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| author | Klaus Weidenbach <Klaus.Weidenbach@gmx.net> | 2014-06-28 22:28:08 +0200 |
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| committer | Klaus Weidenbach <Klaus.Weidenbach@gmx.net> | 2014-06-29 01:17:07 +0200 |
| commit | 03b31d113ea316c8384a4cbf3d27ca22bb528eac (patch) | |
| tree | 92ed87436b09ab806f9effff08145408044d77f4 /vendor/sabre/dav/docs/rfc2616.txt | |
| parent | f49b74c5f6ebe57937fb6dfea7d2e917f4680ce9 (diff) | |
| download | volse-hubzilla-03b31d113ea316c8384a4cbf3d27ca22bb528eac.tar.gz volse-hubzilla-03b31d113ea316c8384a4cbf3d27ca22bb528eac.tar.bz2 volse-hubzilla-03b31d113ea316c8384a4cbf3d27ca22bb528eac.zip | |
Update SabreDAV from 1.8.9 to 1.8.10.
Diffstat (limited to 'vendor/sabre/dav/docs/rfc2616.txt')
| -rw-r--r-- | vendor/sabre/dav/docs/rfc2616.txt | 9859 |
1 files changed, 0 insertions, 9859 deletions
diff --git a/vendor/sabre/dav/docs/rfc2616.txt b/vendor/sabre/dav/docs/rfc2616.txt deleted file mode 100644 index 45d7d08b8..000000000 --- a/vendor/sabre/dav/docs/rfc2616.txt +++ /dev/null @@ -1,9859 +0,0 @@ - - - - - - -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 URI's .................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 other's 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 program's 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 program's 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 message's 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 - <----------------------- response chain - - A more complicated situation occurs when one or more intermediaries - are present in the request/response chain. There are three common - forms of intermediary: proxy, gateway, and tunnel. A proxy is a - forwarding agent, receiving requests for a URI in its absolute form, - rewriting all or part of the message, and forwarding the reformatted - request toward the server identified by the URI. A gateway is a - receiving agent, acting as a layer above some other server(s) and, if - necessary, translating the requests to the underlying server's - protocol. A tunnel acts as a relay point between two connections - without changing the messages; tunnels are used when the - communication needs to pass through an intermediary (such as a - firewall) even when the intermediary cannot understand the contents - of the messages. - - request chain --------------------------------------> - UA -----v----- A -----v----- B -----v----- C -----v----- O - <------------------------------------- response chain - - The figure above shows three intermediaries (A, B, and C) between the - user agent and origin server. A request or response message that - travels the whole chain will pass through four separate connections. - This distinction is important because some HTTP communication options - - - -Fielding, et al. Standards Track [Page 12] - -RFC 2616 HTTP/1.1 June 1999 - - - may apply only to the connection with the nearest, non-tunnel - neighbor, only to the end-points of the chain, or to all connections - along the chain. Although the diagram is linear, each participant may - be engaged in multiple, simultaneous communications. For example, B - may be receiving requests from many clients other than A, and/or - forwarding requests to servers other than C, at the same time that it - is handling A's request. - - Any party to the communication which is not acting as a tunnel may - employ an internal cache for handling requests. The effect of a cache - is that the request/response chain is shortened if one of the - participants along the chain has a cached response applicable to that - request. The following illustrates the resulting chain if B has a - cached copy of an earlier response from O (via C) for a request which - has not been cached by UA or A. - - request chain ----------> - UA -----v----- A -----v----- B - - - - - - C - - - - - - O - <--------- response chain - - Not all responses are usefully cacheable, and some requests may - contain modifiers which place special requirements on cache behavior. - HTTP requirements for cache behavior and cacheable responses are - defined in section 13. - - In fact, there are a wide variety of architectures and configurations - of caches and proxies currently being experimented with or deployed - across the World Wide Web. These systems include national hierarchies - of proxy caches to save transoceanic bandwidth, systems that - broadcast or multicast cache entries, organizations that distribute - subsets of cached data via CD-ROM, and so on. HTTP systems are used - in corporate intranets over high-bandwidth links, and for access via - PDAs with low-power radio links and intermittent connectivity. The - goal of HTTP/1.1 is to support the wide diversity of configurations - already deployed while introducing protocol constructs that meet the - needs of those who build web applications that require high - reliability and, failing that, at least reliable indications of - failure. - - HTTP communication usually takes place over TCP/IP connections. The - default port is TCP 80 [19], but other ports can be used. This does - not preclude HTTP from being implemented on top of any other protocol - on the Internet, or on other networks. HTTP only presumes a reliable - transport; any protocol that provides such guarantees can be used; - the mapping of the HTTP/1.1 request and response structures onto the - transport data units of the protocol in question is outside the scope - of this specification. - - - - -Fielding, et al. Standards Track [Page 13] - -RFC 2616 HTTP/1.1 June 1999 - - - In HTTP/1.0, most implementations used a new connection for each - request/response exchange. In HTTP/1.1, a connection may be used for - one or more request/response exchanges, although connections may be - closed for a variety of reasons (see section 8.1). - -2 Notational Conventions and Generic Grammar - -2.1 Augmented BNF - - All of the mechanisms specified in this document are described in - both prose and an augmented Backus-Naur Form (BNF) similar to that - used by RFC 822 [9]. Implementors will need to be familiar with the - notation in order to understand this specification. The augmented BNF - includes the following constructs: - - name = definition - The name of a rule is simply the name itself (without any - enclosing "<" and ">") and is separated from its definition by the - equal "=" character. White space is only significant in that - indentation of continuation lines is used to indicate a rule - definition that spans more than one line. Certain basic rules are - in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle - brackets are used within definitions whenever their presence will - facilitate discerning the use of rule names. - - "literal" - Quotation marks surround literal text. Unless stated otherwise, - the text is case-insensitive. - - rule1 | rule2 - Elements separated by a bar ("|") are alternatives, e.g., "yes | - no" will accept yes or no. - - (rule1 rule2) - Elements enclosed in parentheses are treated as a single element. - Thus, "(elem (foo | bar) elem)" allows the token sequences "elem - foo elem" and "elem bar elem". - - *rule - The character "*" preceding an element indicates repetition. The - full form is "<n>*<m>element" indicating at least <n> and at most - <m> occurrences of element. Default values are 0 and infinity so - that "*(element)" allows any number, including zero; "1*element" - requires at least one; and "1*2element" allows one or two. - - [rule] - Square brackets enclose optional elements; "[foo bar]" is - equivalent to "*1(foo bar)". - - - -Fielding, et al. Standards Track [Page 14] - -RFC 2616 HTTP/1.1 June 1999 - - - N rule - Specific repetition: "<n>(element)" is equivalent to - "<n>*<n>(element)"; that is, exactly <n> occurrences of (element). - Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three - alphabetic characters. - - #rule - A construct "#" is defined, similar to "*", for defining lists of - elements. The full form is "<n>#<m>element" indicating at least - <n> and at most <m> elements, each separated by one or more commas - (",") and OPTIONAL linear white space (LWS). This makes the usual - form of lists very easy; a rule such as - ( *LWS element *( *LWS "," *LWS element )) - can be shown as - 1#element - Wherever this construct is used, null elements are allowed, but do - not contribute to the count of elements present. That is, - "(element), , (element) " is permitted, but counts as only two - elements. Therefore, where at least one element is required, at - least one non-null element MUST be present. Default values are 0 - and infinity so that "#element" allows any number, including zero; - "1#element" requires at least one; and "1#2element" allows one or - two. - - ; comment - A semi-colon, set off some distance to the right of rule text, - starts a comment that continues to the end of line. This is a - simple way of including useful notes in parallel with the - specifications. - - implied *LWS - The grammar described by this specification is word-based. Except - where noted otherwise, linear white space (LWS) can be included - between any two adjacent words (token or quoted-string), and - between adjacent words and separators, without changing the - interpretation of a field. At least one delimiter (LWS and/or - - separators) MUST exist between any two tokens (for the definition - of "token" below), since they would otherwise be interpreted as a - single token. - -2.2 Basic Rules - - The following rules are used throughout this specification to - describe basic parsing constructs. The US-ASCII coded character set - is defined by ANSI X3.4-1986 [21]. - - - - - -Fielding, et al. Standards Track [Page 15] - -RFC 2616 HTTP/1.1 June 1999 - - - OCTET = <any 8-bit sequence of data> - CHAR = <any US-ASCII character (octets 0 - 127)> - UPALPHA = <any US-ASCII uppercase letter "A".."Z"> - LOALPHA = <any US-ASCII lowercase letter "a".."z"> - ALPHA = UPALPHA | LOALPHA - DIGIT = <any US-ASCII digit "0".."9"> - CTL = <any US-ASCII control character - (octets 0 - 31) and DEL (127)> - CR = <US-ASCII CR, carriage return (13)> - LF = <US-ASCII LF, linefeed (10)> - SP = <US-ASCII SP, space (32)> - HT = <US-ASCII HT, horizontal-tab (9)> - <"> = <US-ASCII double-quote mark (34)> - - HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all - protocol elements except the entity-body (see appendix 19.3 for - tolerant applications). The end-of-line marker within an entity-body - is defined by its associated media type, as described in section 3.7. - - CRLF = CR LF - - HTTP/1.1 header field values can be folded onto multiple lines if the - continuation line begins with a space or horizontal tab. All linear - white space, including folding, has the same semantics as SP. A - recipient MAY replace any linear white space with a single SP before - interpreting the field value or forwarding the message downstream. - - LWS = [CRLF] 1*( SP | HT ) - - The TEXT rule is only used for descriptive field contents and values - that are not intended to be interpreted by the message parser. Words - of *TEXT MAY contain characters from character sets other than ISO- - 8859-1 [22] only when encoded according to the rules of RFC 2047 - [14]. - - TEXT = <any OCTET except CTLs, - but including LWS> - - A CRLF is allowed in the definition of TEXT only as part of a header - field continuation. It is expected that the folding LWS will be - replaced with a single SP before interpretation of the TEXT value. - - Hexadecimal numeric characters are used in several protocol elements. - - HEX = "A" | "B" | "C" | "D" | "E" | "F" - | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT - - - - - -Fielding, et al. Standards Track [Page 16] - -RFC 2616 HTTP/1.1 June 1999 - - - Many HTTP/1.1 header field values consist of words separated by LWS - or special characters. These special characters MUST be in a quoted - string to be used within a parameter value (as defined in section - 3.6). - - token = 1*<any CHAR except CTLs or separators> - separators = "(" | ")" | "<" | ">" | "@" - | "," | ";" | ":" | "\" | <"> - | "/" | "[" | "]" | "?" | "=" - | "{" | "}" | 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 = <any TEXT excluding "(" and ")"> - - A string of text is parsed as a single word if it is quoted using - double-quote marks. - - quoted-string = ( <"> *(qdtext | quoted-pair ) <"> ) - qdtext = <any TEXT except <">> - - The backslash character ("\") MAY be used as a single-character - quoting mechanism only within quoted-string and comment constructs. - - quoted-pair = "\" CHAR - -3 Protocol Parameters - -3.1 HTTP Version - - HTTP uses a "<major>.<minor>" numbering scheme to indicate versions - of the protocol. The protocol versioning policy is intended to allow - the sender to indicate the format of a message and its capacity for - understanding further HTTP communication, rather than the features - obtained via that communication. No change is made to the version - number for the addition of message components which do not affect - communication behavior or which only add to extensible field values. - The <minor> number is incremented when the changes made to the - protocol add features which do not change the general message parsing - algorithm, but which may add to the message semantics and imply - additional capabilities of the sender. The <major> number is - incremented when the format of a message within the protocol is - changed. See RFC 2145 [36] for a fuller explanation. - - - -Fielding, et al. Standards Track [Page 17] - -RFC 2616 HTTP/1.1 June 1999 - - - The version of an HTTP message is indicated by an HTTP-Version field - in the first line of the message. - - HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT - - Note that the major and minor numbers MUST be treated as separate - integers and that each MAY be incremented higher than a single digit. - Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is - lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and - MUST NOT be sent. - - An application that sends a request or response message that includes - HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant - with this specification. Applications that are at least conditionally - compliant with this specification SHOULD use an HTTP-Version of - "HTTP/1.1" in their messages, and MUST do so for any message that is - not compatible with HTTP/1.0. For more details on when to send - specific HTTP-Version values, see RFC 2145 [36]. - - The HTTP version of an application is the highest HTTP version for - which the application is at least conditionally compliant. - - Proxy and gateway applications need to be careful when forwarding - messages in protocol versions different from that of the application. - Since the protocol version indicates the protocol capability of the - sender, a proxy/gateway MUST NOT send a message with a version - indicator which is greater than its actual version. If a higher - version request is received, the proxy/gateway MUST either downgrade - the request version, or respond with an error, or switch to tunnel - behavior. - - Due to interoperability problems with HTTP/1.0 proxies discovered - since the publication of RFC 2068[33], caching proxies MUST, gateways - MAY, and tunnels MUST NOT upgrade the request to the highest version - they support. The proxy/gateway's response to that request MUST be in - the same major version as the request. - - Note: Converting between versions of HTTP may involve modification - of header fields required or forbidden by the versions involved. - -3.2 Uniform Resource Identifiers - - URIs have been known by many names: WWW addresses, Universal Document - Identifiers, Universal Resource Identifiers [3], and finally the - combination of Uniform Resource Locators (URL) [4] and Names (URN) - [20]. As far as HTTP is concerned, Uniform Resource Identifiers are - simply formatted strings which identify--via name, location, or any - other characteristic--a resource. - - - -Fielding, et al. Standards Track [Page 18] - -RFC 2616 HTTP/1.1 June 1999 - - -3.2.1 General Syntax - - URIs in HTTP can be represented in absolute form or relative to some - known base URI [11], depending upon the context of their use. The two - forms are differentiated by the fact that absolute URIs always begin - with a scheme name followed by a colon. For definitive information on - URL syntax and semantics, see "Uniform Resource Identifiers (URI): - Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs - 1738 [4] and RFC 1808 [11]). This specification adopts the - definitions of "URI-reference", "absoluteURI", "relativeURI", "port", - "host","abs_path", "rel_path", and "authority" from that - specification. - - The HTTP protocol does not place any a priori limit on the length of - a URI. Servers MUST be able to handle the URI of any resource they - serve, and SHOULD be able to handle URIs of unbounded length if they - provide GET-based forms that could generate such URIs. A server - SHOULD return 414 (Request-URI Too Long) status if a URI is longer - than the server can handle (see section 10.4.15). - - Note: Servers ought to be cautious about depending on URI lengths - above 255 bytes, because some older client or proxy - implementations might not properly support these lengths. - -3.2.2 http URL - - The "http" scheme is used to locate network resources via the HTTP - protocol. This section defines the scheme-specific syntax and - semantics for http URLs. - - http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]] - - If the port is empty or not given, port 80 is assumed. The semantics - are that the identified resource is located at the server listening - for TCP connections on that port of that host, and the Request-URI - for the resource is abs_path (section 5.1.2). The use of IP addresses - in URLs SHOULD be avoided whenever possible (see RFC 1900 [24]). If - the abs_path is not present in the URL, it MUST be given as "/" when - used as a Request-URI for a resource (section 5.1.2). If a proxy - receives a host name which is not a fully qualified domain name, it - MAY add its domain to the host name it received. If a proxy receives - a fully qualified domain name, the proxy MUST NOT change the host - name. - - - - - - - - -Fielding, et al. Standards Track [Page 19] - -RFC 2616 HTTP/1.1 June 1999 - - -3.2.3 URI Comparison - - When comparing two URIs to decide if they match or not, a client - SHOULD use a case-sensitive octet-by-octet comparison of the entire - URIs, with these exceptions: - - - A port that is empty or not given is equivalent to the default - port for that URI-reference; - - - Comparisons of host names MUST be case-insensitive; - - - Comparisons of scheme names MUST be case-insensitive; - - - An empty abs_path is equivalent to an abs_path of "/". - - Characters other than those in the "reserved" and "unsafe" sets (see - RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding. - - For example, the following three URIs are equivalent: - - http://abc.com:80/~smith/home.html - http://ABC.com/%7Esmith/home.html - http://ABC.com:/%7esmith/home.html - -3.3 Date/Time Formats - -3.3.1 Full Date - - HTTP applications have historically allowed three different formats - for the representation of date/time stamps: - - Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 - Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 - Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format - - The first format is preferred as an Internet standard and represents - a fixed-length subset of that defined by RFC 1123 [8] (an update to - RFC 822 [9]). The second format is in common use, but is based on the - obsolete RFC 850 [12] date format and lacks a four-digit year. - HTTP/1.1 clients and servers that parse the date value MUST accept - all three formats (for compatibility with HTTP/1.0), though they MUST - only generate the RFC 1123 format for representing HTTP-date values - in header fields. See section 19.3 for further information. - - Note: Recipients of date values are encouraged to be robust in - accepting date values that may have been sent by non-HTTP - applications, as is sometimes the case when retrieving or posting - messages via proxies/gateways to SMTP or NNTP. - - - -Fielding, et al. Standards Track [Page 20] - -RFC 2616 HTTP/1.1 June 1999 - - - All HTTP date/time stamps MUST be represented in Greenwich Mean Time - (GMT), without exception. For the purposes of HTTP, GMT is exactly - equal to UTC (Coordinated Universal Time). This is indicated in the - first two formats by the inclusion of "GMT" as the three-letter - abbreviation for time zone, and MUST be assumed when reading the - asctime format. HTTP-date is case sensitive and MUST NOT include - additional LWS beyond that specifically included as SP in the - grammar. - - HTTP-date = rfc1123-date | rfc850-date | asctime-date - rfc1123-date = wkday "," SP date1 SP time SP "GMT" - rfc850-date = weekday "," SP date2 SP time SP "GMT" - asctime-date = wkday SP date3 SP time SP 4DIGIT - date1 = 2DIGIT SP month SP 4DIGIT - ; day month year (e.g., 02 Jun 1982) - date2 = 2DIGIT "-" month "-" 2DIGIT - ; day-month-year (e.g., 02-Jun-82) - date3 = month SP ( 2DIGIT | ( SP 1DIGIT )) - ; month day (e.g., Jun 2) - time = 2DIGIT ":" 2DIGIT ":" 2DIGIT - ; 00:00:00 - 23:59:59 - wkday = "Mon" | "Tue" | "Wed" - | "Thu" | "Fri" | "Sat" | "Sun" - weekday = "Monday" | "Tuesday" | "Wednesday" - | "Thursday" | "Friday" | "Saturday" | "Sunday" - month = "Jan" | "Feb" | "Mar" | "Apr" - | "May" | "Jun" | "Jul" | "Aug" - | "Sep" | "Oct" | "Nov" | "Dec" - - Note: HTTP requirements for the date/time stamp format apply only - to their usage within the protocol stream. Clients and servers are - not required to use these formats for user presentation, request - logging, etc. - -3.3.2 Delta Seconds - - Some HTTP header fields allow a time value to be specified as an - integer number of seconds, represented in decimal, after the time - that the message was received. - - delta-seconds = 1*DIGIT - -3.4 Character Sets - - HTTP uses the same definition of the term "character set" as that - described for MIME: - - - - - -Fielding, et al. Standards Track [Page 21] - -RFC 2616 HTTP/1.1 June 1999 - - - The term "character set" is used in this document to refer to a - method used with one or more tables to convert a sequence of octets - into a sequence of characters. Note that unconditional conversion in - the other direction is not required, in that not all characters may - be available in a given character set and a character set may provide - more than one sequence of octets to represent a particular character. - This definition is intended to allow various kinds of character - encoding, from simple single-table mappings such as US-ASCII to - complex table switching methods such as those that use ISO-2022's - techniques. However, the definition associated with a MIME character - set name MUST fully specify the mapping to be performed from octets - to characters. In particular, use of external profiling information - to determine the exact mapping is not permitted. - - Note: This use of the term "character set" is more commonly - referred to as a "character encoding." However, since HTTP and - MIME share the same registry, it is important that the terminology - also be shared. - - HTTP character sets are identified by case-insensitive tokens. The - complete set of tokens is defined by the IANA Character Set registry - [19]. - - charset = token - - Although HTTP allows an arbitrary token to be used as a charset - value, any token that has a predefined value within the IANA - Character Set registry [19] MUST represent the character set defined - by that registry. Applications SHOULD limit their use of character - sets to those defined by the IANA registry. - - Implementors should be aware of IETF character set requirements [38] - [41]. - -3.4.1 Missing Charset - - Some HTTP/1.0 software has interpreted a Content-Type header without - charset parameter incorrectly to mean "recipient should guess." - Senders wishing to defeat this behavior MAY include a charset - parameter even when the charset is ISO-8859-1 and SHOULD do so when - it is known that it will not confuse the recipient. - - Unfortunately, some older HTTP/1.0 clients did not deal properly with - an explicit charset parameter. HTTP/1.1 recipients MUST respect the - charset label provided by the sender; and those user agents that have - a provision to "guess" a charset MUST use the charset from the - - - - - -Fielding, et al. Standards Track [Page 22] - -RFC 2616 HTTP/1.1 June 1999 - - - content-type field if they support that charset, rather than the - recipient's preference, when initially displaying a document. See - section 3.7.1. - -3.5 Content Codings - - Content coding values indicate an encoding transformation that has - been or can be applied to an entity. Content codings are primarily - used to allow a document to be compressed or otherwise usefully - transformed without losing the identity of its underlying media type - and without loss of information. Frequently, the entity is stored in - coded form, transmitted directly, and only decoded by the recipient. - - content-coding = token - - All content-coding values are case-insensitive. HTTP/1.1 uses - content-coding values in the Accept-Encoding (section 14.3) and - Content-Encoding (section 14.11) header fields. Although the value - describes the content-coding, what is more important is that it - indicates what decoding mechanism will be required to remove the - encoding. - - The Internet Assigned Numbers Authority (IANA) acts as a registry for - content-coding value tokens. Initially, the registry contains the - following tokens: - - gzip An encoding format produced by the file compression program - "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a - Lempel-Ziv coding (LZ77) with a 32 bit CRC. - - compress - The encoding format produced by the common UNIX file compression - program "compress". This format is an adaptive Lempel-Ziv-Welch - coding (LZW). - - Use of program names for the identification of encoding formats - is not desirable and is discouraged for future encodings. Their - use here is representative of historical practice, not good - design. For compatibility with previous implementations of HTTP, - applications SHOULD consider "x-gzip" and "x-compress" to be - equivalent to "gzip" and "compress" respectively. - - deflate - The "zlib" format defined in RFC 1950 [31] in combination with - the "deflate" compression mechanism described in RFC 1951 [29]. - - - - - - -Fielding, et al. Standards Track [Page 23] - -RFC 2616 