Network Working Group R. Fielding Request for Comments: 2616 UC Irvine Obsoletes: 2068 J. Gettys Category: Standards Track Compaq/W3C J. Mogul Compaq H. Frystyk W3C/MIT L. Masinter Xerox P. Leach Microsoft T. Berners-Lee W3C/MIT June 1999 Hypertext Transfer Protocol -- HTTP/1.1 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the Internet Official Protocol Standards (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers [47]. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as HTTP/1.1, and is an update to RFC 2068 [33]. Fielding, et al. Standards Track [Page 1] RFC 2616 HTTP/1.1 June 1999 Table of Contents 1 Introduction ...................................................7 1.1 Purpose......................................................7 1.2 Requirements .................................................8 1.3 Terminology ..................................................8 1.4 Overall Operation ...........................................12 2 Notational Conventions and Generic Grammar ....................14 2.1 Augmented BNF ...............................................14 2.2 Basic Rules .................................................15 3 Protocol Parameters ...........................................17 3.1 HTTP Version ................................................17 3.2 Uniform Resource Identifiers ................................18 3.2.1 General Syntax ...........................................19 3.2.2 http URL .................................................19 3.2.3 URI Comparison ...........................................20 3.3 Date/Time Formats ...........................................20 3.3.1 Full Date ................................................20 3.3.2 Delta Seconds ............................................21 3.4 Character Sets ..............................................21 3.4.1 Missing Charset ..........................................22 3.5 Content Codings .............................................23 3.6 Transfer Codings ............................................24 3.6.1 Chunked Transfer Coding ..................................25 3.7 Media Types .................................................26 3.7.1 Canonicalization and Text Defaults .......................27 3.7.2 Multipart Types ..........................................27 3.8 Product Tokens ..............................................28 3.9 Quality Values ..............................................29 3.10 Language Tags ...............................................29 3.11 Entity Tags .................................................30 3.12 Range Units .................................................30 4 HTTP Message ..................................................31 4.1 Message Types ...............................................31 4.2 Message Headers .............................................31 4.3 Message Body ................................................32 4.4 Message Length ..............................................33 4.5 General Header Fields .......................................34 5 Request .......................................................35 5.1 Request-Line ................................................35 5.1.1 Method ...................................................36 5.1.2 Request-URI ..............................................36 5.2 The Resource Identified by a Request ........................38 5.3 Request Header Fields .......................................38 6 Response ......................................................39 6.1 Status-Line .................................................39 6.1.1 Status Code and Reason Phrase ............................39 6.2 Response Header Fields ......................................41 Fielding, et al. Standards Track [Page 2] RFC 2616 HTTP/1.1 June 1999 7 Entity ........................................................42 7.1 Entity Header Fields ........................................42 7.2 Entity Body .................................................43 7.2.1 Type .....................................................43 7.2.2 Entity Length ............................................43 8 Connections ...................................................44 8.1 Persistent Connections ......................................44 8.1.1 Purpose ..................................................44 8.1.2 Overall Operation ........................................45 8.1.3 Proxy Servers ............................................46 8.1.4 Practical Considerations .................................46 8.2 Message Transmission Requirements ...........................47 8.2.1 Persistent Connections and Flow Control ..................47 8.2.2 Monitoring Connections for Error Status Messages .........48 8.2.3 Use of the 100 (Continue) Status .........................48 8.2.4 Client Behavior if Server Prematurely Closes Connection ..50 9 Method Definitions ............................................51 9.1 Safe and Idempotent Methods .................................51 9.1.1 Safe Methods .............................................51 9.1.2 Idempotent Methods .......................................51 9.2 OPTIONS .....................................................52 9.3 GET .........................................................53 9.4 HEAD ........................................................54 9.5 POST ........................................................54 9.6 PUT .........................................................55 9.7 DELETE ......................................................56 9.8 TRACE .......................................................56 9.9 CONNECT .....................................................57 10 Status Code Definitions ......................................57 10.1 Informational 1xx ...........................................57 10.1.1 100 Continue .............................................58 10.1.2 101 Switching Protocols ..................................58 10.2 Successful 2xx ..............................................58 10.2.1 200 OK ...................................................58 10.2.2 201 Created ..............................................59 10.2.3 202 Accepted .............................................59 10.2.4 203 Non-Authoritative Information ........................59 10.2.5 204 No Content ...........................................60 10.2.6 205 Reset Content ........................................60 10.2.7 206 Partial Content ......................................60 10.3 Redirection 3xx .............................................61 10.3.1 300 Multiple Choices .....................................61 10.3.2 301 Moved Permanently ....................................62 10.3.3 302 Found ................................................62 10.3.4 303 See Other ............................................63 10.3.5 304 Not Modified .........................................63 10.3.6 305 Use Proxy ............................................64 10.3.7 306 (Unused) .............................................64 Fielding, et al. Standards Track [Page 3] RFC 2616 HTTP/1.1 June 1999 10.3.8 307 Temporary Redirect ...................................65 10.4 Client Error 4xx ............................................65 10.4.1 400 Bad Request .........................................65 10.4.2 401 Unauthorized ........................................66 10.4.3 402 Payment Required ....................................66 10.4.4 403 Forbidden ...........................................66 10.4.5 404 Not Found ...........................................66 10.4.6 405 Method Not Allowed ..................................66 10.4.7 406 Not Acceptable ......................................67 10.4.8 407 Proxy Authentication Required .......................67 10.4.9 408 Request Timeout .....................................67 10.4.10 409 Conflict ............................................67 10.4.11 410 Gone ................................................68 10.4.12 411 Length Required .....................................68 10.4.13 412 Precondition Failed .................................68 10.4.14 413 Request Entity Too Large ............................69 10.4.15 414 Request-URI Too Long ................................69 10.4.16 415 Unsupported Media Type ..............................69 10.4.17 416 Requested Range Not Satisfiable .....................69 10.4.18 417 Expectation Failed ..................................70 10.5 Server Error 5xx ............................................70 10.5.1 500 Internal Server Error ................................70 10.5.2 501 Not Implemented ......................................70 10.5.3 502 Bad Gateway ..........................................70 10.5.4 503 Service Unavailable ..................................70 10.5.5 504 Gateway Timeout ......................................71 10.5.6 505 HTTP Version Not Supported ...........................71 11 Access Authentication ........................................71 12 Content Negotiation ..........................................71 12.1 Server-driven Negotiation ...................................72 12.2 Agent-driven Negotiation ....................................73 12.3 Transparent Negotiation .....................................74 13 Caching in HTTP ..............................................74 13.1.1 Cache Correctness ........................................75 13.1.2 Warnings .................................................76 13.1.3 Cache-control Mechanisms .................................77 13.1.4 Explicit User Agent Warnings .............................78 13.1.5 Exceptions to the Rules and Warnings .....................78 13.1.6 Client-controlled Behavior ...............................79 13.2 Expiration Model ............................................79 13.2.1 Server-Specified Expiration ..............................79 13.2.2 Heuristic Expiration .....................................80 13.2.3 Age Calculations .........................................80 13.2.4 Expiration Calculations ..................................83 13.2.5 Disambiguating Expiration Values .........................84 13.2.6 Disambiguating Multiple Responses ........................84 13.3 Validation Model ............................................85 13.3.1 Last-Modified Dates ......................................86 Fielding, et al. Standards Track [Page 4] RFC 2616 HTTP/1.1 June 1999 13.3.2 Entity Tag Cache Validators ..............................86 13.3.3 Weak and Strong Validators ...............................86 13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates.89 13.3.5 Non-validating Conditionals ..............................90 13.4 Response Cacheability .......................................91 13.5 Constructing Responses From Caches ..........................92 13.5.1 End-to-end and Hop-by-hop Headers ........................92 13.5.2 Non-modifiable Headers ...................................92 13.5.3 Combining Headers ........................................94 13.5.4 Combining Byte Ranges ....................................95 13.6 Caching Negotiated Responses ................................95 13.7 Shared and Non-Shared Caches ................................96 13.8 Errors or Incomplete Response Cache Behavior ................97 13.9 Side Effects of GET and HEAD ................................97 13.10 Invalidation After Updates or Deletions ...................97 13.11 Write-Through Mandatory ...................................98 13.12 Cache Replacement .........................................99 13.13 History Lists .............................................99 14 Header Field Definitions ....................................100 14.1 Accept .....................................................100 14.2 Accept-Charset .............................................102 14.3 Accept-Encoding ............................................102 14.4 Accept-Language ............................................104 14.5 Accept-Ranges ..............................................105 14.6 Age ........................................................106 14.7 Allow ......................................................106 14.8 Authorization ..............................................107 14.9 Cache-Control ..............................................108 14.9.1 What is Cacheable .......................................109 14.9.2 What May be Stored by Caches ............................110 14.9.3 Modifications of the Basic Expiration Mechanism .........111 14.9.4 Cache Revalidation and Reload Controls ..................113 14.9.5 No-Transform Directive ..................................115 14.9.6 Cache Control Extensions ................................116 14.10 Connection ...............................................117 14.11 Content-Encoding .........................................118 14.12 Content-Language .........................................118 14.13 Content-Length ...........................................119 14.14 Content-Location .........................................120 14.15 Content-MD5 ..............................................121 14.16 Content-Range ............................................122 14.17 Content-Type .............................................124 14.18 Date .....................................................124 14.18.1 Clockless Origin Server Operation ......................125 14.19 ETag .....................................................126 14.20 Expect ...................................................126 14.21 Expires ..................................................127 14.22 From .....................................................128 Fielding, et al. Standards Track [Page 5] RFC 2616 HTTP/1.1 June 1999 14.23 Host .....................................................128 14.24 If-Match .................................................129 14.25 If-Modified-Since ........................................130 14.26 If-None-Match ............................................132 14.27 If-Range .................................................133 14.28 If-Unmodified-Since ......................................134 14.29 Last-Modified ............................................134 14.30 Location .................................................135 14.31 Max-Forwards .............................................136 14.32 Pragma ...................................................136 14.33 Proxy-Authenticate .......................................137 14.34 Proxy-Authorization ......................................137 14.35 Range ....................................................138 14.35.1 Byte Ranges ...........................................138 14.35.2 Range Retrieval Requests ..............................139 14.36 Referer ..................................................140 14.37 Retry-After ..............................................141 14.38 Server ...................................................141 14.39 TE .......................................................142 14.40 Trailer ..................................................143 14.41 Transfer-Encoding..........................................143 14.42 Upgrade ..................................................144 14.43 User-Agent ...............................................145 14.44 Vary .....................................................145 14.45 Via ......................................................146 14.46 Warning ..................................................148 14.47 WWW-Authenticate .........................................150 15 Security Considerations .......................................150 15.1 Personal Information....................................151 15.1.1 Abuse of Server Log Information .........................151 15.1.2 Transfer of Sensitive Information .......................151 15.1.3 Encoding Sensitive Information in URIs .................152 15.1.4 Privacy Issues Connected to Accept Headers ..............152 15.2 Attacks Based On File and Path Names .......................153 15.3 DNS Spoofing ...............................................154 15.4 Location Headers and Spoofing ..............................154 15.5 Content-Disposition Issues .................................154 15.6 Authentication Credentials and Idle Clients ................155 15.7 Proxies and Caching ........................................155 15.7.1 Denial of Service Attacks on Proxies....................156 16 Acknowledgments .............................................156 17 References ..................................................158 18 Authors Addresses ..........................................162 19 Appendices ..................................................164 19.1 Internet Media Type message/http and application/http ......164 19.2 Internet Media Type multipart/byteranges ...................165 19.3 Tolerant Applications ......................................166 19.4 Differences Between HTTP Entities and RFC 2045 Entities ....167 Fielding, et al. Standards Track [Page 6] RFC 2616 HTTP/1.1 June 1999 19.4.1 MIME-Version ............................................167 19.4.2 Conversion to Canonical Form ............................167 19.4.3 Conversion of Date Formats ..............................168 19.4.4 Introduction of Content-Encoding ........................168 19.4.5 No Content-Transfer-Encoding ............................168 19.4.6 Introduction of Transfer-Encoding .......................169 19.4.7 MHTML and Line Length Limitations .......................169 19.5 Additional Features ........................................169 19.5.1 Content-Disposition .....................................170 19.6 Compatibility with Previous Versions .......................170 19.6.1 Changes from HTTP/1.0 ...................................171 19.6.2 Compatibility with HTTP/1.0 Persistent Connections ......172 19.6.3 Changes from RFC 2068 ...................................172 20 Index .......................................................175 21 Full Copyright Statement ....................................176 1 Introduction 1.1 Purpose The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effects of hierarchical proxies, caching, the need for persistent connections, or virtual hosts. In addition, the proliferation of incompletely-implemented applications calling themselves HTTP/1.0 has necessitated a protocol version change in order for two communicating applications to determine each others true capabilities. This specification defines the protocol referred to as HTTP/1.1. This protocol includes more stringent requirements than HTTP/1.0 in order to ensure reliable implementation of its features. Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods and headers that indicate the purpose of a request [47]. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [3], as a location (URL) [4] or name (URN) [20], for indicating the resource to which a Fielding, et al. Standards Track [Page 7] RFC 2616 HTTP/1.1 June 1999 method is to be applied. Messages are passed in a format similar to that used by Internet mail [9] as defined by the Multipurpose Internet Mail Extensions (MIME) [7]. HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems, including those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10] protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications. 1.2 Requirements The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in RFC 2119 [34]. An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED level requirements for the protocols it implements. An implementation that satisfies all the MUST or REQUIRED level and all the SHOULD level requirements for its protocols is said to be unconditionally compliant; one that satisfies all the MUST level requirements but not all the SHOULD level requirements for its protocols is said to be conditionally compliant. 1.3 Terminology This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication. connection A transport layer virtual circuit established between two programs for the purpose of communication. message The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in section 4 and transmitted via the connection. request An HTTP request message, as defined in section 5. response An HTTP response message, as defined in section 6. Fielding, et al. Standards Track [Page 8] RFC 2616 HTTP/1.1 June 1999 resource A network data object or service that can be identified by a URI, as defined in section 3.2. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions) or vary in other ways. entity The information transferred as the payload of a request or response. An entity consists of metainformation in the form of entity-header fields and content in the form of an entity-body, as described in section 7. representation An entity included with a response that is subject to content negotiation, as described in section 12. There may exist multiple representations associated with a particular response status. content negotiation The mechanism for selecting the appropriate representation when servicing a request, as described in section 12. The representation of entities in any response can be negotiated (including error responses). variant A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `varriant. Use of the term `variant does not necessarily imply that the resource is subject to content negotiation. client A program that establishes connections for the purpose of sending requests. user agent The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools. server An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the programs capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request. Fielding, et al. Standards Track [Page 9] RFC 2616 HTTP/1.1 June 1999 origin server The server on which a given resource resides or is to be created. proxy An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of this specification. A transparent proxy is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A non-transparent proxy is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies. gateway A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway. tunnel An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed. cache A programs local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel. cacheable A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cacheability of HTTP responses are defined in section 13. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular request. Fielding, et al. Standards Track [Page 10] RFC 2616 HTTP/1.1 June 1999 first-hand A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server. explicit expiration time The time at which the origin server intends that an entity should no longer be returned by a cache without further validation. heuristic expiration time An expiration time assigned by a cache when no explicit expiration time is available. age The age of a response is the time since it was sent by, or successfully validated with, the origin server. freshness lifetime The length of time between the generation of a response and its expiration time. fresh A response is fresh if its age has not yet exceeded its freshness lifetime. stale A response is stale if its age has passed its freshness lifetime. semantically transparent A cache behaves in a semantically transparent manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been handled directly by the origin server. validator A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent copy of an entity. upstream/downstream Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream. Fielding, et al. Standards Track [Page 11] RFC 2616 HTTP/1.1 June 1999 inbound/outbound Inbound and outbound refer to the request and response paths for messages: inbound means traveling toward the origin server, and outbound means traveling toward the user agent 1.4 Overall Operation The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the messages protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible entity-body content. The relationship between HTTP and MIME is described in appendix 19.4. Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O). request chain ------------------------> UA -------------------v------------------- O UA -----v----- A -----v----- B - - - - - - C - - - - - - O | / | [ | ] | ? | = | { | } | SP | HT Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing comment as part of their field value definition. In all other fields, parentheses are considered part of the field value. comment = ( *( ctext | quoted-pair | comment ) ) ctext = A string of text is parsed as a single word if it is quoted using double-quote marks. quoted-string = ( ) qdtext =
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