If the request target is not an asterisk, the OPTIONS request applies
to the options that are available when communicating with the target to the options that are available when communicating with the target
resource. resource.
A server generating a successful response to OPTIONS SHOULD send any A server generating a successful response to OPTIONS SHOULD send any
header fields that might indicate optional features implemented by header that might indicate optional features implemented by the
the server and applicable to the target resource (e.g., Allow), server and applicable to the target resource (e.g., Allow), including
including potential extensions not defined by this specification. potential extensions not defined by this specification. The response
The response payload, if any, might also describe the communication content, if any, might also describe the communication options in a
options in a machine or human-readable representation. A standard machine or human-readable representation. A standard format for such
format for such a representation is not defined by this a representation is not defined by this specification, but might be
specification, but might be defined by future extensions to HTTP. A defined by future extensions to HTTP.
server MUST generate a Content-Length field with a value of "0" if no
payload body is to be sent in the response.
A client MAY send a Max-Forwards header field in an OPTIONS request A client MAY send a Max-Forwards header field in an OPTIONS request
to target a specific recipient in the request chain (see to target a specific recipient in the request chain (see
Section 5.1.2). A proxy MUST NOT generate a Max-Forwards header Section 7.6.2). A proxy MUST NOT generate a Max-Forwards header
field while forwarding a request unless that request was received field while forwarding a request unless that request was received
with a Max-Forwards field. with a Max-Forwards field.
A client that generates an OPTIONS request containing a payload body A client that generates an OPTIONS request containing content MUST
MUST send a valid Content-Type header field describing the send a valid Content-Type header field describing the representation
representation media type. Although this specification does not media type. Note that this specification does not define any use for
define any use for such a payload, future extensions to HTTP might such content.
use the OPTIONS body to make more detailed queries about the target
resource.
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 message body of a 200 (OK) response with a back to the client as the content of a 200 (OK) response. The
Content-Type of "message/http" (Section 8.3.1 of [RFC7230]). The "message/http" (Section 10.1 of [Messaging]) format is one way to do
final recipient is either the origin server or the first server to so. The final recipient is either the origin server or the first
receive a Max-Forwards value of zero (0) in th