HTTP/1.1 June 1999 - - - identity - The default (identity) encoding; the use of no transformation - whatsoever. This content-coding is used only in the Accept- - Encoding header, and SHOULD NOT be used in the Content-Encoding - header. - - New content-coding value tokens SHOULD be registered; to allow - interoperability between clients and servers, specifications of the - content coding algorithms needed to implement a new value SHOULD be - publicly available and adequate for independent implementation, and - conform to the purpose of content coding defined in this section. - -3.6 Transfer Codings - - Transfer-coding values are used to indicate an encoding - transformation that has been, can be, or may need to be applied to an - entity-body in order to ensure "safe transport" through the network. - This differs from a content coding in that the transfer-coding is a - property of the message, not of the original entity. - - transfer-coding = "chunked" | transfer-extension - transfer-extension = token *( ";" parameter ) - - Parameters are in the form of attribute/value pairs. - - parameter = attribute "=" value - attribute = token - value = token | quoted-string - - All transfer-coding values are case-insensitive. HTTP/1.1 uses - transfer-coding values in the TE header field (section 14.39) and in - the Transfer-Encoding header field (section 14.41). - - Whenever a transfer-coding is applied to a message-body, the set of - transfer-codings MUST include "chunked", unless the message is - terminated by closing the connection. When the "chunked" transfer- - coding is used, it MUST be the last transfer-coding applied to the - message-body. The "chunked" transfer-coding MUST NOT be applied more - than once to a message-body. These rules allow the recipient to - determine the transfer-length of the message (section 4.4). - - Transfer-codings are analogous to the Content-Transfer-Encoding - values of MIME [7], which were designed to enable safe transport of - binary data over a 7-bit transport service. However, safe transport - has a different focus for an 8bit-clean transfer protocol. In HTTP, - the only unsafe characteristic of message-bodies is the difficulty in - determining the exact body length (section 7.2.2), or the desire to - encrypt data over a shared transport. - - - -Fielding, et al. Standards Track [Page 24] - -RFC 2616 HTTP/1.1 June 1999 - - - The Internet Assigned Numbers Authority (IANA) acts as a registry for - transfer-coding value tokens. Initially, the registry contains the - following tokens: "chunked" (section 3.6.1), "identity" (section - 3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate" - (section 3.5). - - New transfer-coding value tokens SHOULD be registered in the same way - as new content-coding value tokens (section 3.5). - - A server which receives an entity-body with a transfer-coding it does - not understand SHOULD return 501 (Unimplemented), and close the - connection. A server MUST NOT send transfer-codings to an HTTP/1.0 - client. - -3.6.1 Chunked Transfer Coding - - The chunked encoding modifies the body of a message in order to - transfer it as a series of chunks, each with its own size indicator, - followed by an OPTIONAL trailer containing entity-header fields. This - allows dynamically produced content to be transferred along with the - information necessary for the recipient to verify that it has - received the full message. - - Chunked-Body = *chunk - last-chunk - trailer - CRLF - - chunk = chunk-size [ chunk-extension ] CRLF - chunk-data CRLF - chunk-size = 1*HEX - last-chunk = 1*("0") [ chunk-extension ] CRLF - - chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] ) - chunk-ext-name = token - chunk-ext-val = token | quoted-string - chunk-data = chunk-size(OCTET) - trailer = *(entity-header CRLF) - - The chunk-size field is a string of hex digits indicating the size of - the chunk. The chunked encoding is ended by any chunk whose size is - zero, followed by the trailer, which is terminated by an empty line. - - The trailer allows the sender to include additional HTTP header - fields at the end of the message. The Trailer header field can be - used to indicate which header fields are included in a trailer (see - section 14.40). - - - - -Fielding, et al. Standards Track [Page 25] - -RFC 2616 HTTP/1.1 June 1999 - - - A server using chunked transfer-coding in a response MUST NOT use the - trailer for any header fields unless at least one of the following is - true: - - a)the request included a TE header field that indicates "trailers" is - acceptable in the transfer-coding of the response, as described in - section 14.39; or, - - b)the server is the origin server for the response, the trailer - fields consist entirely of optional metadata, and the recipient - could use the message (in a manner acceptable to the origin server) - without receiving this metadata. In other words, the origin server - is willing to accept the possibility that the trailer fields might - be silently discarded along the path to the client. - - This requirement prevents an interoperability failure when the - message is being received by an HTTP/1.1 (or later) proxy and - forwarded to an HTTP/1.0 recipient. It avoids a situation where - compliance with the protocol would have necessitated a possibly - infinite buffer on the proxy. - - An example process for decoding a Chunked-Body is presented in - appendix 19.4.6. - - All HTTP/1.1 applications MUST be able to receive and decode the - "chunked" transfer-coding, and MUST ignore chunk-extension extensions - they do not understand. - -3.7 Media Types - - HTTP uses Internet Media Types [17] in the Content-Type (section - 14.17) and Accept (section 14.1) header fields in order to provide - open and extensible data typing and type negotiation. - - media-type = type "/" subtype *( ";" parameter ) - type = token - subtype = token - - Parameters MAY follow the type/subtype in the form of attribute/value - pairs (as defined in section 3.6). - - The type, subtype, and parameter attribute names are case- - insensitive. Parameter values might or might not be case-sensitive, - depending on the semantics of the parameter name. Linear white space - (LWS) MUST NOT be used between the type and subtype, nor between an - attribute and its value. The presence or absence of a parameter might - be significant to the processing of a media-type, depending on its - definition within the media type registry. - - - -Fielding, et al. Standards Track [Page 26] - -RFC 2616 HTTP/1.1 June 1999 - - - Note that some older HTTP applications do not recognize media type - parameters. When sending data to older HTTP applications, - implementations SHOULD only use media type parameters when they are - required by that type/subtype definition. - - Media-type values are registered with the Internet Assigned Number - Authority (IANA [19]). The media type registration process is - outlined in RFC 1590 [17]. Use of non-registered media types is - discouraged. - -3.7.1 Canonicalization and Text Defaults - - Internet media types are registered with a canonical form. An - entity-body transferred via HTTP messages MUST be represented in the - appropriate canonical form prior to its transmission except for - "text" types, as defined in the next paragraph. - - When in canonical form, media subtypes of the "text" type use CRLF as - the text line break. HTTP relaxes this requirement and allows the - transport of text media with plain CR or LF alone representing a line - break when it is done consistently for an entire entity-body. HTTP - applications MUST accept CRLF, bare CR, and bare LF as being - representative of a line break in text media received via HTTP. In - addition, if the text is represented in a character set that does not - use octets 13 and 10 for CR and LF respectively, as is the case for - some multi-byte character sets, HTTP allows the use of whatever octet - sequences are defined by that character set to represent the - equivalent of CR and LF for line breaks. This flexibility regarding - line breaks applies only to text media in the entity-body; a bare CR - or LF MUST NOT be substituted for CRLF within any of the HTTP control - structures (such as header fields and multipart boundaries). - - If an entity-body is encoded with a content-coding, the underlying - data MUST be in a form defined above prior to being encoded. - - The "charset" parameter is used with some media types to define the - character set (section 3.4) of the data. When no explicit charset - parameter is provided by the sender, media subtypes of the "text" - type are defined to have a default charset value of "ISO-8859-1" when - received via HTTP. Data in character sets other than "ISO-8859-1" or - its subsets MUST be labeled with an appropriate charset value. See - section 3.4.1 for compatibility problems. - -3.7.2 Multipart Types - - MIME provides for a number of "multipart" types -- encapsulations of - one or more entities within a single message-body. All multipart - types share a common syntax, as defined in section 5.1.1 of RFC 2046 - - - -Fielding, et al. Standards Track [Page 27] - -RFC 2616 HTTP/1.1 June 1999 - - - [40], and MUST include a boundary parameter as part of the media type - value. The message body is itself a protocol element and MUST - therefore use only CRLF to represent line breaks between body-parts. - Unlike in RFC 2046, the epilogue of any multipart message MUST be - empty; HTTP applications MUST NOT transmit the epilogue (even if the - original multipart contains an epilogue). These restrictions exist in - order to preserve the self-delimiting nature of a multipart message- - body, wherein the "end" of the message-body is indicated by the - ending multipart boundary. - - In general, HTTP treats a multipart message-body no differently than - any other media type: strictly as payload. The one exception is the - "multipart/byteranges" type (appendix 19.2) when it appears in a 206 - (Partial Content) response, which will be interpreted by some HTTP - caching mechanisms as described in sections 13.5.4 and 14.16. In all - other cases, an HTTP user agent SHOULD follow the same or similar - behavior as a MIME user agent would upon receipt of a multipart type. - The MIME header fields within each body-part of a multipart message- - body do not have any significance to HTTP beyond that defined by - their MIME semantics. - - In general, an HTTP user agent SHOULD follow the same or similar - behavior as a MIME user agent would upon receipt of a multipart type. - If an application receives an unrecognized multipart subtype, the - application MUST treat it as being equivalent to "multipart/mixed". - - Note: The "multipart/form-data" type has been specifically defined - for carrying form data suitable for processing via the POST - request method, as described in RFC 1867 [15]. - -3.8 Product Tokens - - Product tokens are used to allow communicating applications to - identify themselves by software name and version. Most fields using - product tokens also allow sub-products which form a significant part - of the application to be listed, separated by white space. By - convention, the products are listed in order of their significance - for identifying the application. - - product = token ["/" product-version] - product-version = token - - Examples: - - User-Agent: CERN-LineMode/2.15 libwww/2.17b3 - Server: Apache/0.8.4 - - - - - -Fielding, et al. Standards Track [Page 28] - -RFC 2616 HTTP/1.1 June 1999 - - - Product tokens SHOULD be short and to the point. They MUST NOT be - used for advertising or other non-essential information. Although any - token character MAY appear in a product-version, this token SHOULD - only be used for a version identifier (i.e., successive versions of - the same product SHOULD only differ in the product-version portion of - the product value). - -3.9 Quality Values - - HTTP content negotiation (section 12) uses short "floating point" - numbers to indicate the relative importance ("weight") of various - negotiable parameters. A weight is normalized to a real number in - the range 0 through 1, where 0 is the minimum and 1 the maximum - value. If a parameter has a quality value of 0, then content with - this parameter is `not acceptable' for the client. HTTP/1.1 - applications MUST NOT generate more than three digits after the - decimal point. User configuration of these values SHOULD also be - limited in this fashion. - - qvalue = ( "0" [ "." 0*3DIGIT ] ) - | ( "1" [ "." 0*3("0") ] ) - - "Quality values" is a misnomer, since these values merely represent - relative degradation in desired quality. - -3.10 Language Tags - - A language tag identifies a natural language spoken, written, or - otherwise conveyed by human beings for communication of information - to other human beings. Computer languages are explicitly excluded. - HTTP uses language tags within the Accept-Language and Content- - Language fields. - - The syntax and registry of HTTP language tags is the same as that - defined by RFC 1766 [1]. In summary, a language tag is composed of 1 - or more parts: A primary language tag and a possibly empty series of - subtags: - - language-tag = primary-tag *( "-" subtag ) - primary-tag = 1*8ALPHA - subtag = 1*8ALPHA - - White space is not allowed within the tag and all tags are case- - insensitive. The name space of language tags is administered by the - IANA. Example tags include: - - en, en-US, en-cockney, i-cherokee, x-pig-latin - - - - -Fielding, et al. Standards Track [Page 29] - -RFC 2616 HTTP/1.1 June 1999 - - - where any two-letter primary-tag is an ISO-639 language abbreviation - and any two-letter initial subtag is an ISO-3166 country code. (The - last three tags above are not registered tags; all but the last are - examples of tags which could be registered in future.) - -3.11 Entity Tags - - Entity tags are used for comparing two or more entities from the same - requested resource. HTTP/1.1 uses entity tags in the ETag (section - 14.19), If-Match (section 14.24), If-None-Match (section 14.26), and - If-Range (section 14.27) header fields. The definition of how they - are used and compared as cache validators is in section 13.3.3. An - entity tag consists of an opaque quoted string, possibly prefixed by - a weakness indicator. - - entity-tag = [ weak ] opaque-tag - weak = "W/" - opaque-tag = quoted-string - - A "strong entity tag" MAY be shared by two entities of a resource - only if they are equivalent by octet equality. - - A "weak entity tag," indicated by the "W/" prefix, MAY be shared by - two entities of a resource only if the entities are equivalent and - could be substituted for each other with no significant change in - semantics. A weak entity tag can only be used for weak comparison. - - An entity tag MUST be unique across all versions of all entities - associated with a particular resource. A given entity tag value MAY - be used for entities obtained by requests on different URIs. The use - of the same entity tag value in conjunction with entities obtained by - requests on different URIs does not imply the equivalence of those - entities. - -3.12 Range Units - - HTTP/1.1 allows a client to request that only part (a range of) the - response entity be included within the response. HTTP/1.1 uses range - units in the Range (section 14.35) and Content-Range (section 14.16) - header fields. An entity can be broken down into subranges according - to various structural units. - - range-unit = bytes-unit | other-range-unit - bytes-unit = "bytes" - other-range-unit = token - - The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 - implementations MAY ignore ranges specified using other units. - - - -Fielding, et al. Standards Track [Page 30] - -RFC 2616 HTTP/1.1 June 1999 - - - HTTP/1.1 has been designed to allow implementations of applications - that do not depend on knowledge of ranges. - -4 HTTP Message - -4.1 Message Types - - HTTP messages consist of requests from client to server and responses - from server to client. - - HTTP-message = Request | Response ; HTTP/1.1 messages - - Request (section 5) and Response (section 6) messages use the generic - message format of RFC 822 [9] for transferring entities (the payload - of the message). Both types of message consist of a start-line, zero - or more header fields (also known as "headers"), an empty line (i.e., - a line with nothing preceding the CRLF) indicating the end of the - header fields, and possibly a message-body. - - generic-message = start-line - *(message-header CRLF) - CRLF - [ message-body ] - start-line = Request-Line | Status-Line - - In the interest of robustness, servers SHOULD ignore any empty - line(s) received where a Request-Line is expected. In other words, if - the server is reading the protocol stream at the beginning of a - message and receives a CRLF first, it should ignore the CRLF. - - Certain buggy HTTP/1.0 client implementations generate extra CRLF's - after a POST request. To restate what is explicitly forbidden by the - BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an - extra CRLF. - -4.2 Message Headers - - HTTP header fields, which include general-header (section 4.5), - request-header (section 5.3), response-header (section 6.2), and - entity-header (section 7.1) fields, follow the same generic format as - that given in Section 3.1 of RFC 822 [9]. Each header field consists - of a name followed by a colon (":") and the field value. Field names - are case-insensitive. The field value MAY be preceded by any amount - of LWS, though a single SP is preferred. Header fields can be - extended over multiple lines by preceding each extra line with at - least one SP or HT. Applications ought to follow "common form", where - one is known or indicated, when generating HTTP constructs, since - there might exist some implementations that fail to accept anything - - - -Fielding, et al. Standards Track [Page 31] - -RFC 2616 HTTP/1.1 June 1999 - - - beyond the common forms. - - message-header = field-name ":" [ field-value ] - field-name = token - field-value = *( field-content | LWS ) - field-content = <the OCTETs making up the field-value - and consisting of either *TEXT or combinations - of token, separators, and quoted-string> - - The field-content does not include any leading or trailing LWS: - linear white space occurring before the first non-whitespace - character of the field-value or after the last non-whitespace - character of the field-value. Such leading or trailing LWS MAY be - removed without changing the semantics of the field value. Any LWS - that occurs between field-content MAY be replaced with a single SP - before interpreting the field value or forwarding the message - downstream. - - The order in which header fields with differing field names are - received is not significant. However, it is "good practice" to send - general-header fields first, followed by request-header or response- - header fields, and ending with the entity-header fields. - - Multiple message-header fields with the same field-name MAY be - present in a message if and only if the entire field-value for that - header field is defined as a comma-separated list [i.e., #(values)]. - It MUST be possible to combine the multiple header fields into one - "field-name: field-value" pair, without changing the semantics of the - message, by appending each subsequent field-value to the first, each - separated by a comma. The order in which header fields with the same - field-name are received is therefore significant to the - interpretation of the combined field value, and thus a proxy MUST NOT - change the order of these field values when a message is forwarded. - -4.3 Message Body - - The message-body (if any) of an HTTP message is used to carry the - entity-body associated with the request or response. The message-body - differs from the entity-body only when a transfer-coding has been - applied, as indicated by the Transfer-Encoding header field (section - 14.41). - - message-body = entity-body - | <entity-body encoded as per Transfer-Encoding> - - Transfer-Encoding MUST be used to indicate any transfer-codings - applied by an application to ensure safe and proper transfer of the - message. Transfer-Encoding is a property of the message, not of the - - - -Fielding, et al. Standards Track [Page 32] - -RFC 2616 HTTP/1.1 June 1999 - - - entity, and thus MAY be added or removed by any application along the - request/response chain. (However, section 3.6 places restrictions on - when certain transfer-codings may be used.) - - The rules for when a message-body is allowed in a message differ for - requests and responses. - - The presence of a message-body in a request is signaled by the - inclusion of a Content-Length or Transfer-Encoding header field in - the request's message-headers. A message-body MUST NOT be included in - a request if the specification of the request method (section 5.1.1) - does not allow sending an entity-body in requests. A server SHOULD - read and forward a message-body on any request; if the request method - does not include defined semantics for an entity-body, then the - message-body SHOULD be ignored when handling the request. - - For response messages, whether or not a message-body is included with - a message is dependent on both the request method and the response - status code (section 6.1.1). All responses to the HEAD request method - MUST NOT include a message-body, even though the presence of entity- - header fields might lead one to believe they do. All 1xx - (informational), 204 (no content), and 304 (not modified) responses - MUST NOT include a message-body. All other responses do include a - message-body, although it MAY be of zero length. - -4.4 Message Length - - The transfer-length of a message is the length of the message-body as - it appears in the message; that is, after any transfer-codings have - been applied. When a message-body is included with a message, the - transfer-length of that body is determined by one of the following - (in order of precedence): - - 1.Any response message which "MUST NOT" include a message-body (such - as the 1xx, 204, and 304 responses and any response to a HEAD - request) is always terminated by the first empty line after the - header fields, regardless of the entity-header fields present in - the message. - - 2.If a Transfer-Encoding header field (section 14.41) is present and - has any value other than "identity", then the transfer-length is - defined by use of the "chunked" transfer-coding (section 3.6), - unless the message is terminated by closing the connection. - - 3.If a Content-Length header field (section 14.13) is present, its - decimal value in OCTETs represents both the entity-length and the - transfer-length. The Content-Length header field MUST NOT be sent - if these two lengths are different (i.e., if a Transfer-Encoding - - - -Fielding, et al. Standards Track [Page 33] - -RFC 2616 HTTP/1.1 June 1999 - - - header field is present). If a message is received with both a - Transfer-Encoding header field and a Content-Length header field, - the latter MUST be ignored. - - 4.If the message uses the media type "multipart/byteranges", and the - ransfer-length is not otherwise specified, then this self- - elimiting media type defines the transfer-length. This media type - UST NOT be used unless the sender knows that the recipient can arse - it; the presence in a request of a Range header with ultiple byte- - range specifiers from a 1.1 client implies that the lient can parse - multipart/byteranges responses. - - A range header might be forwarded by a 1.0 proxy that does not - understand multipart/byteranges; in this case the server MUST - delimit the message using methods defined in items 1,3 or 5 of - this section. - - 5.By the server closing the connection. (Closing the connection - cannot be used to indicate the end of a request body, since that - would leave no possibility for the server to send back a response.) - - For compatibility with HTTP/1.0 applications, HTTP/1.1 requests - containing a message-body MUST include a valid Content-Length header - field unless the server is known to be HTTP/1.1 compliant. If a - request contains a message-body and a Content-Length is not given, - the server SHOULD respond with 400 (bad request) if it cannot - determine the length of the message, or with 411 (length required) if - it wishes to insist on receiving a valid Content-Length. - - All HTTP/1.1 applications that receive entities MUST accept the - "chunked" transfer-coding (section 3.6), thus allowing this mechanism - to be used for messages when the message length cannot be determined - in advance. - - Messages MUST NOT include both a Content-Length header field and a - non-identity transfer-coding. If the message does include a non- - identity transfer-coding, the Content-Length MUST be ignored. - - When a Content-Length is given in a message where a message-body is - allowed, its field value MUST exactly match the number of OCTETs in - the message-body. HTTP/1.1 user agents MUST notify the user when an - invalid length is received and detected. - -4.5 General Header Fields - - There are a few header fields which have general applicability for - both request and response messages, but which do not apply to the - entity being transferred. These header fields apply only to the - - - -Fielding, et al. Standards Track [Page 34] - -RFC 2616 HTTP/1.1 June 1999 - - - message being transmitted. - - general-header = Cache-Control ; Section 14.9 - | Connection ; Section 14.10 - | Date ; Section 14.18 - | Pragma ; Section 14.32 - | Trailer ; Section 14.40 - | Transfer-Encoding ; Section 14.41 - | Upgrade ; Section 14.42 - | Via ; Section 14.45 - | Warning ; Section 14.46 - - General-header field names can be extended reliably only in - combination with a change in the protocol version. However, new or - experimental header fields may be given the semantics of general - header fields if all parties in the communication recognize them to - be general-header fields. Unrecognized header fields are treated as - entity-header fields. - -5 Request - - A request message from a client to a server includes, within the - first line of that message, the method to be applied to the resource, - the identifier of the resource, and the protocol version in use. - - Request = Request-Line ; Section 5.1 - *(( general-header ; Section 4.5 - | request-header ; Section 5.3 - | entity-header ) CRLF) ; Section 7.1 - CRLF - [ message-body ] ; Section 4.3 - -5.1 Request-Line - - The Request-Line begins with a method token, followed by the - Request-URI and the protocol version, and ending with CRLF. The - elements are separated by SP characters. No CR or LF is allowed - except in the final CRLF sequence. - - Request-Line = Method SP Request-URI SP HTTP-Version CRLF - - - - - - - - - - - -Fielding, et al. Standards Track [Page 35] - -RFC 2616 HTTP/1.1 June 1999 - - -5.1.1 Method - - The Method token indicates the method to be performed on the - resource identified by the Request-URI. The method is case-sensitive. - - Method = "OPTIONS" ; Section 9.2 - | "GET" ; Section 9.3 - | "HEAD" ; Section 9.4 - | "POST" ; Section 9.5 - | "PUT" ; Section 9.6 - | "DELETE" ; Section 9.7 - | "TRACE" ; Section 9.8 - | "CONNECT" ; Section 9.9 - | extension-method - extension-method = token - - The list of methods allowed by a resource can be specified in an - Allow header field (section 14.7). The return code of the response - always notifies the client whether a method is currently allowed on a - resource, since the set of allowed methods can change dynamically. An - origin server SHOULD return the status code 405 (Method Not Allowed) - if the method is known by the origin server but not allowed for the - requested resource, and 501 (Not Implemented) if the method is - unrecognized or not implemented by the origin server. The methods GET - and HEAD MUST be supported by all general-purpose servers. All other - methods are OPTIONAL; however, if the above methods are implemented, - they MUST be implemented with the same semantics as those specified - in section 9. - -5.1.2 Request-URI - - The Request-URI is a Uniform Resource Identifier (section 3.2) and - identifies the resource upon which to apply the request. - - Request-URI = "*" | absoluteURI | abs_path | authority - - The four options for Request-URI are dependent on the nature of the - request. The asterisk "*" means that the request does not apply to a - particular resource, but to the server itself, and is only allowed - when the method used does not necessarily apply to a resource. One - example would be - - OPTIONS * HTTP/1.1 - - The absoluteURI form is REQUIRED when the request is being made to a - proxy. The proxy is requested to forward the request or service it - from a valid cache, and return the response. Note that the proxy MAY - forward the request on to another proxy or directly to the server - - - -Fielding, et al. Standards Track [Page 36] - -RFC 2616 HTTP/1.1 June 1999 - - - specified by the absoluteURI. In order to avoid request loops, a - proxy MUST be able to recognize all of its server names, including - any aliases, local variations, and the numeric IP address. An example - Request-Line would be: - - GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1 - - To allow for transition to absoluteURIs in all requests in future - versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI - form in requests, even though HTTP/1.1 clients will only generate - them in requests to proxies. - - The authority form is only used by the CONNECT method (section 9.9). - - The most common form of Request-URI is that used to identify a - resource on an origin server or gateway. In this case the absolute - path of the URI MUST be transmitted (see section 3.2.1, abs_path) as - the Request-URI, and the network location of the URI (authority) MUST - be transmitted in a Host header field. For example, a client wishing - to retrieve the resource above directly from the origin server would - create a TCP connection to port 80 of the host "www.w3.org" and send - the lines: - - GET /pub/WWW/TheProject.html HTTP/1.1 - Host: www.w3.org - - followed by the remainder of the Request. Note that the absolute path - cannot be empty; if none is present in the original URI, it MUST be - given as "/" (the server root). - - The Request-URI is transmitted in the format specified in section - 3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding - [42], the origin server MUST decode the Request-URI in order to - properly interpret the request. Servers SHOULD respond to invalid - Request-URIs with an appropriate status code. - - A transparent proxy MUST NOT rewrite the "abs_path" part of the - received Request-URI when forwarding it to the next inbound server, - except as noted above to replace a null abs_path with "/". - - Note: The "no rewrite" rule prevents the proxy from changing the - meaning of the request when the origin server is improperly using - a non-reserved URI character for a reserved purpose. Implementors - should be aware that some pre-HTTP/1.1 proxies have been known to - rewrite the Request-URI. - - - - - - -Fielding, et al. Standards Track [Page 37] - -RFC 2616 HTTP/1.1 June 1999 - - -5.2 The Resource Identified by a Request - - The exact resource identified by an Internet request is determined by - examining both the Request-URI and the Host header field. - - An origin server that does not allow resources to differ by the - requested host MAY ignore the Host header field value when - determining the resource identified by an HTTP/1.1 request. (But see - section 19.6.1.1 for other requirements on Host support in HTTP/1.1.) - - An origin server that does differentiate resources based on the host - requested (sometimes referred to as virtual hosts or vanity host - names) MUST use the following rules for determining the requested - resource on an HTTP/1.1 request: - - 1. If Request-URI is an absoluteURI, the host is part of the - Request-URI. Any Host header field value in the request MUST be - ignored. - - 2. If the Request-URI is not an absoluteURI, and the request includes - a Host header field, the host is determined by the Host header - field value. - - 3. If the host as determined by rule 1 or 2 is not a valid host on - the server, the response MUST be a 400 (Bad Request) error message. - - Recipients of an HTTP/1.0 request that lacks a Host header field MAY - attempt to use heuristics (e.g., examination of the URI path for - something unique to a particular host) in order to determine what - exact resource is being requested. - -5.3 Request Header Fields - - The request-header fields allow the client to pass additional - information about the request, and about the client itself, to the - server. These fields act as request modifiers, with semantics - equivalent to the parameters on a programming language method - invocation. - - request-header = Accept ; Section 14.1 - | Accept-Charset ; Section 14.2 - | Accept-Encoding ; Section 14.3 - | Accept-Language ; Section 14.4 - | Authorization ; Section 14.8 - | Expect ; Section 14.20 - | From ; Section 14.22 - | Host ; Section 14.23 - | If-Match ; Section 14.24 - - - -Fielding, et al. Standards Track [Page 38] - -RFC 2616 HTTP/1.1 June 1999 - - - | If-Modified-Since ; Section 14.25 - | If-None-Match ; Section 14.26 - | If-Range ; Section 14.27 - | If-Unmodified-Since ; Section 14.28 - | Max-Forwards ; Section 14.31 - | Proxy-Authorization ; Section 14.34 - | Range ; Section 14.35 - | Referer ; Section 14.36 - | TE ; Section 14.39 - | User-Agent ; Section 14.43 - - Request-header field names can be extended reliably only in - combination with a change in the protocol version. However, new or - experimental header fields MAY be given the semantics of request- - header fields if all parties in the communication recognize them to - be request-header fields. Unrecognized header fields are treated as - entity-header fields. - -6 Response - - After receiving and interpreting a request message, a server responds - with an HTTP response message. - - Response = Status-Line ; Section 6.1 - *(( general-header ; Section 4.5 - | response-header ; Section 6.2 - | entity-header ) CRLF) ; Section 7.1 - CRLF - [ message-body ] ; Section 7.2 - -6.1 Status-Line - - The first line of a Response message is the Status-Line, consisting - of the protocol version followed by a numeric status code and its - associated textual phrase, with each element separated by SP - characters. No CR or LF is allowed except in the final CRLF sequence. - - Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF - -6.1.1 Status Code and Reason Phrase - - The Status-Code element is a 3-digit integer result code of the - attempt to understand and satisfy the request. These codes are fully - defined in section 10. The Reason-Phrase is intended to give a short - textual description of the Status-Code. The Status-Code is intended - for use by automata and the Reason-Phrase is intended for the human - user. The client is not required to examine or display the Reason- - Phrase. - - - -Fielding, et al. Standards Track [Page 39] - -RFC 2616 HTTP/1.1 June 1999 - - - The first digit of the Status-Code defines the class of response. The - last two digits do not have any categorization role. There are 5 - values for the first digit: - - - 1xx: Informational - Request received, continuing process - - - 2xx: Success - The action was successfully received, - understood, and accepted - - - 3xx: Redirection - Further action must be taken in order to - complete the request - - - 4xx: Client Error - The request contains bad syntax or cannot - be fulfilled - - - 5xx: Server Error - The server failed to fulfill an apparently - valid request - - The individual values of the numeric status codes defined for - HTTP/1.1, and an example set of corresponding Reason-Phrase's, are - presented below. The reason phrases listed here are only - recommendations -- they MAY be replaced by local equivalents without - affecting the protocol. - - Status-Code = - "100" ; Section 10.1.1: Continue - | "101" ; Section 10.1.2: Switching Protocols - | "200" ; Section 10.2.1: OK - | "201" ; Section 10.2.2: Created - | "202" ; Section 10.2.3: Accepted - | "203" ; Section 10.2.4: Non-Authoritative Information - | "204" ; Section 10.2.5: No Content - | "205" ; Section 10.2.6: Reset Content - | "206" ; Section 10.2.7: Partial Content - | "300" ; Section 10.3.1: Multiple Choices - | "301" ; Section 10.3.2: Moved Permanently - | "302" ; Section 10.3.3: Found - | "303" ; Section 10.3.4: See Other - | "304" ; Section 10.3.5: Not Modified - | "305" ; Section 10.3.6: Use Proxy - | "307" ; Section 10.3.8: Temporary Redirect - | "400" ; Section 10.4.1: Bad Request - | "401" ; Section 10.4.2: Unauthorized - | "402" ; Section 10.4.3: Payment Required - | "403" ; Section 10.4.4: Forbidden - | "404" ; Section 10.4.5: Not Found - | "405" ; Section 10.4.6: Method Not Allowed - | "406" ; Section 10.4.7: Not Acceptable - - - -Fielding, et al. Standards Track [Page 40] - -RFC 2616 HTTP/1.1 June 1999 - - - | "407" ; Section 10.4.8: Proxy Authentication Required - | "408" ; Section 10.4.9: Request Time-out - | "409" ; Section 10.4.10: Conflict - | "410" ; Section 10.4.11: Gone - | "411" ; Section 10.4.12: Length Required - | "412" ; Section 10.4.13: Precondition Failed - | "413" ; Section 10.4.14: Request Entity Too Large - | "414" ; Section 10.4.15: Request-URI Too Large - | "415" ; Section 10.4.16: Unsupported Media Type - | "416" ; Section 10.4.17: Requested range not satisfiable - | "417" ; Section 10.4.18: Expectation Failed - | "500" ; Section 10.5.1: Internal Server Error - | "501" ; Section 10.5.2: Not Implemented - | "502" ; Section 10.5.3: Bad Gateway - | "503" ; Section 10.5.4: Service Unavailable - | "504" ; Section 10.5.5: Gateway Time-out - | "505" ; Section 10.5.6: HTTP Version not supported - | extension-code - - extension-code = 3DIGIT - Reason-Phrase = *<TEXT, excluding CR, LF> - - HTTP status codes are extensible. HTTP applications are not required - to understand the meaning of all registered status codes, though such - understanding is obviously desirable. However, applications MUST - understand the class of any status code, as indicated by the first - digit, and treat any unrecognized response as being equivalent to the - x00 status code of that class, with the exception that an - unrecognized response MUST NOT be cached. For example, if an - unrecognized status code of 431 is received by the client, it can - safely assume that there was something wrong with its request and - treat the response as if it had received a 400 status code. In such - cases, user agents SHOULD present to the user the entity returned - with the response, since that entity is likely to include human- - readable information which will explain the unusual status. - -6.2 Response Header Fields - - The response-header fields allow the server to pass additional - information about the response which cannot be placed in the Status- - Line. These header fields give information about the server and about - further access to the resource identified by the Request-URI. - - response-header = Accept-Ranges ; Section 14.5 - | Age ; Section 14.6 - | ETag ; Section 14.19 - | Location ; Section 14.30 - | Proxy-Authenticate ; Section 14.33 - - - -Fielding, et al. Standards Track [Page 41] - -RFC 2616 HTTP/1.1 June 1999 - - - | Retry-After ; Section 14.37 - | Server ; Section 14.38 - | Vary ; Section 14.44 - | WWW-Authenticate ; Section 14.47 - - Response-header field names can be extended reliably only in - combination with a change in the protocol version. However, new or - experimental header fields MAY be given the semantics of response- - header fields if all parties in the communication recognize them to - be response-header fields. Unrecognized header fields are treated as - entity-header fields. - -7 Entity - - Request and Response messages MAY transfer an entity if not otherwise - restricted by the request method or response status code. An entity - consists of entity-header fields and an entity-body, although some - responses will only include the entity-headers. - - In this section, both sender and recipient refer to either the client - or the server, depending on who sends and who receives the entity. - -7.1 Entity Header Fields - - Entity-header fields define metainformation about the entity-body or, - if no body is present, about the resource identified by the request. - Some of this metainformation is OPTIONAL; some might be REQUIRED by - portions of this specification. - - entity-header = Allow ; Section 14.7 - | Content-Encoding ; Section 14.11 - | Content-Language ; Section 14.12 - | Content-Length ; Section 14.13 - | Content-Location ; Section 14.14 - | Content-MD5 ; Section 14.15 - | Content-Range ; Section 14.16 - | Content-Type ; Section 14.17 - | Expires ; Section 14.21 - | Last-Modified ; Section 14.29 - | extension-header - - extension-header = message-header - - The extension-header mechanism allows additional entity-header fields - to be defined without changing the protocol, but these fields cannot - be assumed to be recognizable by the recipient. Unrecognized header - fields SHOULD be ignored by the recipient and MUST be forwarded by - transparent proxies. - - - -Fielding, et al. Standards Track [Page 42] - -RFC 2616 HTTP/1.1 June 1999 - - -7.2 Entity Body - - The entity-body (if any) sent with an HTTP request or response is in - a format and encoding defined by the entity-header fields. - - entity-body = *OCTET - - An entity-body is only present in a message when a message-body is - present, as described in section 4.3. The entity-body is obtained - from the message-body by decoding any Transfer-Encoding that might - have been applied to ensure safe and proper transfer of the message. - -7.2.1 Type - - When an entity-body is included with a message, the data type of that - body is determined via the header fields Content-Type and Content- - Encoding. These define a two-layer, ordered encoding model: - - entity-body := Content-Encoding( Content-Type( data ) ) - - Content-Type specifies the media type of the underlying data. - Content-Encoding may be used to indicate any additional content - codings applied to the data, usually for the purpose of data - compression, that are a property of the requested resource. There is - no default encoding. - - Any HTTP/1.1 message containing an entity-body SHOULD include a - Content-Type header field defining the media type of that body. If - and only if the media type is not given by a Content-Type field, the - recipient MAY attempt to guess the media type via inspection of its - content and/or the name extension(s) of the URI used to identify the - resource. If the media type remains unknown, the recipient SHOULD - treat it as type "application/octet-stream". - -7.2.2 Entity Length - - The entity-length of a message is the length of the message-body - before any transfer-codings have been applied. Section 4.4 defines - how the transfer-length of a message-body is determined. - - - - - - - - - - - - -Fielding, et al. Standards Track [Page 43] - -RFC 2616 HTTP/1.1 June 1999 - - -8 Connections - -8.1 Persistent Connections - -8.1.1 Purpose - - Prior to persistent connections, a separate TCP connection was - established to fetch each URL, increasing the load on HTTP servers - and causing congestion on the Internet. The use of inline images and - other associated data often require a client to make multiple - requests of the same server in a short amount of time. Analysis of - these performance problems and results from a prototype - implementation are available [26] [30]. Implementation experience and - measurements of actual HTTP/1.1 (RFC 2068) implementations show good - results [39]. Alternatives have also been explored, for example, - T/TCP [27]. - - Persistent HTTP connections have a number of advantages: - - - By opening and closing fewer TCP connections, CPU time is saved - in routers and hosts (clients, servers, proxies, gateways, - tunnels, or caches), and memory used for TCP protocol control - blocks can be saved in hosts. - - - HTTP requests and responses can be pipelined on a connection. - Pipelining allows a client to make multiple requests without - waiting for each response, allowing a single TCP connection to - be used much more efficiently, with much lower elapsed time. - - - Network congestion is reduced by reducing the number of packets - caused by TCP opens, and by allowing TCP sufficient time to - determine the congestion state of the network. - - - Latency on subsequent requests is reduced since there is no time - spent in TCP's connection opening handshake. - - - HTTP can evolve more gracefully, since errors can be reported - without the penalty of closing the TCP connection. Clients using - future versions of HTTP might optimistically try a new feature, - but if communicating with an older server, retry with old - semantics after an error is reported. - - HTTP implementations SHOULD implement persistent connections. - - - - - - - - -Fielding, et al. Standards Track [Page 44] - -RFC 2616 HTTP/1.1 June 1999 - - -8.1.2 Overall Operation - - A significant difference between HTTP/1.1 and earlier versions of - HTTP is that persistent connections are the default behavior of any - HTTP connection. That is, unless otherwise indicated, the client - SHOULD assume that the server will maintain a persistent connection, - even after error responses from the server. - - Persistent connections provide a mechanism by which a client and a - server can signal the close of a TCP connection. This signaling takes - place using the Connection header field (section 14.10). Once a close - has been signaled, the client MUST NOT send any more requests on that - connection. - -8.1.2.1 Negotiation - - An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to - maintain a persistent connection unless a Connection header including - the connection-token "close" was sent in the request. If the server - chooses to close the connection immediately after sending the - response, it SHOULD send a Connection header including the - connection-token close. - - An HTTP/1.1 client MAY expect a connection to remain open, but would - decide to keep it open based on whether the response from a server - contains a Connection header with the connection-token close. In case - the client does not want to maintain a connection for more than that - request, it SHOULD send a Connection header including the - connection-token close. - - If either the client or the server sends the close token in the - Connection header, that request becomes the last one for the - connection. - - Clients and servers SHOULD NOT assume that a persistent connection is - maintained for HTTP versions less than 1.1 unless it is explicitly - signaled. See section 19.6.2 for more information on backward - compatibility with HTTP/1.0 clients. - - In order to remain persistent, all messages on the connection MUST - have a self-defined message length (i.e., one not defined by closure - of the connection), as described in section 4.4. - - - - - - - - - -Fielding, et al. Standards Track [Page 45] - -RFC 2616 HTTP/1.1 June 1999 - - -8.1.2.2 Pipelining - - A client that supports persistent connections MAY "pipeline" its - requests (i.e., send multiple requests without waiting for each - response). A server MUST send its responses to those requests in the - same order that the requests were received. - - Clients which assume persistent connections and pipeline immediately - after connection establishment SHOULD be prepared to retry their - connection if the first pipelined attempt fails. If a client does - such a retry, it MUST NOT pipeline before it knows the connection is - persistent. Clients MUST also be prepared to resend their requests if - the server closes the connection before sending all of the - corresponding responses. - - Clients SHOULD NOT pipeline requests using non-idempotent methods or - non-idempotent sequences of methods (see section 9.1.2). Otherwise, a - premature termination of the transport connection could lead to - indeterminate results. A client wishing to send a non-idempotent - request SHOULD wait to send that request until it has received the - response status for the previous request. - -8.1.3 Proxy Servers - - It is especially important that proxies correctly implement the - properties of the Connection header field as specified in section - 14.10. - - The proxy server MUST signal persistent connections separately with - its clients and the origin servers (or other proxy servers) that it - connects to. Each persistent connection applies to only one transport - link. - - A proxy server MUST NOT establish a HTTP/1.1 persistent connection - with an HTTP/1.0 client (but see RFC 2068 [33] for information and - discussion of the problems with the Keep-Alive header implemented by - many HTTP/1.0 clients). - -8.1.4 Practical Considerations - - Servers will usually have some time-out value beyond which they will - no longer maintain an inactive connection. Proxy servers might make - this a higher value since it is likely that the client will be making - more connections through the same server. The use of persistent - connections places no requirements on the length (or existence) of - this time-out for either the client or the server. - - - - - -Fielding, et al. Standards Track [Page 46] - -RFC 2616 HTTP/1.1 June 1999 - - - When a client or server wishes to time-out it SHOULD issue a graceful - close on the transport connection. Clients and servers SHOULD both - constantly watch for the other side of the transport close, and - respond to it as appropriate. If a client or server does not detect - the other side's close promptly it could cause unnecessary resource - drain on the network. - - A client, server, or proxy MAY close the transport connection at any - time. For example, a client might have started to send a new request - at the same time that the server has decided to close the "idle" - connection. From the server's point of view, the connection is being - closed while it was idle, but from the client's point of view, a - request is in progress. - - This means that clients, servers, and proxies MUST be able to recover - from asynchronous close events. Client software SHOULD reopen the - transport connection and retransmit the aborted sequence of requests - without user interaction so long as the request sequence is - idempotent (see section 9.1.2). Non-idempotent methods or sequences - MUST NOT be automatically retried, although user agents MAY offer a - human operator the choice of retrying the request(s). Confirmation by - user-agent software with semantic understanding of the application - MAY substitute for user confirmation. The automatic retry SHOULD NOT - be repeated if the second sequence of requests fails. - - Servers SHOULD always respond to at least one request per connection, - if at all possible. Servers SHOULD NOT close a connection in the - middle of transmitting a response, unless a network or client failure - is suspected. - - Clients that use persistent connections SHOULD limit the number of - simultaneous connections that they maintain to a given server. A - single-user client SHOULD NOT maintain more than 2 connections with - any server or proxy. A proxy SHOULD use up to 2*N connections to - another server or proxy, where N is the number of simultaneously - active users. These guidelines are intended to improve HTTP response - times and avoid congestion. - -8.2 Message Transmission Requirements - -8.2.1 Persistent Connections and Flow Control - - HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's - flow control mechanisms to resolve temporary overloads, rather than - terminating connections with the expectation that clients will retry. - The latter technique can exacerbate network congestion. - - - - - -Fielding, et al. Standards Track [Page 47] - -RFC 2616 HTTP/1.1 June 1999 - - -8.2.2 Monitoring Connections for Error Status Messages - - An HTTP/1.1 (or later) client sending a message-body SHOULD monitor - the network connection for an error status while it is transmitting - the request. If the client sees an error status, it SHOULD - immediately cease transmitting the body. If the body is being sent - using a "chunked" encoding (section 3.6), a zero length chunk and - empty trailer MAY be used to prematurely mark the end of the message. - If the body was preceded by a Content-Length header, the client MUST - close the connection. - -8.2.3 Use of the 100 (Continue) Status - - The purpose of the 100 (Continue) status (see section 10.1.1) is to - allow a client that is sending a request message with a request body - to determine if the origin server is willing to accept the request - (based on the request headers) before the client sends the request - body. In some cases, it might either be inappropriate or highly - inefficient for the client to send the body if the server will reject - the message without looking at the body. - - Requirements for HTTP/1.1 clients: - - - If a client will wait for a 100 (Continue) response before - sending the request body, it MUST send an Expect request-header - field (section 14.20) with the "100-continue" expectation. - - - A client MUST NOT send an Expect request-header field (section - 14.20) with the "100-continue" expectation if it does not intend - to send a request body. - - Because of the presence of older implementations, the protocol allows - ambiguous situations in which a client may send "Expect: 100- - continue" without receiving either a 417 (Expectation Failed) status - or a 100 (Continue) status. Therefore, when a client sends this - header field to an origin server (possibly via a proxy) from which it - has never seen a 100 (Continue) status, the client SHOULD NOT wait - for an indefinite period before sending the request body. - - Requirements for HTTP/1.1 origin servers: - - - Upon receiving a request which includes an Expect request-header - field with the "100-continue" expectation, an origin server MUST - either respond with 100 (Continue) status and continue to read - from the input stream, or respond with a final status code. The - origin server MUST NOT wait for the request body before sending - the 100 (Continue) response. If it responds with a final status - code, it MAY close the transport connection or it MAY continue - - - -Fielding, et al. Standards Track [Page 48] - -RFC 2616 HTTP/1.1 June 1999 - - - to read and discard the rest of the request. It MUST NOT - perform the requested method if it returns a final status code. - - - An origin server SHOULD NOT send a 100 (Continue) response if - the request message does not include an Expect request-header - field with the "100-continue" expectation, and MUST NOT send a - 100 (Continue) response if such a request comes from an HTTP/1.0 - (or earlier) client. There is an exception to this rule: for - compatibility with RFC 2068, a server MAY send a 100 (Continue) - status in response to an HTTP/1.1 PUT or POST request that does - not include an Expect request-header field with the "100- - continue" expectation. This exception, the purpose of which is - to minimize any client processing delays associated with an - undeclared wait for 100 (Continue) status, applies only to - HTTP/1.1 requests, and not to requests with any other HTTP- - version value. - - - An origin server MAY omit a 100 (Continue) response if it has - already received some or all of the request body for the - corresponding request. - - - An origin server that sends a 100 (Continue) response MUST - ultimately send a final status code, once the request body is - received and processed, unless it terminates the transport - connection prematurely. - - - If an origin server receives a request that does not include an - Expect request-header field with the "100-continue" expectation, - the request includes a request body, and the server responds - with a final status code before reading the entire request body - from the transport connection, then the server SHOULD NOT close - the transport connection until it has read the entire request, - or until the client closes the connection. Otherwise, the client - might not reliably receive the response message. However, this - requirement is not be construed as preventing a server from - defending itself against denial-of-service attacks, or from - badly broken client implementations. - - Requirements for HTTP/1.1 proxies: - - - If a proxy receives a request that includes an Expect request- - header field with the "100-continue" expectation, and the proxy - either knows that the next-hop server complies with HTTP/1.1 or - higher, or does not know the HTTP version of the next-hop - server, it MUST forward the request, including the Expect header - field. - - - - - -Fielding, et al. Standards Track [Page 49] - -RFC 2616 HTTP/1.1 June 1999 - - - - If the proxy knows that the version of the next-hop server is - HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST - respond with a 417 (Expectation Failed) status. - - - Proxies SHOULD maintain a cache recording the HTTP version - numbers received from recently-referenced next-hop servers. - - - A proxy MUST NOT forward a 100 (Continue) response if the - request message was received from an HTTP/1.0 (or earlier) - client and did not include an Expect request-header field with - the "100-continue" expectation. This requirement overrides the - general rule for forwarding of 1xx responses (see section 10.1). - -8.2.4 Client Behavior if Server Prematurely Closes Connection - - If an HTTP/1.1 client sends a request which includes a request body, - but which does not include an Expect request-header field with the - "100-continue" expectation, and if the client is not directly - connected to an HTTP/1.1 origin server, and if the client sees the - connection close before receiving any status from the server, the - client SHOULD retry the request. If the client does retry this - request, it MAY use the following "binary exponential backoff" - algorithm to be assured of obtaining a reliable response: - - 1. Initiate a new connection to the server - - 2. Transmit the request-headers - - 3. Initialize a variable R to the estimated round-trip time to the - server (e.g., based on the time it took to establish the - connection), or to a constant value of 5 seconds if the round- - trip time is not available. - - 4. Compute T = R * (2**N), where N is the number of previous - retries of this request. - - 5. Wait either for an error response from the server, or for T - seconds (whichever comes first) - - 6. If no error response is received, after T seconds transmit the - body of the request. - - 7. If client sees that the connection is closed prematurely, - repeat from step 1 until the request is accepted, an error - response is received, or the user becomes impatient and - terminates the retry process. - - - - - -Fielding, et al. Standards Track [Page 50] - -RFC 2616 HTTP/1.1 June 1999 - - - If at any point an error status is received, the client - - - SHOULD NOT continue and - - - SHOULD close the connection if it has not completed sending the - request message. - -9 Method Definitions - - The set of common methods for HTTP/1.1 is defined below. Although - this set can be expanded, additional methods cannot be assumed to - share the same semantics for separately extended clients and servers. - - The Host request-header field (section 14.23) MUST accompany all - HTTP/1.1 requests. - -9.1 Safe and Idempotent Methods - -9.1.1 Safe Methods - - Implementors should be aware that the software represents the user in - their interactions over the Internet, and should be careful to allow - the user to be aware of any actions they might take which may have an - unexpected significance to themselves or others. - - In particular, the convention has been established that the GET and - HEAD methods SHOULD NOT have the significance of taking an action - other than retrieval. These methods ought to be considered "safe". - This allows user agents to represent other methods, such as POST, PUT - and DELETE, in a special way, so that the user is made aware of the - fact that a possibly unsafe action is being requested. - - Naturally, it is not possible to ensure that the server does not - generate side-effects as a result of performing a GET request; in - fact, some dynamic resources consider that a feature. The important - distinction here is that the user did not request the side-effects, - so therefore cannot be held accountable for them. - -9.1.2 Idempotent Methods - - Methods can also have the property of "idempotence" in that (aside - from error or expiration issues) the side-effects of N > 0 identical - requests is the same as for a single request. The methods GET, HEAD, - PUT and DELETE share this property. Also, the methods OPTIONS and - TRACE SHOULD NOT have side effects, and so are inherently idempotent. - - - - - - -Fielding, et al. Standards Track [Page 51] - -RFC 2616 HTTP/1.1 June 1999 - - - However, it is possible that a sequence of several requests is non- - idempotent, even if all of the methods executed in that sequence are - idempotent. (A sequence is idempotent if a single execution of the - entire sequence always yields a result that is not changed by a - reexecution of all, or part, of that sequence.) For example, a - sequence is non-idempotent if its result depends on a value that is - later modified in the same sequence. - - A sequence that never has side effects is idempotent, by definition - (provided that no concurrent operations are being executed on the - same set of resources). - -9.2 OPTIONS - - The OPTIONS method represents a request for information about the - communication options available on the request/response chain - identified by the Request-URI. This method allows the client to - determine the options and/or requirements associated with a resource, - or the capabilities of a server, without implying a resource action - or initiating a resource retrieval. - - Responses to this method are not cacheable. - - If the OPTIONS request includes an entity-body (as indicated by the - presence of Content-Length or Transfer-Encoding), then the media type - MUST be indicated by a Content-Type field. Although this - specification does not define any use for such a body, future - extensions to HTTP might use the OPTIONS body to make more detailed - queries on the server. A server that does not support such an - extension MAY discard the request body. - - If the Request-URI is an asterisk ("*"), the OPTIONS request is - intended to apply to the server in general rather than to a specific - resource. Since a server's communication options typically depend on - the resource, the "*" request is only useful as a "ping" or "no-op" - type of method; it does nothing beyond allowing the client to test - the capabilities of the server. For example, this can be used to test - a proxy for HTTP/1.1 compliance (or lack thereof). - - If the Request-URI is not an asterisk, the OPTIONS request applies - only to the options that are available when communicating with that - resource. - - A 200 response SHOULD include any header fields that indicate - optional features implemented by the server and applicable to that - resource (e.g., Allow), possibly including extensions not defined by - this specification. The response body, if any, SHOULD also include - information about the communication options. The format for such a - - - -Fielding, et al. Standards Track [Page 52] - -RFC 2616 HTTP/1.1 June 1999 - - - body is not defined by this specification, but might be defined by - future extensions to HTTP. Content negotiation MAY be used to select - the appropriate response format. If no response body is included, the - response MUST include a Content-Length field with a field-value of - "0". - - The Max-Forwards request-header field MAY be used to target a - specific proxy in the request chain. When a proxy receives an OPTIONS - request on an absoluteURI for which request forwarding is permitted, - the proxy MUST check for a Max-Forwards field. If the Max-Forwards - field-value is zero ("0"), the proxy MUST NOT forward the message; - instead, the proxy SHOULD respond with its own communication options. - If the Max-Forwards field-value is an integer greater than zero, the - proxy MUST decrement the field-value when it forwards the request. If - no Max-Forwards field is present in the request, then the forwarded - request MUST NOT include a Max-Forwards field. - -9.3 GET - - The GET method means retrieve whatever information (in the form of an - entity) is identified by the Request-URI. If the Request-URI refers - to a data-producing process, it is the produced data which shall be - returned as the entity in the response and not the source text of the - process, unless that text happens to be the output of the process. - - The semantics of the GET method change to a "conditional GET" if the - request message includes an If-Modified-Since, If-Unmodified-Since, - If-Match, If-None-Match, or If-Range header field. A conditional GET - method requests that the entity be transferred only under the - circumstances described by the conditional header field(s). The - conditional GET method is intended to reduce unnecessary network - usage by allowing cached entities to be refreshed without requiring - multiple requests or transferring data already held by the client. - - The semantics of the GET method change to a "partial GET" if the - request message includes a Range header field. A partial GET requests - that only part of the entity be transferred, as described in section - 14.35. The partial GET method is intended to reduce unnecessary - network usage by allowing partially-retrieved entities to be - completed without transferring data already held by the client. - - The response to a GET request is cacheable if and only if it meets - the requirements for HTTP caching described in section 13. - - See section 15.1.3 for security considerations when used for forms. - - - - - - -Fielding, et al. Standards Track [Page 53] - -RFC 2616 HTTP/1.1 June 1999 - - -9.4 HEAD - - The HEAD method is identical to GET except that the server MUST NOT - return a message-body in the response. The metainformation contained - in the HTTP headers in response to a HEAD request SHOULD be identical - to the information sent in response to a GET request. This method can - be used for obtaining metainformation about the entity implied by the - request without transferring the entity-body itself. This method is - often used for testing hypertext links for validity, accessibility, - and recent modification. - - The response to a HEAD request MAY be cacheable in the sense that the - information contained in the response MAY be used to update a - previously cached entity from that resource. If the new field values - indicate that the cached entity differs from the current entity (as - would be indicated by a change in Content-Length, Content-MD5, ETag - or Last-Modified), then the cache MUST treat the cache entry as - stale. - -9.5 POST - - The POST method is used to request that the origin server accept the - entity enclosed in the request as a new subordinate of the resource - identified by the Request-URI in the Request-Line. POST is designed - to allow a uniform method to cover the following functions: - - - Annotation of existing resources; - - - Posting a message to a bulletin board, newsgroup, mailing list, - or similar group of articles; - - - Providing a block of data, such as the result of submitting a - form, to a data-handling process; - - - Extending a database through an append operation. - - The actual function performed by the POST method is determined by the - server and is usually dependent on the Request-URI. The posted entity - is subordinate to that URI in the same way that a file is subordinate - to a directory containing it, a news article is subordinate to a - newsgroup to which it is posted, or a record is subordinate to a - database. - - The action performed by the POST method might not result in a - resource that can be identified by a URI. In this case, either 200 - (OK) or 204 (No Content) is the appropriate response status, - depending on whether or not the response includes an entity that - describes the result. - - - -Fielding, et al. Standards Track [Page 54] - -RFC 2616 HTTP/1.1 June 1999 - - - If a resource has been created on the origin server, the response - SHOULD be 201 (Created) and contain an entity which describes the - status of the request and refers to the new resource, and a Location - header (see section 14.30). - - Responses to this method are not cacheable, unless the response - includes appropriate Cache-Control or Expires header fields. However, - the 303 (See Other) response can be used to direct the user agent to - retrieve a cacheable resource. - - POST requests MUST obey the message transmission requirements set out - in section 8.2. - - See section 15.1.3 for security considerations. - -9.6 PUT - - The PUT method requests that the enclosed entity be stored under the - supplied Request-URI. If the Request-URI refers to an already - existing resource, the enclosed entity SHOULD be considered as a - modified version of the one residing on the origin server. If the - Request-URI does not point to an existing resource, and that URI is - capable of being defined as a new resource by the requesting user - agent, the origin server can create the resource with that URI. If a - new resource is created, the origin server MUST inform the user agent - via the 201 (Created) response. If an existing resource is modified, - either the 200 (OK) or 204 (No Content) response codes SHOULD be sent - to indicate successful completion of the request. If the resource - could not be created or modified with the Request-URI, an appropriate - error response SHOULD be given that reflects the nature of the - problem. The recipient of the entity MUST NOT ignore any Content-* - (e.g. Content-Range) headers that it does not understand or implement - and MUST return a 501 (Not Implemented) response in such cases. - - If the request passes through a cache and the Request-URI identifies - one or more currently cached entities, those entries SHOULD be - treated as stale. Responses to this method are not cacheable. - - The fundamental difference between the POST and PUT requests is - reflected in the different meaning of the Request-URI. The URI in a - POST request identifies the resource that will handle the enclosed - entity. That resource might be a data-accepting process, a gateway to - some other protocol, or a separate entity that accepts annotations. - In contrast, the URI in a PUT request identifies the entity enclosed - with the request -- the user agent knows what URI is intended and the - server MUST NOT attempt to apply the request to some other resource. - If the server desires that the request be applied to a different URI, - - - - -Fielding, et al. Standards Track [Page 55] - -RFC 2616 HTTP/1.1 June 1999 - - - it MUST send a 301 (Moved Permanently) response; the user agent MAY - then make its own decision regarding whether or not to redirect the - request. - - A single resource MAY be identified by many different URIs. For - example, an article might have a URI for identifying "the current - version" which is separate from the URI identifying each particular - version. In this case, a PUT request on a general URI might result in - several other URIs being defined by the origin server. - - HTTP/1.1 does not define how a PUT method affects the state of an - origin server. - - PUT requests MUST obey the message transmission requirements set out - in section 8.2. - - Unless otherwise specified for a particular entity-header, the - entity-headers in the PUT request SHOULD be applied to the resource - created or modified by the PUT. - -9.7 DELETE - - The DELETE method requests that the origin server delete the resource - identified by the Request-URI. This method MAY be overridden by human - intervention (or other means) on the origin server. The client cannot - be guaranteed that the operation has been carried out, even if the - status code returned from the origin server indicates that the action - has been completed successfully. However, the server SHOULD NOT - indicate success unless, at the time the response is given, it - intends to delete the resource or move it to an inaccessible - location. - - A successful response SHOULD be 200 (OK) if the response includes an - entity describing the status, 202 (Accepted) if the action has not - yet been enacted, or 204 (No Content) if the action has been enacted - but the response does not include an entity. - - If the request passes through a cache and the Request-URI identifies - one or more currently cached entities, those entries SHOULD be - treated as stale. Responses to this method are not cacheable. - -9.8 TRACE - - The TRACE method is used to invoke a remote, application-layer loop- - back of the request message. The final recipient of the request - SHOULD reflect the message received back to the client as the - entity-body of a 200 (OK) response. The final recipient is either the - - - - -Fielding, et al. Standards Track [Page 56] - -RFC 2616 HTTP/1.1 June 1999 - - - origin server or the first proxy or gateway to receive a Max-Forwards - value of zero (0) in the request (see section 14.31). A TRACE request - MUST NOT include an entity. - - TRACE allows the client to see what is being received at the other - end of the request chain and use that data for testing or diagnostic - information. The value of the Via header field (section 14.45) is of - particular interest, since it acts as a trace of the request chain. - Use of the Max-Forwards header field allows the client to limit the - length of the request chain, which is useful for testing a chain of - proxies forwarding messages in an infinite loop. - - If the request is valid, the response SHOULD contain the entire - request message in the entity-body, with a Content-Type of - "message/http". Responses to this method MUST NOT be cached. - -9.9 CONNECT - - This specification reserves the method name CONNECT for use with a - proxy that can dynamically switch to being a tunnel (e.g. SSL - tunneling [44]). - -10 Status Code Definitions - - Each Status-Code is described below, including a description of which - method(s) it can follow and any metainformation required in the - response. - -10.1 Informational 1xx - - This class of status code indicates a provisional response, - consisting only of the Status-Line and optional headers, and is - terminated by an empty line. There are no required headers for this - class of status code. Since HTTP/1.0 did not define any 1xx status - codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client - except under experimental conditions. - - A client MUST be prepared to accept one or more 1xx status responses - prior to a regular response, even if the client does not expect a 100 - (Continue) status message. Unexpected 1xx status responses MAY be - ignored by a user agent. - - Proxies MUST forward 1xx responses, unless the connection between the - proxy and its client has been closed, or unless the proxy itself - requested the generation of the 1xx response. (For example, if a - - - - - - -Fielding, et al. Standards Track [Page 57] - -RFC 2616 HTTP/1.1 June 1999 - - - proxy adds a "Expect: 100-continue" field when it forwards a request, - then it need not forward the corresponding 100 (Continue) - response(s).) - -10.1.1 100 Continue - - The client SHOULD continue with its request. This interim response is - used to inform the client that the initial part of the request has - been received and has not yet been rejected by the server. The client - SHOULD continue by sending the remainder of the request or, if the - request has already been completed, ignore this response. The server - MUST send a final response after the request has been completed. See - section 8.2.3 for detailed discussion of the use and handling of this - status code. - -10.1.2 101 Switching Protocols - - The server understands and is willing to comply with the client's - request, via the Upgrade message header field (section 14.42), for a - change in the application protocol being used on this connection. The - server will switch protocols to those defined by the response's - Upgrade header field immediately after the empty line which - terminates the 101 response. - - The protocol SHOULD be switched only when it is advantageous to do - so. For example, switching to a newer version of HTTP is advantageous - over older versions, and switching to a real-time, synchronous - protocol might be advantageous when delivering resources that use - such features. - -10.2 Successful 2xx - - This class of status code indicates that the client's request was - successfully received, understood, and accepted. - -10.2.1 200 OK - - The request has succeeded. The information returned with the response - is dependent on the method used in the request, for example: - - GET an entity corresponding to the requested resource is sent in - the response; - - HEAD the entity-header fields corresponding to the requested - resource are sent in the response without any message-body; - - POST an entity describing or containing the result of the action; - - - - -Fielding, et al. Standards Track [Page 58] - -RFC 2616 HTTP/1.1 June 1999 - - - TRACE an entity containing the request message as received by the - end server. - -10.2.2 201 Created - - The request has been fulfilled and resulted in a new resource being - created. The newly created resource can be referenced by the URI(s) - returned in the entity of the response, with the most specific URI - for the resource given by a Location header field. The response - SHOULD include an entity containing a list of resource - characteristics and location(s) from which the user or user agent can - choose the one most appropriate. The entity format is specified by - the media type given in the Content-Type header field. The origin - server MUST create the resource before returning the 201 status code. - If the action cannot be carried out immediately, the server SHOULD - respond with 202 (Accepted) response instead. - - A 201 response MAY contain an ETag response header field indicating - the current value of the entity tag for the requested variant just - created, see section 14.19. - -10.2.3 202 Accepted - - The request has been accepted for processing, but the processing has - not been completed. The request might or might not eventually be - acted upon, as it might be disallowed when processing actually takes - place. There is no facility for re-sending a status code from an - asynchronous operation such as this. - - The 202 response is intentionally non-committal. Its purpose is to - allow a server to accept a request for some other process (perhaps a - batch-oriented process that is only run once per day) without - requiring that the user agent's connection to the server persist - until the process is completed. The entity returned with this - response SHOULD include an indication of the request's current status - and either a pointer to a status monitor or some estimate of when the - user can expect the request to be fulfilled. - -10.2.4 203 Non-Authoritative Information - - The returned metainformation in the entity-header is not the - definitive set as available from the origin server, but is gathered - from a local or a third-party copy. The set presented MAY be a subset - or superset of the original version. For example, including local - annotation information about the resource might result in a superset - of the metainformation known by the origin server. Use of this - response code is not required and is only appropriate when the - response would otherwise be 200 (OK). - - - -Fielding, et al. Standards Track [Page 59] - -RFC 2616 HTTP/1.1 June 1999 - - -10.2.5 204 No Content - - The server has fulfilled the request but does not need to return an - entity-body, and might want to return updated metainformation. The - response MAY include new or updated metainformation in the form of - entity-headers, which if present SHOULD be associated with the - requested variant. - - If the client is a user agent, it SHOULD NOT change its document view - from that which caused the request to be sent. This response is - primarily intended to allow input for actions to take place without - causing a change to the user agent's active document view, although - any new or updated metainformation SHOULD be applied to the document - currently in the user agent's active view. - - The 204 response MUST NOT include a message-body, and thus is always - terminated by the first empty line after the header fields. - -10.2.6 205 Reset Content - - The server has fulfilled the request and the user agent SHOULD reset - the document view which caused the request to be sent. This response - is primarily intended to allow input for actions to take place via - user input, followed by a clearing of the form in which the input is - given so that the user can easily initiate another input action. The - response MUST NOT include an entity. - -10.2.7 206 Partial Content - - The server has fulfilled the partial GET request for the resource. - The request MUST have included a Range header field (section 14.35) - indicating the desired range, and MAY have included an If-Range - header field (section 14.27) to make the request conditional. - - The response MUST include the following header fields: - - - Either a Content-Range header field (section 14.16) indicating - the range included with this response, or a multipart/byteranges - Content-Type including Content-Range fields for each part. If a - Content-Length header field is present in the response, its - value MUST match the actual number of OCTETs transmitted in the - message-body. - - - Date - - - ETag and/or Content-Location, if the header would have been sent - in a 200 response to the same request - - - - -Fielding, et al. Standards Track [Page 60] - -RFC 2616 HTTP/1.1 June 1999 - - - - Expires, Cache-Control, and/or Vary, if the field-value might - differ from that sent in any previous response for the same - variant - - If the 206 response is the result of an If-Range request that used a - strong cache validator (see section 13.3.3), the response SHOULD NOT - include other entity-headers. If the response is the result of an - If-Range request that used a weak validator, the response MUST NOT - include other entity-headers; this prevents inconsistencies between - cached entity-bodies and updated headers. Otherwise, the response - MUST include all of the entity-headers that would have been returned - with a 200 (OK) response to the same request. - - A cache MUST NOT combine a 206 response with other previously cached - content if the ETag or Last-Modified headers do not match exactly, - see 13.5.4. - - A cache that does not support the Range and Content-Range headers - MUST NOT cache 206 (Partial) responses. - -10.3 Redirection 3xx - - This class of status code indicates that further action needs to be - taken by the user agent in order to fulfill the request. The action - required MAY be carried out by the user agent without interaction - with the user if and only if the method used in the second request is - GET or HEAD. A client SHOULD detect infinite redirection loops, since - such loops generate network traffic for each redirection. - - Note: previous versions of this specification recommended a - maximum of five redirections. Content developers should be aware - that there might be clients that implement such a fixed - limitation. - -10.3.1 300 Multiple Choices - - The requested resource corresponds to any one of a set of - representations, each with its own specific location, and agent- - driven negotiation information (section 12) is being provided so that - the user (or user agent) can select a preferred representation and - redirect its request to that location. - - Unless it was a HEAD request, the response SHOULD include an entity - containing a list of resource characteristics and location(s) from - which the user or user agent can choose the one most appropriate. The - entity format is specified by the media type given in the Content- - Type header field. Depending upon the format and the capabilities of - - - - -Fielding, et al. Standards Track [Page 61] - -RFC 2616 HTTP/1.1 June 1999 - - - the user agent, selection of the most appropriate choice MAY be - performed automatically. However, this specification does not define - any standard for such automatic selection. - - If the server has a preferred choice of representation, it SHOULD - include the specific URI for that representation in the Location - field; user agents MAY use the Location field value for automatic - redirection. This response is cacheable unless indicated otherwise. - -10.3.2 301 Moved Permanently - - The requested resource has been assigned a new permanent URI and any - future references to this resource SHOULD use one of the returned - URIs. Clients with link editing capabilities ought to automatically - re-link references to the Request-URI to one or more of the new - references returned by the server, where possible. This response is - cacheable unless indicated otherwise. - - The new permanent URI SHOULD be given by the Location field in the - response. Unless the request method was HEAD, the entity of the - response SHOULD contain a short hypertext note with a hyperlink to - the new URI(s). - - If the 301 status code is received in response to a request other - than GET or HEAD, the user agent MUST NOT automatically redirect the - request unless it can be confirmed by the user, since this might - change the conditions under which the request was issued. - - Note: When automatically redirecting a POST request after - receiving a 301 status code, some existing HTTP/1.0 user agents - will erroneously change it into a GET request. - -10.3.3 302 Found - - The requested resource resides temporarily under a different URI. - Since the redirection might be altered on occasion, the client SHOULD - continue to use the Request-URI for future requests. This response - is only cacheable if indicated by a Cache-Control or Expires header - field. - - The temporary URI SHOULD be given by the Location field in the - response. Unless the request method was HEAD, the entity of the - response SHOULD contain a short hypertext note with a hyperlink to - the new URI(s). - - - - - - - -Fielding, et al. Standards Track [Page 62] - -RFC 2616 HTTP/1.1 June 1999 - - - If the 302 status code is received in response to a request other - than GET or HEAD, the user agent MUST NOT automatically redirect the - request unless it can be confirmed by the user, since this might - change the conditions under which the request was issued. - - Note: RFC 1945 and RFC 2068 specify that the client is not allowed - to change the method on the redirected request. However, most - existing user agent implementations treat 302 as if it were a 303 - response, performing a GET on the Location field-value regardless - of the original request method. The status codes 303 and 307 have - been added for servers that wish to make unambiguously clear which - kind of reaction is expected of the client. - -10.3.4 303 See Other - - The response to the request can be found under a different URI and - SHOULD be retrieved using a GET method on that resource. This method - exists primarily to allow the output of a POST-activated script to - redirect the user agent to a selected resource. The new URI is not a - substitute reference for the originally requested resource. The 303 - response MUST NOT be cached, but the response to the second - (redirected) request might be cacheable. - - The different URI SHOULD be given by the Location field in the - response. Unless the request method was HEAD, the entity of the - response SHOULD contain a short hypertext note with a hyperlink to - the new URI(s). - - Note: Many pre-HTTP/1.1 user agents do not understand the 303 - status. When interoperability with such clients is a concern, the - 302 status code may be used instead, since most user agents react - to a 302 response as described here for 303. - -10.3.5 304 Not Modified - - If the client has performed a conditional GET request and access is - allowed, but the document has not been modified, the server SHOULD - respond with this status code. The 304 response MUST NOT contain a - message-body, and thus is always terminated by the first empty line - after the header fields. - - The response MUST include the following header fields: - - - Date, unless its omission is required by section 14.18.1 - - - - - - - -Fielding, et al. Standards Track [Page 63] - -RFC 2616 HTTP/1.1 June 1999 - - - If a clockless origin server obeys these rules, and proxies and - clients add their own Date to any response received without one (as - already specified by [RFC 2068], section 14.19), caches will operate - correctly. - - - ETag and/or Content-Location, if the header would have been sent - in a 200 response to the same request - - - Expires, Cache-Control, and/or Vary, if the field-value might - differ from that sent in any previous response for the same - variant - - If the conditional GET used a strong cache validator (see section - 13.3.3), the response SHOULD NOT include other entity-headers. - Otherwise (i.e., the conditional GET used a weak validator), the - response MUST NOT include other entity-headers; this prevents - inconsistencies between cached entity-bodies and updated headers. - - If a 304 response indicates an entity not currently cached, then the - cache MUST disregard the response and repeat the request without the - conditional. - - If a cache uses a received 304 response to update a cache entry, the - cache MUST update the entry to reflect any new field values given in - the response. - -10.3.6 305 Use Proxy - - The requested resource MUST be accessed through the proxy given by - the Location field. The Location field gives the URI of the proxy. - The recipient is expected to repeat this single request via the - proxy. 305 responses MUST only be generated by origin servers. - - Note: RFC 2068 was not clear that 305 was intended to redirect a - single request, and to be generated by origin servers only. Not - observing these limitations has significant security consequences. - -10.3.7 306 (Unused) - - The 306 status code was used in a previous version of the - specification, is no longer used, and the code is reserved. - - - - - - - - - - -Fielding, et al. Standards Track [Page 64] - -RFC 2616 HTTP/1.1 June 1999 - - -10.3.8 307 Temporary Redirect - - The requested resource resides temporarily under a different URI. - Since the redirection MAY be altered on occasion, the client SHOULD - continue to use the Request-URI for future requests. This response - is only cacheable if indicated by a Cache-Control or Expires header - field. - - The temporary URI SHOULD be given by the Location field in the - response. Unless the request method was HEAD, the entity of the - response SHOULD contain a short hypertext note with a hyperlink to - the new URI(s) , since many pre-HTTP/1.1 user agents do not - understand the 307 status. Therefore, the note SHOULD contain the - information necessary for a user to repeat the original request on - the new URI. - - If the 307 status code is received in response to a request other - than GET or HEAD, the user agent MUST NOT automatically redirect the - request unless it can be confirmed by the user, since this might - change the conditions under which the request was issued. - -10.4 Client Error 4xx - - The 4xx class of status code is intended for cases in which the - client seems to have erred. Except when responding to a HEAD request, - the server SHOULD include an entity containing an explanation of the - error situation, and whether it is a temporary or permanent - condition. These status codes are applicable to any request method. - User agents SHOULD display any included entity to the user. - - If the client is sending data, a server implementation using TCP - SHOULD be careful to ensure that the client acknowledges receipt of - the packet(s) containing the response, before the server closes the - input connection. If the client continues sending data to the server - after the close, the server's TCP stack will send a reset packet to - the client, which may erase the client's unacknowledged input buffers - before they can be read and interpreted by the HTTP application. - -10.4.1 400 Bad Request - - The request could not be understood by the server due to malformed - syntax. The client SHOULD NOT repeat the request without - modifications. - - - - - - - - -Fielding, et al. Standards Track [Page 65] - -RFC 2616 HTTP/1.1 June 1999 - - -10.4.2 401 Unauthorized - - The request requires user authentication. The response MUST include a - WWW-Authenticate header field (section 14.47) containing a challenge - applicable to the requested resource. The client MAY repeat the - request with a suitable Authorization header field (section 14.8). If - the request already included Authorization credentials, then the 401 - response indicates that authorization has been refused for those - credentials. If the 401 response contains the same challenge as the - prior response, and the user agent has already attempted - authentication at least once, then the user SHOULD be presented the - entity that was given in the response, since that entity might - include relevant diagnostic information. HTTP access authentication - is explained in "HTTP Authentication: Basic and Digest Access - Authentication" [43]. - -10.4.3 402 Payment Required - - This code is reserved for future use. - -10.4.4 403 Forbidden - - The server understood the request, but is refusing to fulfill it. - Authorization will not help and the request SHOULD NOT be repeated. - If the request method was not HEAD and the server wishes to make - public why the request has not been fulfilled, it SHOULD describe the - reason for the refusal in the entity. If the server does not wish to - make this information available to the client, the status code 404 - (Not Found) can be used instead. - -10.4.5 404 Not Found - - The server has not found anything matching the Request-URI. No - indication is given of whether the condition is temporary or - permanent. The 410 (Gone) status code SHOULD be used if the server - knows, through some internally configurable mechanism, that an old - resource is permanently unavailable and has no forwarding address. - This status code is commonly used when the server does not wish to - reveal exactly why the request has been refused, or when no other - response is applicable. - -10.4.6 405 Method Not Allowed - - The method specified in the Request-Line is not allowed for the - resource identified by the Request-URI. The response MUST include an - Allow header containing a list of valid methods for the requested - resource. - - - - -Fielding, et al. Standards Track [Page 66] - -RFC 2616 HTTP/1.1 June 1999 - - -10.4.7 406 Not Acceptable - - The resource identified by the request is only capable of generating - response entities which have content characteristics not acceptable - according to the accept headers sent in the request. - - Unless it was a HEAD request, the response SHOULD include an entity - containing a list of available entity characteristics and location(s) - from which the user or user agent can choose the one most - appropriate. The entity format is specified by the media type given - in the Content-Type header field. Depending upon the format and the - capabilities of the user agent, selection of the most appropriate - choice MAY be performed automatically. However, this specification - does not define any standard for such automatic selection. - - Note: HTTP/1.1 servers are allowed to return responses which are - not acceptable according to the accept headers sent in the - request. In some cases, this may even be preferable to sending a - 406 response. User agents are encouraged to inspect the headers of - an incoming response to determine if it is acceptable. - - If the response could be unacceptable, a user agent SHOULD - temporarily stop receipt of more data and query the user for a - decision on further actions. - -10.4.8 407 Proxy Authentication Required - - This code is similar to 401 (Unauthorized), but indicates that the - client must first authenticate itself with the proxy. The proxy MUST - return a Proxy-Authenticate header field (section 14.33) containing a - challenge applicable to the proxy for the requested resource. The - client MAY repeat the request with a suitable Proxy-Authorization - header field (section 14.34). HTTP access authentication is explained - in "HTTP Authentication: Basic and Digest Access Authentication" - [43]. - -10.4.9 408 Request Timeout - - The client did not produce a request within the time that the server - was prepared to wait. The client MAY repeat the request without - modifications at any later time. - -10.4.10 409 Conflict - - The request could not be completed due to a conflict with the current - state of the resource. This code is only allowed in situations where - it is expected that the user might be able to resolve the conflict - and resubmit the request. The response body SHOULD include enough - - - -Fielding, et al. Standards Track [Page 67] - -RFC 2616 HTTP/1.1 June 1999 - - - information for the user to recognize the source of the conflict. - Ideally, the response entity would include enough information for the - user or user agent to fix the problem; however, that might not be - possible and is not required. - - Conflicts are most likely to occur in response to a PUT request. For - example, if versioning were being used and the entity being PUT - included changes to a resource which conflict with those made by an - earlier (third-party) request, the server might use the 409 response - to indicate that it can't complete the request. In this case, the - response entity would likely contain a list of the differences - between the two versions in a format defined by the response - Content-Type. - -10.4.11 410 Gone - - The requested resource is no longer available at the server and no - forwarding address is known. This condition is expected to be - considered permanent. Clients with link editing capabilities SHOULD - delete references to the Request-URI after user approval. If the - server does not know, or has no facility to determine, whether or not - the condition is permanent, the status code 404 (Not Found) SHOULD be - used instead. This response is cacheable unless indicated otherwise. - - The 410 response is primarily intended to assist the task of web - maintenance by notifying the recipient that the resource is - intentionally unavailable and that the server owners desire that - remote links to that resource be removed. Such an event is common for - limited-time, promotional services and for resources belonging to - individuals no longer working at the server's site. It is not - necessary to mark all permanently unavailable resources as "gone" or - to keep the mark for any length of time -- that is left to the - discretion of the server owner. - -10.4.12 411 Length Required - - The server refuses to accept the request without a defined Content- - Length. The client MAY repeat the request if it adds a valid - Content-Length header field containing the length of the message-body - in the request message. - -10.4.13 412 Precondition Failed - - The precondition given in one or more of the request-header fields - evaluated to false when it was tested on the server. This response - code allows the client to place preconditions on the current resource - metainformation (header field data) and thus prevent the requested - method from being applied to a resource other than the one intended. - - - -Fielding, et al. Standards Track [Page 68] - -RFC 2616 HTTP/1.1 June 1999 - - -10.4.14 413 Request Entity Too Large - - The server is refusing to process a request because the request - entity is larger than the server is willing or able to process. The - server MAY close the connection to prevent the client from continuing - the request. - - If the condition is temporary, the server SHOULD include a Retry- - After header field to indicate that it is temporary and after what - time the client MAY try again. - -10.4.15 414 Request-URI Too Long - - The server is refusing to service the request because the Request-URI - is longer than the server is willing to interpret. This rare - condition is only likely to occur when a client has improperly - converted a POST request to a GET request with long query - information, when the client has descended into a URI "black hole" of - redirection (e.g., a redirected URI prefix that points to a suffix of - itself), or when the server is under attack by a client attempting to - exploit security holes present in some servers using fixed-length - buffers for reading or manipulating the Request-URI. - -10.4.16 415 Unsupported Media Type - - The server is refusing to service the request because the entity of - the request is in a format not supported by the requested resource - for the requested method. - -10.4.17 416 Requested Range Not Satisfiable - - A server SHOULD return a response with this status code if a request - included a Range request-header field (section 14.35), and none of - the range-specifier values in this field overlap the current extent - of the selected resource, and the request did not include an If-Range - request-header field. (For byte-ranges, this means that the first- - byte-pos of all of the byte-range-spec values were greater than the - current length of the selected resource.) - - When this status code is returned for a byte-range request, the - response SHOULD include a Content-Range entity-header field - specifying the current length of the selected resource (see section - 14.16). This response MUST NOT use the multipart/byteranges content- - type. - - - - - - - -Fielding, et al. Standards Track [Page 69] - -RFC 2616 HTTP/1.1 June 1999 - - -10.4.18 417 Expectation Failed - - The expectation given in an Expect request-header field (see section - 14.20) could not be met by this server, or, if the server is a proxy, - the server has unambiguous evidence that the request could not be met - by the next-hop server. - -10.5 Server Error 5xx - - Response status codes beginning with the digit "5" indicate cases in - which the server is aware that it has erred or is incapable of - performing the request. Except when responding to a HEAD request, the - server SHOULD include an entity containing an explanation of the - error situation, and whether it is a temporary or permanent - condition. User agents SHOULD display any included entity to the - user. These response codes are applicable to any request method. - -10.5.1 500 Internal Server Error - - The server encountered an unexpected condition which prevented it - from fulfilling the request. - -10.5.2 501 Not Implemented - - The server does not support the functionality required to fulfill the - request. This is the appropriate response when the server does not - recognize the request method and is not capable of supporting it for - any resource. - -10.5.3 502 Bad Gateway - - The server, while acting as a gateway or proxy, received an invalid - response from the upstream server it accessed in attempting to - fulfill the request. - -10.5.4 503 Service Unavailable - - The server is currently unable to handle the request due to a - temporary overloading or maintenance of the server. The implication - is that this is a temporary condition which will be alleviated after - some delay. If known, the length of the delay MAY be indicated in a - Retry-After header. If no Retry-After is given, the client SHOULD - handle the response as it would for a 500 response. - - Note: The existence of the 503 status code does not imply that a - server must use it when becoming overloaded. Some servers may wish - to simply refuse the connection. - - - - -Fielding, et al. Standards Track [Page 70] - -RFC 2616 HTTP/1.1 June 1999 - - -10.5.5 504 Gateway Timeout - - The server, while acting as a gateway or proxy, did not receive a - timely response from the upstream server specified by the URI (e.g. - HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed - to access in attempting to complete the request. - - Note: Note to implementors: some deployed proxies are known to - return 400 or 500 when DNS lookups time out. - -10.5.6 505 HTTP Version Not Supported - - The server does not support, or refuses to support, the HTTP protocol - version that was used in the request message. The server is - indicating that it is unable or unwilling to complete the request - using the same major version as the client, as described in section - 3.1, other than with this error message. The response SHOULD contain - an entity describing why that version is not supported and what other - protocols are supported by that server. - -11 Access Authentication - - HTTP provides several OPTIONAL challenge-response authentication - mechanisms which can be used by a server to challenge a client - request and by a client to provide authentication information. The - general framework for access authentication, and the specification of - "basic" and "digest" authentication, are specified in "HTTP - Authentication: Basic and Digest Access Authentication" [43]. This - specification adopts the definitions of "challenge" and "credentials" - from that specification. - -12 Content Negotiation - - Most HTTP responses include an entity which contains information for - interpretation by a human user. Naturally, it is desirable to supply - the user with the "best available" entity corresponding to the - request. Unfortunately for servers and caches, not all users have the - same preferences for what is "best," and not all user agents are - equally capable of rendering all entity types. For that reason, HTTP - has provisions for several mechanisms for "content negotiation" -- - the process of selecting the best representation for a given response - when there are multiple representations available. - - Note: This is not called "format negotiation" because the - alternate representations may be of the same media type, but use - different capabilities of that type, be in different languages, - etc. - - - - -Fielding, et al. Standards Track [Page 71] - -RFC 2616 HTTP/1.1 June 1999 - - - Any response containing an entity-body MAY be subject to negotiation, - including error responses. - - There are two kinds of content negotiation which are possible in - HTTP: server-driven and agent-driven negotiation. These two kinds of - negotiation are orthogonal and thus may be used separately or in - combination. One method of combination, referred to as transparent - negotiation, occurs when a cache uses the agent-driven negotiation - information provided by the origin server in order to provide - server-driven negotiation for subsequent requests. - -12.1 Server-driven Negotiation - - If the selection of the best representation for a response is made by - an algorithm located at the server, it is called server-driven - negotiation. Selection is based on the available representations of - the response (the dimensions over which it can vary; e.g. language, - content-coding, etc.) and the contents of particular header fields in - the request message or on other information pertaining to the request - (such as the network address of the client). - - Server-driven negotiation is advantageous when the algorithm for - selecting from among the available representations is difficult to - describe to the user agent, or when the server desires to send its - "best guess" to the client along with the first response (hoping to - avoid the round-trip delay of a subsequent request if the "best - guess" is good enough for the user). In order to improve the server's - guess, the user agent MAY include request header fields (Accept, - Accept-Language, Accept-Encoding, etc.) which describe its - preferences for such a response. - - Server-driven negotiation has disadvantages: - - 1. It is impossible for the server to accurately determine what - might be "best" for any given user, since that would require - complete knowledge of both the capabilities of the user agent - and the intended use for the response (e.g., does the user want - to view it on screen or print it on paper?). - - 2. Having the user agent describe its capabilities in every - request can be both very inefficient (given that only a small - percentage of responses have multiple representations) and a - potential violation of the user's privacy. - - 3. It complicates the implementation of an origin server and the - algorithms for generating responses to a request. - - - - - -Fielding, et al. Standards Track [Page 72] - -RFC 2616 HTTP/1.1 June 1999 - - - 4. It may limit a public cache's ability to use the same response - for multiple user's requests. - - HTTP/1.1 includes the following request-header fields for enabling - server-driven negotiation through description of user agent - capabilities and user preferences: Accept (section 14.1), Accept- - Charset (section 14.2), Accept-Encoding (section 14.3), Accept- - Language (section 14.4), and User-Agent (section 14.43). However, an - origin server is not limited to these dimensions and MAY vary the - response based on any aspect of the request, including information - outside the request-header fields or within extension header fields - not defined by this specification. - - The Vary header field can be used to express the parameters the - server uses to select a representation that is subject to server- - driven negotiation. See section 13.6 for use of the Vary header field - by caches and section 14.44 for use of the Vary header field by - servers. - -12.2 Agent-driven Negotiation - - With agent-driven negotiation, selection of the best representation - for a response is performed by the user agent after receiving an - initial response from the origin server. Selection is based on a list - of the available representations of the response included within the - header fields or entity-body of the initial response, with each - representation identified by its own URI. Selection from among the - representations may be performed automatically (if the user agent is - capable of doing so) or manually by the user selecting from a - generated (possibly hypertext) menu. - - Agent-driven negotiation is advantageous when the response would vary - over commonly-used dimensions (such as type, language, or encoding), - when the origin server is unable to determine a user agent's - capabilities from examining the request, and generally when public - caches are used to distribute server load and reduce network usage. - - Agent-driven negotiation suffers from the disadvantage of needing a - second request to obtain the best alternate representation. This - second request is only efficient when caching is used. In addition, - this specification does not define any mechanism for supporting - automatic selection, though it also does not prevent any such - mechanism from being developed as an extension and used within - HTTP/1.1. - - - - - - - -Fielding, et al. Standards Track [Page 73] - -RFC 2616 HTTP/1.1 June 1999 - - - HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) - status codes for enabling agent-driven negotiation when the server is - unwilling or unable to provide a varying response using server-driven - negotiation. - -12.3 Transparent Negotiation - - Transparent negotiation is a combination of both server-driven and - agent-driven negotiation. When a cache is supplied with a form of the - list of available representations of the response (as in agent-driven - negotiation) and the dimensions of variance are completely understood - by the cache, then the cache becomes capable of performing server- - driven negotiation on behalf of the origin server for subsequent - requests on that resource. - - Transparent negotiation has the advantage of distributing the - negotiation work that would otherwise be required of the origin - server and also removing the second request delay of agent-driven - negotiation when the cache is able to correctly guess the right - response. - - This specification does not define any mechanism for transparent - negotiation, though it also does not prevent any such mechanism from - being developed as an extension that could be used within HTTP/1.1. - -13 Caching in HTTP - - HTTP is typically used for distributed information systems, where - performance can be improved by the use of response caches. The - HTTP/1.1 protocol includes a number of elements intended to make - caching work as well as possible. Because these elements are - inextricable from other aspects of the protocol, and because they - interact with each other, it is useful to describe the basic caching - design of HTTP separately from the detailed descriptions of methods, - headers, response codes, etc. - - Caching would be useless if it did not significantly improve - performance. The goal of caching in HTTP/1.1 is to eliminate the need - to send requests in many cases, and to eliminate the need to send - full responses in many other cases. The former reduces the number of - network round-trips required for many operations; we use an - "expiration" mechanism for this purpose (see section 13.2). The - latter reduces network bandwidth requirements; we use a "validation" - mechanism for this purpose (see section 13.3). - - Requirements for performance, availability, and disconnected - operation require us to be able to relax the goal of semantic - transparency. The HTTP/1.1 protocol allows origin servers, caches, - - - -Fielding, et al. Standards Track [Page 74] - -RFC 2616 HTTP/1.1 June 1999 - - - and clients to explicitly reduce transparency when necessary. - However, because non-transparent operation may confuse non-expert - users, and might be incompatible with certain server applications - (such as those for ordering merchandise), the protocol requires that - transparency be relaxed - - - only by an explicit protocol-level request when relaxed by - client or origin server - - - only with an explicit warning to the end user when relaxed by - cache or client - - Therefore, the HTTP/1.1 protocol provides these important elements: - - 1. Protocol features that provide full semantic transparency when - this is required by all parties. - - 2. Protocol features that allow an origin server or user agent to - explicitly request and control non-transparent operation. - - 3. Protocol features that allow a cache to attach warnings to - responses that do not preserve the requested approximation of - semantic transparency. - - A basic principle is that it must be possible for the clients to - detect any potential relaxation of semantic transparency. - - Note: The server, cache, or client implementor might be faced with - design decisions not explicitly discussed in this specification. - If a decision might affect semantic transparency, the implementor - ought to err on the side of maintaining transparency unless a - careful and complete analysis shows significant benefits in - breaking transparency. - -13.1.1 Cache Correctness - - A correct cache MUST respond to a request with the most up-to-date - response held by the cache that is appropriate to the request (see - sections 13.2.5, 13.2.6, and 13.12) which meets one of the following - conditions: - - 1. It has been checked for equivalence with what the origin server - would have returned by revalidating the response with the - origin server (section 13.3); - - - - - - - -Fielding, et al. Standards Track [Page 75] - -RFC 2616 HTTP/1.1 June 1999 - - - 2. It is "fresh enough" (see section 13.2). In the default case, - this means it meets the least restrictive freshness requirement - of the client, origin server, and cache (see section 14.9); if - the origin server so specifies, it is the freshness requirement - of the origin server alone. - - If a stored response is not "fresh enough" by the most - restrictive freshness requirement of both the client and the - origin server, in carefully considered circumstances the cache - MAY still return the response with the appropriate Warning - header (see section 13.1.5 and 14.46), unless such a response - is prohibited (e.g., by a "no-store" cache-directive, or by a - "no-cache" cache-request-directive; see section 14.9). - - 3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), - or error (4xx or 5xx) response message. - - If the cache can not communicate with the origin server, then a - correct cache SHOULD respond as above if the response can be - correctly served from the cache; if not it MUST return an error or - warning indicating that there was a communication failure. - - If a cache receives a response (either an entire response, or a 304 - (Not Modified) response) that it would normally forward to the - requesting client, and the received response is no longer fresh, the - cache SHOULD forward it to the requesting client without adding a new - Warning (but without removing any existing Warning headers). A cache - SHOULD NOT attempt to revalidate a response simply because that - response became stale in transit; this might lead to an infinite - loop. A user agent that receives a stale response without a Warning - MAY display a warning indication to the user. - -13.1.2 Warnings - - Whenever a cache returns a response that is neither first-hand nor - "fresh enough" (in the sense of condition 2 in section 13.1.1), it - MUST attach a warning to that effect, using a Warning general-header. - The Warning header and the currently defined warnings are described - in section 14.46. The warning allows clients to take appropriate - action. - - Warnings MAY be used for other purposes, both cache-related and - otherwise. The use of a warning, rather than an error status code, - distinguish these responses from true failures. - - Warnings are assigned three digit warn-codes. The first digit - indicates whether the Warning MUST or MUST NOT be deleted from a - stored cache entry after a successful revalidation: - - - -Fielding, et al. Standards Track [Page 76] - -RFC 2616 HTTP/1.1 June 1999 - - - 1xx Warnings that describe the freshness or revalidation status of - the response, and so MUST be deleted after a successful - revalidation. 1XX warn-codes MAY be generated by a cache only when - validating a cached entry. It MUST NOT be generated by clients. - - 2xx Warnings that describe some aspect of the entity body or entity - headers that is not rectified by a revalidation (for example, a - lossy compression of the entity bodies) and which MUST NOT be - deleted after a successful revalidation. - - See section 14.46 for the definitions of the codes themselves. - - HTTP/1.0 caches will cache all Warnings in responses, without - deleting the ones in the first category. Warnings in responses that - are passed to HTTP/1.0 caches carry an extra warning-date field, - which prevents a future HTTP/1.1 recipient from believing an - erroneously cached Warning. - - Warnings also carry a warning text. The text MAY be in any - appropriate natural language (perhaps based on the client's Accept - headers), and include an OPTIONAL indication of what character set is - used. - - Multiple warnings MAY be attached to a response (either by the origin - server or by a cache), including multiple warnings with the same code - number. For example, a server might provide the same warning with - texts in both English and Basque. - - When multiple warnings are attached to a response, it might not be - practical or reasonable to display all of them to the user. This - version of HTTP does not specify strict priority rules for deciding - which warnings to display and in what order, but does suggest some - heuristics. - -13.1.3 Cache-control Mechanisms - - The basic cache mechanisms in HTTP/1.1 (server-specified expiration - times and validators) are implicit directives to caches. In some - cases, a server or client might need to provide explicit directives - to the HTTP caches. We use the Cache-Control header for this purpose. - - The Cache-Control header allows a client or server to transmit a - variety of directives in either requests or responses. These - directives typically override the default caching algorithms. As a - general rule, if there is any apparent conflict between header - values, the most restrictive interpretation is applied (that is, the - one that is most likely to preserve semantic transparency). However, - - - - -Fielding, et al. Standards Track [Page 77] - -RFC 2616 HTTP/1.1 June 1999 - - - in some cases, cache-control directives are explicitly specified as - weakening the approximation of semantic transparency (for example, - "max-stale" or "public"). - - The cache-control directives are described in detail in section 14.9. - -13.1.4 Explicit User Agent Warnings - - Many user agents make it possible for users to override the basic - caching mechanisms. For example, the user agent might allow the user - to specify that cached entities (even explicitly stale ones) are - never validated. Or the user agent might habitually add "Cache- - Control: max-stale=3600" to every request. The user agent SHOULD NOT - default to either non-transparent behavior, or behavior that results - in abnormally ineffective caching, but MAY be explicitly configured - to do so by an explicit action of the user. - - If the user has overridden the basic caching mechanisms, the user - agent SHOULD explicitly indicate to the user whenever this results in - the display of information that might not meet the server's - transparency requirements (in particular, if the displayed entity is - known to be stale). Since the protocol normally allows the user agent - to determine if responses are stale or not, this indication need only - be displayed when this actually happens. The indication need not be a - dialog box; it could be an icon (for example, a picture of a rotting - fish) or some other indicator. - - If the user has overridden the caching mechanisms in a way that would - abnormally reduce the effectiveness of caches, the user agent SHOULD - continually indicate this state to the user (for example, by a - display of a picture of currency in flames) so that the user does not - inadvertently consume excess resources or suffer from excessive - latency. - -13.1.5 Exceptions to the Rules and Warnings - - In some cases, the operator of a cache MAY choose to configure it to - return stale responses even when not requested by clients. This - decision ought not be made lightly, but may be necessary for reasons - of availability or performance, especially when the cache is poorly - connected to the origin server. Whenever a cache returns a stale - response, it MUST mark it as such (using a Warning header) enabling - the client software to alert the user that there might be a potential - problem. - - - - - - - -Fielding, et al. Standards Track [Page 78] - -RFC 2616 HTTP/1.1 June 1999 - - - It also allows the user agent to take steps to obtain a first-hand or - fresh response. For this reason, a cache SHOULD NOT return a stale - response if the client explicitly requests a first-hand or fresh one, - unless it is impossible to comply for technical or policy reasons. - -13.1.6 Client-controlled Behavior - - While the origin server (and to a lesser extent, intermediate caches, - by their contribution to the age of a response) are the primary - source of expiration information, in some cases the client might need - to control a cache's decision about whether to return a cached - response without validating it. Clients do this using several - directives of the Cache-Control header. - - A client's request MAY specify the maximum age it is willing to - accept of an unvalidated response; specifying a value of zero forces - the cache(s) to revalidate all responses. A client MAY also specify - the minimum time remaining before a response expires. Both of these - options increase constraints on the behavior of caches, and so cannot - further relax the cache's approximation of semantic transparency. - - A client MAY also specify that it will accept stale responses, up to - some maximum amount of staleness. This loosens the constraints on the - caches, and so might violate the origin server's specified - constraints on semantic transparency, but might be necessary to - support disconnected operation, or high availability in the face of - poor connectivity. - -13.2 Expiration Model - -13.2.1 Server-Specified Expiration - - HTTP caching works best when caches can entirely avoid making - requests to the origin server. The primary mechanism for avoiding - requests is for an origin server to provide an explicit expiration - time in the future, indicating that a response MAY be used to satisfy - subsequent requests. In other words, a cache can return a fresh - response without first contacting the server. - - Our expectation is that servers will assign future explicit - expiration times to responses in the belief that the entity is not - likely to change, in a semantically significant way, before the - expiration time is reached. This normally preserves semantic - transparency, as long as the server's expiration times are carefully - chosen. - - - - - - -Fielding, et al. Standards Track [Page 79] - -RFC 2616 HTTP/1.1 June 1999 - - - The expiration mechanism applies only to responses taken from a cache - and not to first-hand responses forwarded immediately to the - requesting client. - - If an origin server wishes to force a semantically transparent cache - to validate every request, it MAY assign an explicit expiration time - in the past. This means that the response is always stale, and so the - cache SHOULD validate it before using it for subsequent requests. See - section 14.9.4 for a more restrictive way to force revalidation. - - If an origin server wishes to force any HTTP/1.1 cache, no matter how - it is configured, to validate every request, it SHOULD use the "must- - revalidate" cache-control directive (see section 14.9). - - Servers specify explicit expiration times using either the Expires - header, or the max-age directive of the Cache-Control header. - - An expiration time cannot be used to force a user agent to refresh - its display or reload a resource; its semantics apply only to caching - mechanisms, and such mechanisms need only check a resource's - expiration status when a new request for that resource is initiated. - See section 13.13 for an explanation of the difference between caches - and history mechanisms. - -13.2.2 Heuristic Expiration - - Since origin servers do not always provide explicit expiration times, - HTTP caches typically assign heuristic expiration times, employing - algorithms that use other header values (such as the Last-Modified - time) to estimate a plausible expiration time. The HTTP/1.1 - specification does not provide specific algorithms, but does impose - worst-case constraints on their results. Since heuristic expiration - times might compromise semantic transparency, they ought to used - cautiously, and we encourage origin servers to provide explicit - expiration times as much as possible. - -13.2.3 Age Calculations - - In order to know if a cached entry is fresh, a cache needs to know if - its age exceeds its freshness lifetime. We discuss how to calculate - the latter in section 13.2.4; this section describes how to calculate - the age of a response or cache entry. - - In this discussion, we use the term "now" to mean "the current value - of the clock at the host performing the calculation." Hosts that use - HTTP, but especially hosts running origin servers and caches, SHOULD - use NTP [28] or some similar protocol to synchronize their clocks to - a globally accurate time standard. - - - -Fielding, et al. Standards Track [Page 80] - -RFC 2616 HTTP/1.1 June 1999 - - - HTTP/1.1 requires origin servers to send a Date header, if possible, - with every response, giving the time at which the response was - generated (see section 14.18). We use the term "date_value" to denote - the value of the Date header, in a form appropriate for arithmetic - operations. - - HTTP/1.1 uses the Age response-header to convey the estimated age of - the response message when obtained from a cache. The Age field value - is the cache's estimate of the amount of time since the response was - generated or revalidated by the origin server. - - In essence, the Age value is the sum of the time that the response - has been resident in each of the caches along the path from the - origin server, plus the amount of time it has been in transit along - network paths. - - We use the term "age_value" to denote the value of the Age header, in - a form appropriate for arithmetic operations. - - A response's age can be calculated in two entirely independent ways: - - 1. now minus date_value, if the local clock is reasonably well - synchronized to the origin server's clock. If the result is - negative, the result is replaced by zero. - - 2. age_value, if all of the caches along the response path - implement HTTP/1.1. - - Given that we have two independent ways to compute the age of a - response when it is received, we can combine these as - - corrected_received_age = max(now - date_value, age_value) - - and as long as we have either nearly synchronized clocks or all- - HTTP/1.1 paths, one gets a reliable (conservative) result. - - Because of network-imposed delays, some significant interval might - pass between the time that a server generates a response and the time - it is received at the next outbound cache or client. If uncorrected, - this delay could result in improperly low ages. - - Because the request that resulted in the returned Age value must have - been initiated prior to that Age value's generation, we can correct - for delays imposed by the network by recording the time at which the - request was initiated. Then, when an Age value is received, it MUST - be interpreted relative to the time the request was initiated, not - - - - - -Fielding, et al. Standards Track [Page 81] - -RFC 2616 HTTP/1.1 June 1999 - - - the time that the response was received. This algorithm results in - conservative behavior no matter how much delay is experienced. So, we - compute: - - corrected_initial_age = corrected_received_age - + (now - request_time) - - where "request_time" is the time (according to the local clock) when - the request that elicited this response was sent. - - Summary of age calculation algorithm, when a cache receives a - response: - - /* - * age_value - * is the value of Age: header received by the cache with - * this response. - * date_value - * is the value of the origin server's Date: header - * request_time - * is the (local) time when the cache made the request - * that resulted in this cached response - * response_time - * is the (local) time when the cache received the - * response - * now - * is the current (local) time - */ - - apparent_age = max(0, response_time - date_value); - corrected_received_age = max(apparent_age, age_value); - response_delay = response_time - request_time; - corrected_initial_age = corrected_received_age + response_delay; - resident_time = now - response_time; - current_age = corrected_initial_age + resident_time; - - The current_age of a cache entry is calculated by adding the amount - of time (in seconds) since the cache entry was last validated by the - origin server to the corrected_initial_age. When a response is - generated from a cache entry, the cache MUST include a single Age - header field in the response with a value equal to the cache entry's - current_age. - - The presence of an Age header field in a response implies that a - response is not first-hand. However, the converse is not true, since - the lack of an Age header field in a response does not imply that the - - - - - -Fielding, et al. Standards Track [Page 82] - -RFC 2616 HTTP/1.1 June 1999 - - - response is first-hand unless all caches along the request path are - compliant with HTTP/1.1 (i.e., older HTTP caches did not implement - the Age header field). - -13.2.4 Expiration Calculations - - In order to decide whether a response is fresh or stale, we need to - compare its freshness lifetime to its age. The age is calculated as - described in section 13.2.3; this section describes how to calculate - the freshness lifetime, and to determine if a response has expired. - In the discussion below, the values can be represented in any form - appropriate for arithmetic operations. - - We use the term "expires_value" to denote the value of the Expires - header. We use the term "max_age_value" to denote an appropriate - value of the number of seconds carried by the "max-age" directive of - the Cache-Control header in a response (see section 14.9.3). - - The max-age directive takes priority over Expires, so if max-age is - present in a response, the calculation is simply: - - freshness_lifetime = max_age_value - - Otherwise, if Expires is present in the response, the calculation is: - - freshness_lifetime = expires_value - date_value - - Note that neither of these calculations is vulnerable to clock skew, - since all of the information comes from the origin server. - - If none of Expires, Cache-Control: max-age, or Cache-Control: s- - maxage (see section 14.9.3) appears in the response, and the response - does not include other restrictions on caching, the cache MAY compute - a freshness lifetime using a heuristic. The cache MUST attach Warning - 113 to any response whose age is more than 24 hours if such warning - has not already been added. - - Also, if the response does have a Last-Modified time, the heuristic - expiration value SHOULD be no more than some fraction of the interval - since that time. A typical setting of this fraction might be 10%. - - The calculation to determine if a response has expired is quite - simple: - - response_is_fresh = (freshness_lifetime > current_age) - - - - - - -Fielding, et al. Standards Track [Page 83] - -RFC 2616 HTTP/1.1 June 1999 - - -13.2.5 Disambiguating Expiration Values - - Because expiration values are assigned optimistically, it is possible - for two caches to contain fresh values for the same resource that are - different. - - If a client performing a retrieval receives a non-first-hand response - for a request that was already fresh in its own cache, and the Date - header in its existing cache entry is newer than the Date on the new - response, then the client MAY ignore the response. If so, it MAY - retry the request with a "Cache-Control: max-age=0" directive (see - section 14.9), to force a check with the origin server. - - If a cache has two fresh responses for the same representation with - different validators, it MUST use the one with the more recent Date - header. This situation might arise because the cache is pooling - responses from other caches, or because a client has asked for a - reload or a revalidation of an apparently fresh cache entry. - -13.2.6 Disambiguating Multiple Responses - - Because a client might be receiving responses via multiple paths, so - that some responses flow through one set of caches and other - responses flow through a different set of caches, a client might - receive responses in an order different from that in which the origin - server sent them. We would like the client to use the most recently - generated response, even if older responses are still apparently - fresh. - - Neither the entity tag nor the expiration value can impose an - ordering on responses, since it is possible that a later response - intentionally carries an earlier expiration time. The Date values are - ordered to a granularity of one second. - - When a client tries to revalidate a cache entry, and the response it - receives contains a Date header that appears to be older than the one - for the existing entry, then the client SHOULD repeat the request - unconditionally, and include - - Cache-Control: max-age=0 - - to force any intermediate caches to validate their copies directly - with the origin server, or - - Cache-Control: no-cache - - to force any intermediate caches to obtain a new copy from the origin - server. - - - -Fielding, et al. Standards Track [Page 84] - -RFC 2616 HTTP/1.1 June 1999 - - - If the Date values are equal, then the client MAY use either response - (or MAY, if it is being extremely prudent, request a new response). - Servers MUST NOT depend on clients being able to choose - deterministically between responses generated during the same second, - if their expiration times overlap. - -13.3 Validation Model - - When a cache has a stale entry that it would like to use as a - response to a client's request, it first has to check with the origin - server (or possibly an intermediate cache with a fresh response) to - see if its cached entry is still usable. We call this "validating" - the cache entry. Since we do not want to have to pay the overhead of - retransmitting the full response if the cached entry is good, and we - do not want to pay the overhead of an extra round trip if the cached - entry is invalid, the HTTP/1.1 protocol supports the use of - conditional methods. - - The key protocol features for supporting conditional methods are - those concerned with "cache validators." When an origin server - generates a full response, it attaches some sort of validator to it, - which is kept with the cache entry. When a client (user agent or - proxy cache) makes a conditional request for a resource for which it - has a cache entry, it includes the associated validator in the - request. - - The server then checks that validator against the current validator - for the entity, and, if they match (see section 13.3.3), it responds - with a special status code (usually, 304 (Not Modified)) and no - entity-body. Otherwise, it returns a full response (including - entity-body). Thus, we avoid transmitting the full response if the - validator matches, and we avoid an extra round trip if it does not - match. - - In HTTP/1.1, a conditional request looks exactly the same as a normal - request for the same resource, except that it carries a special - header (which includes the validator) that implicitly turns the - method (usually, GET) into a conditional. - - The protocol includes both positive and negative senses of cache- - validating conditions. That is, it is possible to request either that - a method be performed if and only if a validator matches or if and - only if no validators match. - - - - - - - - -Fielding, et al. Standards Track [Page 85] - -RFC 2616 HTTP/1.1 June 1999 - - - Note: a response that lacks a validator may still be cached, and - served from cache until it expires, unless this is explicitly - prohibited by a cache-control directive. However, a cache cannot - do a conditional retrieval if it does not have a validator for the - entity, which means it will not be refreshable after it expires. - -13.3.1 Last-Modified Dates - - The Last-Modified entity-header field value is often used as a cache - validator. In simple terms, a cache entry is considered to be valid - if the entity has not been modified since the Last-Modified value. - -13.3.2 Entity Tag Cache Validators - - The ETag response-header field value, an entity tag, provides for an - "opaque" cache validator. This might allow more reliable validation - in situations where it is inconvenient to store modification dates, - where the one-second resolution of HTTP date values is not - sufficient, or where the origin server wishes to avoid certain - paradoxes that might arise from the use of modification dates. - - Entity Tags are described in section 3.11. The headers used with - entity tags are described in sections 14.19, 14.24, 14.26 and 14.44. - -13.3.3 Weak and Strong Validators - - Since both origin servers and caches will compare two validators to - decide if they represent the same or different entities, one normally - would expect that if the entity (the entity-body or any entity- - headers) changes in any way, then the associated validator would - change as well. If this is true, then we call this validator a - "strong validator." - - However, there might be cases when a server prefers to change the - validator only on semantically significant changes, and not when - insignificant aspects of the entity change. A validator that does not - always change when the resource changes is a "weak validator." - - Entity tags are normally "strong validators," but the protocol - provides a mechanism to tag an entity tag as "weak." One can think of - a strong validator as one that changes whenever the bits of an entity - changes, while a weak value changes whenever the meaning of an entity - changes. Alternatively, one can think of a strong validator as part - of an identifier for a specific entity, while a weak validator is - part of an identifier for a set of semantically equivalent entities. - - Note: One example of a strong validator is an integer that is - incremented in stable storage every time an entity is changed. - - - -Fielding, et al. Standards Track [Page 86] - -RFC 2616 HTTP/1.1 June 1999 - - - An entity's modification time, if represented with one-second - resolution, could be a weak validator, since it is possible that - the resource might be modified twice during a single second. - - Support for weak validators is optional. However, weak validators - allow for more efficient caching of equivalent objects; for - example, a hit counter on a site is probably good enough if it is - updated every few days or weeks, and any value during that period - is likely "good enough" to be equivalent. - - A "use" of a validator is either when a client generates a request - and includes the validator in a validating header field, or when a - server compares two validators. - - Strong validators are usable in any context. Weak validators are only - usable in contexts that do not depend on exact equality of an entity. - For example, either kind is usable for a conditional GET of a full - entity. However, only a strong validator is usable for a sub-range - retrieval, since otherwise the client might end up with an internally - inconsistent entity. - - Clients MAY issue simple (non-subrange) GET requests with either weak - validators or strong validators. Clients MUST NOT use weak validators - in other forms of request. - - The only function that the HTTP/1.1 protocol defines on validators is - comparison. There are two validator comparison functions, depending - on whether the comparison context allows the use of weak validators - or not: - - - The strong comparison function: in order to be considered equal, - both validators MUST be identical in every way, and both MUST - NOT be weak. - - - The weak comparison function: in order to be considered equal, - both validators MUST be identical in every way, but either or - both of them MAY be tagged as "weak" without affecting the - result. - - An entity tag is strong unless it is explicitly tagged as weak. - Section 3.11 gives the syntax for entity tags. - - A Last-Modified time, when used as a validator in a request, is - implicitly weak unless it is possible to deduce that it is strong, - using the following rules: - - - The validator is being compared by an origin server to the - actual current validator for the entity and, - - - -Fielding, et al. Standards Track [Page 87] - -RFC 2616 HTTP/1.1 June 1999 - - - - That origin server reliably knows that the associated entity did - not change twice during the second covered by the presented - validator. - - or - - - The validator is about to be used by a client in an If- - Modified-Since or If-Unmodified-Since header, because the client - has a cache entry for the associated entity, and - - - That cache entry includes a Date value, which gives the time - when the origin server sent the original response, and - - - The presented Last-Modified time is at least 60 seconds before - the Date value. - - or - - - The validator is being compared by an intermediate cache to the - validator stored in its cache entry for the entity, and - - - That cache entry includes a Date value, which gives the time - when the origin server sent the original response, and - - - The presented Last-Modified time is at least 60 seconds before - the Date value. - - This method relies on the fact that if two different responses were - sent by the origin server during the same second, but both had the - same Last-Modified time, then at least one of those responses would - have a Date value equal to its Last-Modified time. The arbitrary 60- - second limit guards against the possibility that the Date and Last- - Modified values are generated from different clocks, or at somewhat - different times during the preparation of the response. An - implementation MAY use a value larger than 60 seconds, if it is - believed that 60 seconds is too short. - - If a client wishes to perform a sub-range retrieval on a value for - which it has only a Last-Modified time and no opaque validator, it - MAY do this only if the Last-Modified time is strong in the sense - described here. - - A cache or origin server receiving a conditional request, other than - a full-body GET request, MUST use the strong comparison function to - evaluate the condition. - - These rules allow HTTP/1.1 caches and clients to safely perform sub- - range retrievals on values that have been obtained from HTTP/1.0 - - - -Fielding, et al. Standards Track [Page 88] - -RFC 2616 HTTP/1.1 June 1999 - - - servers. - -13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates - - We adopt a set of rules and recommendations for origin servers, - clients, and caches regarding when various validator types ought to - be used, and for what purposes. - - HTTP/1.1 origin servers: - - - SHOULD send an entity tag validator unless it is not feasible to - generate one. - - - MAY send a weak entity tag instead of a strong entity tag, if - performance considerations support the use of weak entity tags, - or if it is unfeasible to send a strong entity tag. - - - SHOULD send a Last-Modified value if it is feasible to send one, - unless the risk of a breakdown in semantic transparency that - could result from using this date in an If-Modified-Since header - would lead to serious problems. - - In other words, the preferred behavior for an HTTP/1.1 origin server - is to send both a strong entity tag and a Last-Modified value. - - In order to be legal, a strong entity tag MUST change whenever the - associated entity value changes in any way. A weak entity tag SHOULD - change whenever the associated entity changes in a semantically - significant way. - - Note: in order to provide semantically transparent caching, an - origin server must avoid reusing a specific strong entity tag - value for two different entities, or reusing a specific weak - entity tag value for two semantically different entities. Cache - entries might persist for arbitrarily long periods, regardless of - expiration times, so it might be inappropriate to expect that a - cache will never again attempt to validate an entry using a - validator that it obtained at some point in the past. - - HTTP/1.1 clients: - - - If an entity tag has been provided by the origin server, MUST - use that entity tag in any cache-conditional request (using If- - Match or If-None-Match). - - - If only a Last-Modified value has been provided by the origin - server, SHOULD use that value in non-subrange cache-conditional - requests (using If-Modified-Since). - - - -Fielding, et al. Standards Track [Page 89] - -RFC 2616 HTTP/1.1 June 1999 - - - - If only a Last-Modified value has been provided by an HTTP/1.0 - origin server, MAY use that value in subrange cache-conditional - requests (using If-Unmodified-Since:). The user agent SHOULD - provide a way to disable this, in case of difficulty. - - - If both an entity tag and a Last-Modified value have been - provided by the origin server, SHOULD use both validators in - cache-conditional requests. This allows both HTTP/1.0 and - HTTP/1.1 caches to respond appropriately. - - An HTTP/1.1 origin server, upon receiving a conditional request that - includes both a Last-Modified date (e.g., in an If-Modified-Since or - If-Unmodified-Since header field) and one or more entity tags (e.g., - in an If-Match, If-None-Match, or If-Range header field) as cache - validators, MUST NOT return a response status of 304 (Not Modified) - unless doing so is consistent with all of the conditional header - fields in the request. - - An HTTP/1.1 caching proxy, upon receiving a conditional request that - includes both a Last-Modified date and one or more entity tags as - cache validators, MUST NOT return a locally cached response to the - client unless that cached response is consistent with all of the - conditional header fields in the request. - - Note: The general principle behind these rules is that HTTP/1.1 - servers and clients should transmit as much non-redundant - information as is available in their responses and requests. - HTTP/1.1 systems receiving this information will make the most - conservative assumptions about the validators they receive. - - HTTP/1.0 clients and caches will ignore entity tags. Generally, - last-modified values received or used by these systems will - support transparent and efficient caching, and so HTTP/1.1 origin - servers should provide Last-Modified values. In those rare cases - where the use of a Last-Modified value as a validator by an - HTTP/1.0 system could result in a serious problem, then HTTP/1.1 - origin servers should not provide one. - -13.3.5 Non-validating Conditionals - - The principle behind entity tags is that only the service author - knows the semantics of a resource well enough to select an - appropriate cache validation mechanism, and the specification of any - validator comparison function more complex than byte-equality would - open up a can of worms. Thus, comparisons of any other headers - (except Last-Modified, for compatibility with HTTP/1.0) are never - used for purposes of validating a cache entry. - - - - -Fielding, et al. Standards Track [Page 90] - -RFC 2616 HTTP/1.1 June 1999 - - -13.4 Response Cacheability - - Unless specifically constrained by a cache-control (section 14.9) - directive, a caching system MAY always store a successful response - (see section 13.8) as a cache entry, MAY return it without validation - if it is fresh, and MAY return it after successful validation. If - there is neither a cache validator nor an explicit expiration time - associated with a response, we do not expect it to be cached, but - certain caches MAY violate this expectation (for example, when little - or no network connectivity is available). A client can usually detect - that such a response was taken from a cache by comparing the Date - header to the current time. - - Note: some HTTP/1.0 caches are known to violate this expectation - without providing any Warning. - - However, in some cases it might be inappropriate for a cache to - retain an entity, or to return it in response to a subsequent - request. This might be because absolute semantic transparency is - deemed necessary by the service author, or because of security or - privacy considerations. Certain cache-control directives are - therefore provided so that the server can indicate that certain - resource entities, or portions thereof, are not to be cached - regardless of other considerations. - - Note that section 14.8 normally prevents a shared cache from saving - and returning a response to a previous request if that request - included an Authorization header. - - A response received with a status code of 200, 203, 206, 300, 301 or - 410 MAY be stored by a cache and used in reply to a subsequent - request, subject to the expiration mechanism, unless a cache-control - directive prohibits caching. However, a cache that does not support - the Range and Content-Range headers MUST NOT cache 206 (Partial - Content) responses. - - A response received with any other status code (e.g. status codes 302 - and 307) MUST NOT be returned in a reply to a subsequent request - unless there are cache-control directives or another header(s) that - explicitly allow it. For example, these include the following: an - Expires header (section 14.21); a "max-age", "s-maxage", "must- - revalidate", "proxy-revalidate", "public" or "private" cache-control - directive (section 14.9). - - - - - - - - -Fielding, et al. Standards Track [Page 91] - -RFC 2616 HTTP/1.1 June 1999 - - -13.5 Constructing Responses From Caches - - The purpose of an HTTP cache is to store information received in - response to requests for use in responding to future requests. In - many cases, a cache simply returns the appropriate parts of a - response to the requester. However, if the cache holds a cache entry - based on a previous response, it might have to combine parts of a new - response with what is held in the cache entry. - -13.5.1 End-to-end and Hop-by-hop Headers - - For the purpose of defining the behavior of caches and non-caching - proxies, we divide HTTP headers into two categories: - - - End-to-end headers, which are transmitted to the ultimate - recipient of a request or response. End-to-end headers in - responses MUST be stored as part of a cache entry and MUST be - transmitted in any response formed from a cache entry. - - - Hop-by-hop headers, which are meaningful only for a single - transport-level connection, and are not stored by caches or - forwarded by proxies. - - The following HTTP/1.1 headers are hop-by-hop headers: - - - Connection - - Keep-Alive - - Proxy-Authenticate - - Proxy-Authorization - - TE - - Trailers - - Transfer-Encoding - - Upgrade - - All other headers defined by HTTP/1.1 are end-to-end headers. - - Other hop-by-hop headers MUST be listed in a Connection header, - (section 14.10) to be introduced into HTTP/1.1 (or later). - -13.5.2 Non-modifiable Headers - - Some features of the HTTP/1.1 protocol, such as Digest - Authentication, depend on the value of certain end-to-end headers. A - transparent proxy SHOULD NOT modify an end-to-end header unless the - definition of that header requires or specifically allows that. - - - - - - -Fielding, et al. Standards Track [Page 92] - -RFC 2616 HTTP/1.1 June 1999 - - - A transparent proxy MUST NOT modify any of the following fields in a - request or response, and it MUST NOT add any of these fields if not - already present: - - - Content-Location - - - Content-MD5 - - - ETag - - - Last-Modified - - A transparent proxy MUST NOT modify any of the following fields in a - response: - - - Expires - - but it MAY add any of these fields if not already present. If an - Expires header is added, it MUST be given a field-value identical to - that of the Date header in that response. - - A proxy MUST NOT modify or add any of the following fields in a - message that contains the no-transform cache-control directive, or in - any request: - - - Content-Encoding - - - Content-Range - - - Content-Type - - A non-transparent proxy MAY modify or add these fields to a message - that does not include no-transform, but if it does so, it MUST add a - Warning 214 (Transformation applied) if one does not already appear - in the message (see section 14.46). - - Warning: unnecessary modification of end-to-end headers might - cause authentication failures if stronger authentication - mechanisms are introduced in later versions of HTTP. Such - authentication mechanisms MAY rely on the values of header fields - not listed here. - - The Content-Length field of a request or response is added or deleted - according to the rules in section 4.4. A transparent proxy MUST - preserve the entity-length (section 7.2.2) of the entity-body, - although it MAY change the transfer-length (section 4.4). - - - - - -Fielding, et al. Standards Track [Page 93] - -RFC 2616 HTTP/1.1 June 1999 - - -13.5.3 Combining Headers - - When a cache makes a validating request to a server, and the server - provides a 304 (Not Modified) response or a 206 (Partial Content) - response, the cache then constructs a response to send to the - requesting client. - - If the status code is 304 (Not Modified), the cache uses the entity- - body stored in the cache entry as the entity-body of this outgoing - response. If the status code is 206 (Partial Content) and the ETag or - Last-Modified headers match exactly, the cache MAY combine the - contents stored in the cache entry with the new contents received in - the response and use the result as the entity-body of this outgoing - response, (see 13.5.4). - - The end-to-end headers stored in the cache entry are used for the - constructed response, except that - - - any stored Warning headers with warn-code 1xx (see section - 14.46) MUST be deleted from the cache entry and the forwarded - response. - - - any stored Warning headers with warn-code 2xx MUST be retained - in the cache entry and the forwarded response. - - - any end-to-end headers provided in the 304 or 206 response MUST - replace the corresponding headers from the cache entry. - - Unless the cache decides to remove the cache entry, it MUST also - replace the end-to-end headers stored with the cache entry with - corresponding headers received in the incoming response, except for - Warning headers as described immediately above. If a header field- - name in the incoming response matches more than one header in the - cache entry, all such old headers MUST be replaced. - - In other words, the set of end-to-end headers received in the - incoming response overrides all corresponding end-to-end headers - stored with the cache entry (except for stored Warning headers with - warn-code 1xx, which are deleted even if not overridden). - - Note: this rule allows an origin server to use a 304 (Not - Modified) or a 206 (Partial Content) response to update any header - associated with a previous response for the same entity or sub- - ranges thereof, although it might not always be meaningful or - correct to do so. This rule does not allow an origin server to use - a 304 (Not Modified) or a 206 (Partial Content) response to - entirely delete a header that it had provided with a previous - response. - - - -Fielding, et al. Standards Track [Page 94] - -RFC 2616 HTTP/1.1 June 1999 - - -13.5.4 Combining Byte Ranges - - A response might transfer only a subrange of the bytes of an entity- - body, either because the request included one or more Range - specifications, or because a connection was broken prematurely. After - several such transfers, a cache might have received several ranges of - the same entity-body. - - If a cache has a stored non-empty set of subranges for an entity, and - an incoming response transfers another subrange, the cache MAY - combine the new subrange with the existing set if both the following - conditions are met: - - - Both the incoming response and the cache entry have a cache - validator. - - - The two cache validators match using the strong comparison - function (see section 13.3.3). - - If either requirement is not met, the cache MUST use only the most - recent partial response (based on the Date values transmitted with - every response, and using the incoming response if these values are - equal or missing), and MUST discard the other partial information. - -13.6 Caching Negotiated Responses - - Use of server-driven content negotiation (section 12.1), as indicated - by the presence of a Vary header field in a response, alters the - conditions and procedure by which a cache can use the response for - subsequent requests. See section 14.44 for use of the Vary header - field by servers. - - A server SHOULD use the Vary header field to inform a cache of what - request-header fields were used to select among multiple - representations of a cacheable response subject to server-driven - negotiation. The set of header fields named by the Vary field value - is known as the "selecting" request-headers. - - When the cache receives a subsequent request whose Request-URI - specifies one or more cache entries including a Vary header field, - the cache MUST NOT use such a cache entry to construct a response to - the new request unless all of the selecting request-headers present - in the new request match the corresponding stored request-headers in - the original request. - - The selecting request-headers from two requests are defined to match - if and only if the selecting request-headers in the first request can - be transformed to the selecting request-headers in the second request - - - -Fielding, et al. Standards Track [Page 95] - -RFC 2616 HTTP/1.1 June 1999 - - - by adding or removing linear white space (LWS) at places where this - is allowed by the corresponding BNF, and/or combining multiple - message-header fields with the same field name following the rules - about message headers in section 4.2. - - A Vary header field-value of "*" always fails to match and subsequent - requests on that resource can only be properly interpreted by the - origin server. - - If the selecting request header fields for the cached entry do not - match the selecting request header fields of the new request, then - the cache MUST NOT use a cached entry to satisfy the request unless - it first relays the new request to the origin server in a conditional - request and the server responds with 304 (Not Modified), including an - entity tag or Content-Location that indicates the entity to be used. - - If an entity tag was assigned to a cached representation, the - forwarded request SHOULD be conditional and include the entity tags - in an If-None-Match header field from all its cache entries for the - resource. This conveys to the server the set of entities currently - held by the cache, so that if any one of these entities matches the - requested entity, the server can use the ETag header field in its 304 - (Not Modified) response to tell the cache which entry is appropriate. - If the entity-tag of the new response matches that of an existing - entry, the new response SHOULD be used to update the header fields of - the existing entry, and the result MUST be returned to the client. - - If any of the existing cache entries contains only partial content - for the associated entity, its entity-tag SHOULD NOT be included in - the If-None-Match header field unless the request is for a range that - would be fully satisfied by that entry. - - If a cache receives a successful response whose Content-Location - field matches that of an existing cache entry for the same Request- - ]URI, whose entity-tag differs from that of the existing entry, and - whose Date is more recent than that of the existing entry, the - existing entry SHOULD NOT be returned in response to future requests - and SHOULD be deleted from the cache. - -13.7 Shared and Non-Shared Caches - - For reasons of security and privacy, it is necessary to make a - distinction between "shared" and "non-shared" caches. A non-shared - cache is one that is accessible only to a single user. Accessibility - in this case SHOULD be enforced by appropriate security mechanisms. - All other caches are considered to be "shared." Other sections of - - - - - -Fielding, et al. Standards Track [Page 96] - -RFC 2616 HTTP/1.1 June 1999 - - - this specification place certain constraints on the operation of - shared caches in order to prevent loss of privacy or failure of - access controls. - -13.8 Errors or Incomplete Response Cache Behavior - - A cache that receives an incomplete response (for example, with fewer - bytes of data than specified in a Content-Length header) MAY store - the response. However, the cache MUST treat this as a partial - response. Partial responses MAY be combined as described in section - 13.5.4; the result might be a full response or might still be - partial. A cache MUST NOT return a partial response to a client - without explicitly marking it as such, using the 206 (Partial - Content) status code. A cache MUST NOT return a partial response - using a status code of 200 (OK). - - If a cache receives a 5xx response while attempting to revalidate an - entry, it MAY either forward this response to the requesting client, - or act as if the server failed to respond. In the latter case, it MAY - return a previously received response unless the cached entry - includes the "must-revalidate" cache-control directive (see section - 14.9). - -13.9 Side Effects of GET and HEAD - - Unless the origin server explicitly prohibits the caching of their - responses, the application of GET and HEAD methods to any resources - SHOULD NOT have side effects that would lead to erroneous behavior if - these responses are taken from a cache. They MAY still have side - effects, but a cache is not required to consider such side effects in - its caching decisions. Caches are always expected to observe an - origin server's explicit restrictions on caching. - - We note one exception to this rule: since some applications have - traditionally used GETs and HEADs with query URLs (those containing a - "?" in the rel_path part) to perform operations with significant side - effects, caches MUST NOT treat responses to such URIs as fresh unless - the server provides an explicit expiration time. This specifically - means that responses from HTTP/1.0 servers for such URIs SHOULD NOT - be taken from a cache. See section 9.1.1 for related information. - -13.10 Invalidation After Updates or Deletions - - The effect of certain methods performed on a resource at the origin - server might cause one or more existing cache entries to become non- - transparently invalid. That is, although they might continue to be - "fresh," they do not accurately reflect what the origin server would - return for a new request on that resource. - - - -Fielding, et al. Standards Track [Page 97] - -RFC 2616 HTTP/1.1 June 1999 - - - There is no way for the HTTP protocol to guarantee that all such - cache entries are marked invalid. For example, the request that - caused the change at the origin server might not have gone through - the proxy where a cache entry is stored. However, several rules help - reduce the likelihood of erroneous behavior. - - In this section, the phrase "invalidate an entity" means that the - cache will either remove all instances of that entity from its - storage, or will mark these as "invalid" and in need of a mandatory - revalidation before they can be returned in response to a subsequent - request. - - Some HTTP methods MUST cause a cache to invalidate an entity. This is - either the entity referred to by the Request-URI, or by the Location - or Content-Location headers (if present). These methods are: - - - PUT - - - DELETE - - - POST - - In order to prevent denial of service attacks, an invalidation based - on the URI in a Location or Content-Location header MUST only be - performed if the host part is the same as in the Request-URI. - - A cache that passes through requests for methods it does not - understand SHOULD invalidate any entities referred to by the - Request-URI. - -13.11 Write-Through Mandatory - - All methods that might be expected to cause modifications to the - origin server's resources MUST be written through to the origin - server. This currently includes all methods except for GET and HEAD. - A cache MUST NOT reply to such a request from a client before having - transmitted the request to the inbound server, and having received a - corresponding response from the inbound server. This does not prevent - a proxy cache from sending a 100 (Continue) response before the - inbound server has sent its final reply. - - The alternative (known as "write-back" or "copy-back" caching) is not - allowed in HTTP/1.1, due to the difficulty of providing consistent - updates and the problems arising from server, cache, or network - failure prior to write-back. - - - - - - -Fielding, et al. Standards Track [Page 98] - -RFC 2616 HTTP/1.1 June 1999 - - -13.12 Cache Replacement - - If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) - response is received from a resource while any existing responses for - the same resource are cached, the cache SHOULD use the new response - to reply to the current request. It MAY insert it into cache storage - and MAY, if it meets all other requirements, use it to respond to any - future requests that would previously have caused the old response to - be returned. If it inserts the new response into cache storage the - rules in section 13.5.3 apply. - - Note: a new response that has an older Date header value than - existing cached responses is not cacheable. - -13.13 History Lists - - User agents often have history mechanisms, such as "Back" buttons and - history lists, which can be used to redisplay an entity retrieved - earlier in a session. - - History mechanisms and caches are different. In particular history - mechanisms SHOULD NOT try to show a semantically transparent view of - the current state of a resource. Rather, a history mechanism is meant - to show exactly what the user saw at the time when the resource was - retrieved. - - By default, an expiration time does not apply to history mechanisms. - If the entity is still in storage, a history mechanism SHOULD display - it even if the entity has expired, unless the user has specifically - configured the agent to refresh expired history documents. - - This is not to be construed to prohibit the history mechanism from - telling the user that a view might be stale. - - Note: if history list mechanisms unnecessarily prevent users from - viewing stale resources, this will tend to force service authors - to avoid using HTTP expiration controls and cache controls when - they would otherwise like to. Service authors may consider it - important that users not be presented with error messages or - warning messages when they use navigation controls (such as BACK) - to view previously fetched resources. Even though sometimes such - resources ought not to cached, or ought to expire quickly, user - interface considerations may force service authors to resort to - other means of preventing caching (e.g. "once-only" URLs) in order - not to suffer the effects of improperly functioning history - mechanisms. - - - - - -Fielding, et al. Standards Track [Page 99] - -RFC 2616 HTTP/1.1 June 1999 - - -14 Header Field Definitions - - This section defines the syntax and semantics of all standard - HTTP/1.1 header fields. For entity-header fields, both sender and - recipient refer to either the client or the server, depending on who - sends and who receives the entity. - -14.1 Accept - - The Accept request-header field can be used to specify certain media - types which are acceptable for the response. Accept headers can be - used to indicate that the request is specifically limited to a small - set of desired types, as in the case of a request for an in-line - image. - - Accept = "Accept" ":" - #( media-range [ accept-params ] ) - - media-range = ( "*/*" - | ( type "/" "*" ) - | ( type "/" subtype ) - ) *( ";" parameter ) - accept-params = ";" "q" "=" qvalue *( accept-extension ) - accept-extension = ";" token [ "=" ( token | quoted-string ) ] - - The asterisk "*" character is used to group media types into ranges, - with "*/*" indicating all media types and "type/*" indicating all - subtypes of that type. The media-range MAY include media type - parameters that are applicable to that range. - - Each media-range MAY be followed by one or more accept-params, - beginning with the "q" parameter for indicating a relative quality - factor. The first "q" parameter (if any) separates the media-range - parameter(s) from the accept-params. Quality factors allow the user - or user agent to indicate the relative degree of preference for that - media-range, using the qvalue scale from 0 to 1 (section 3.9). The - default value is q=1. - - Note: Use of the "q" parameter name to separate media type - parameters from Accept extension parameters is due to historical - practice. Although this prevents any media type parameter named - "q" from being used with a media range, such an event is believed - to be unlikely given the lack of any "q" parameters in the IANA - media type registry and the rare usage of any media type - parameters in Accept. Future media types are discouraged from - registering any parameter named "q". - - - - - -Fielding, et al. Standards Track [Page 100] - -RFC 2616 HTTP/1.1 June 1999 - - - The example - - Accept: audio/*; q=0.2, audio/basic - - SHOULD be interpreted as "I prefer audio/basic, but send me any audio - type if it is the best available after an 80% mark-down in quality." - - If no Accept header field is present, then it is assumed that the - client accepts all media types. If an Accept header field is present, - and if the server cannot send a response which is acceptable - according to the combined Accept field value, then the server SHOULD - send a 406 (not acceptable) response. - - A more elaborate example is - - Accept: text/plain; q=0.5, text/html, - text/x-dvi; q=0.8, text/x-c - - Verbally, this would be interpreted as "text/html and text/x-c are - the preferred media types, but if they do not exist, then send the - text/x-dvi entity, and if that does not exist, send the text/plain - entity." - - Media ranges can be overridden by more specific media ranges or - specific media types. If more than one media range applies to a given - type, the most specific reference has precedence. For example, - - Accept: text/*, text/html, text/html;level=1, */* - - have the following precedence: - - 1) text/html;level=1 - 2) text/html - 3) text/* - 4) */* - - The media type quality factor associated with a given type is - determined by finding the media range with the highest precedence - which matches that type. For example, - - Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1, - text/html;level=2;q=0.4, */*;q=0.5 - - would cause the following values to be associated: - - text/html;level=1 = 1 - text/html = 0.7 - text/plain = 0.3 - - - -Fielding, et al. Standards Track [Page 101] - -RFC 2616 HTTP/1.1 June 1999 - - - image/jpeg = 0.5 - text/html;level=2 = 0.4 - text/html;level=3 = 0.7 - - Note: A user agent might be provided with a default set of quality - values for certain media ranges. However, unless the user agent is - a closed system which cannot interact with other rendering agents, - this default set ought to be configurable by the user. - -14.2 Accept-Charset - - The Accept-Charset request-header field can be used to indicate what - character sets are acceptable for the response. This field allows - clients capable of understanding more comprehensive or special- - purpose character sets to signal that capability to a server which is - capable of representing documents in those character sets. - - Accept-Charset = "Accept-Charset" ":" - 1#( ( charset | "*" )[ ";" "q" "=" qvalue ] ) - - - Character set values are described in section 3.4. Each charset MAY - be given an associated quality value which represents the user's - preference for that charset. The default value is q=1. An example is - - Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 - - The special value "*", if present in the Accept-Charset field, - matches every character set (including ISO-8859-1) which is not - mentioned elsewhere in the Accept-Charset field. If no "*" is present - in an Accept-Charset field, then all character sets not explicitly - mentioned get a quality value of 0, except for ISO-8859-1, which gets - a quality value of 1 if not explicitly mentioned. - - If no Accept-Charset header is present, the default is that any - character set is acceptable. If an Accept-Charset header is present, - and if the server cannot send a response which is acceptable - according to the Accept-Charset header, then the server SHOULD send - an error response with the 406 (not acceptable) status code, though - the sending of an unacceptable response is also allowed. - -14.3 Accept-Encoding - - The Accept-Encoding request-header field is similar to Accept, but - restricts the content-codings (section 3.5) that are acceptable in - the response. - - Accept-Encoding = "Accept-Encoding" ":" - - - -Fielding, et al. Standards Track [Page 102] - -RFC 2616 HTTP/1.1 June 1999 - - - 1#( codings [ ";" "q" "=" qvalue ] ) - codings = ( content-coding | "*" ) - - Examples of its use are: - - Accept-Encoding: compress, gzip - Accept-Encoding: - Accept-Encoding: * - Accept-Encoding: compress;q=0.5, gzip;q=1.0 - Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0 - - A server tests whether a content-coding is acceptable, according to - an Accept-Encoding field, using these rules: - - 1. If the content-coding is one of the content-codings listed in - the Accept-Encoding field, then it is acceptable, unless it is - accompanied by a qvalue of 0. (As defined in section 3.9, a - qvalue of 0 means "not acceptable.") - - 2. The special "*" symbol in an Accept-Encoding field matches any - available content-coding not explicitly listed in the header - field. - - 3. If multiple content-codings are acceptable, then the acceptable - content-coding with the highest non-zero qvalue is preferred. - - 4. The "identity" content-coding is always acceptable, unless - specifically refused because the Accept-Encoding field includes - "identity;q=0", or because the field includes "*;q=0" and does - not explicitly include the "identity" content-coding. If the - Accept-Encoding field-value is empty, then only the "identity" - encoding is acceptable. - - If an Accept-Encoding field is present in a request, and if the - server cannot send a response which is acceptable according to the - Accept-Encoding header, then the server SHOULD send an error response - with the 406 (Not Acceptable) status code. - - If no Accept-Encoding field is present in a request, the server MAY - assume that the client will accept any content coding. In this case, - if "identity" is one of the available content-codings, then the - server SHOULD use the "identity" content-coding, unless it has - additional information that a different content-coding is meaningful - to the client. - - Note: If the request does not include an Accept-Encoding field, - and if the "identity" content-coding is unavailable, then - content-codings commonly understood by HTTP/1.0 clients (i.e., - - - -Fielding, et al. Standards Track [Page 103] - -RFC 2616 HTTP/1.1 June 1999 - - - "gzip" and "compress") are preferred; some older clients - improperly display messages sent with other content-codings. The - server might also make this decision based on information about - the particular user-agent or client. - - Note: Most HTTP/1.0 applications do not recognize or obey qvalues - associated with content-codings. This means that qvalues will not - work and are not permitted with x-gzip or x-compress. - -14.4 Accept-Language - - The Accept-Language request-header field is similar to Accept, but - restricts the set of natural languages that are preferred as a - response to the request. Language tags are defined in section 3.10. - - Accept-Language = "Accept-Language" ":" - 1#( language-range [ ";" "q" "=" qvalue ] ) - language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) - - Each language-range MAY be given an associated quality value which - represents an estimate of the user's preference for the languages - specified by that range. The quality value defaults to "q=1". For - example, - - Accept-Language: da, en-gb;q=0.8, en;q=0.7 - - would mean: "I prefer Danish, but will accept British English and - other types of English." A language-range matches a language-tag if - it exactly equals the tag, or if it exactly equals a prefix of the - tag such that the first tag character following the prefix is "-". - The special range "*", if present in the Accept-Language field, - matches every tag not matched by any other range present in the - Accept-Language field. - - Note: This use of a prefix matching rule does not imply that - language tags are assigned to languages in such a way that it is - always true that if a user understands a language with a certain - tag, then this user will also understand all languages with tags - for which this tag is a prefix. The prefix rule simply allows the - use of prefix tags if this is the case. - - The language quality factor assigned to a language-tag by the - Accept-Language field is the quality value of the longest language- - range in the field that matches the language-tag. If no language- - range in the field matches the tag, the language quality factor - assigned is 0. If no Accept-Language header is present in the - request, the server - - - - -Fielding, et al. Standards Track [Page 104] - -RFC 2616 HTTP/1.1 June 1999 - - - SHOULD assume that all languages are equally acceptable. If an - Accept-Language header is present, then all languages which are - assigned a quality factor greater than 0 are acceptable. - - It might be contrary to the privacy expectations of the user to send - an Accept-Language header with the complete linguistic preferences of - the user in every request. For a discussion of this issue, see - section 15.1.4. - - As intelligibility is highly dependent on the individual user, it is - recommended that client applications make the choice of linguistic - preference available to the user. If the choice is not made - available, then the Accept-Language header field MUST NOT be given in - the request. - - Note: When making the choice of linguistic preference available to - the user, we remind implementors of the fact that users are not - familiar with the details of language matching as described above, - and should provide appropriate guidance. As an example, users - might assume that on selecting "en-gb", they will be served any - kind of English document if British English is not available. A - user agent might suggest in such a case to add "en" to get the - best matching behavior. - -14.5 Accept-Ranges - - The Accept-Ranges response-header field allows the server to - indicate its acceptance of range requests for a resource: - - Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges - acceptable-ranges = 1#range-unit | "none" - - Origin servers that accept byte-range requests MAY send - - Accept-Ranges: bytes - - but are not required to do so. Clients MAY generate byte-range - requests without having received this header for the resource - involved. Range units are defined in section 3.12. - - Servers that do not accept any kind of range request for a - resource MAY send - - Accept-Ranges: none - - to advise the client not to attempt a range request. - - - - - -Fielding, et al. Standards Track [Page 105] - -RFC 2616 HTTP/1.1 June 1999 - - -14.6 Age - - The Age response-header field conveys the sender's estimate of the - amount of time since the response (or its revalidation) was - generated at the origin server. A cached response is "fresh" if - its age does not exceed its freshness lifetime. Age values are - calculated as specified in section 13.2.3. - - Age = "Age" ":" age-value - age-value = delta-seconds - - Age values are non-negative decimal integers, representing time in - seconds. - - If a cache receives a value larger than the largest positive - integer it can represent, or if any of its age calculations - overflows, it MUST transmit an Age header with a value of - 2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST - include an Age header field in every response generated from its - own cache. Caches SHOULD use an arithmetic type of at least 31 - bits of range. - -14.7 Allow - - The Allow entity-header field lists the set of methods supported - by the resource identified by the Request-URI. The purpose of this - field is strictly to inform the recipient of valid methods - associated with the resource. An Allow header field MUST be - present in a 405 (Method Not Allowed) response. - - Allow = "Allow" ":" #Method - - Example of use: - - Allow: GET, HEAD, PUT - - This field cannot prevent a client from trying other methods. - However, the indications given by the Allow header field value - SHOULD be followed. The actual set of allowed methods is defined - by the origin server at the time of each request. - - The Allow header field MAY be provided with a PUT request to - recommend the methods to be supported by the new or modified - resource. The server is not required to support these methods and - SHOULD include an Allow header in the response giving the actual - supported methods. - - - - - -Fielding, et al. Standards Track [Page 106] - -RFC 2616 HTTP/1.1 June 1999 - - - A proxy MUST NOT modify the Allow header field even if it does not - understand all the methods specified, since the user agent might - have other means of communicating with the origin server. - -14.8 Authorization - - A user agent that wishes to authenticate itself with a server-- - usually, but not necessarily, after receiving a 401 response--does - so by including an Authorization request-header field with the - request. The Authorization field value consists of credentials - containing the authentication information of the user agent for - the realm of the resource being requested. - - Authorization = "Authorization" ":" credentials - - HTTP access authentication is described in "HTTP Authentication: - Basic and Digest Access Authentication" [43]. If a request is - authenticated and a realm specified, the same credentials SHOULD - be valid for all other requests within this realm (assuming that - the authentication scheme itself does not require otherwise, such - as credentials that vary according to a challenge value or using - synchronized clocks). - - When a shared cache (see section 13.7) receives a request - containing an Authorization field, it MUST NOT return the - corresponding response as a reply to any other request, unless one - of the following specific exceptions holds: - - 1. If the response includes the "s-maxage" cache-control - directive, the cache MAY use that response in replying to a - subsequent request. But (if the specified maximum age has - passed) a proxy cache MUST first revalidate it with the origin - server, using the request-headers from the new request to allow - the origin server to authenticate the new request. (This is the - defined behavior for s-maxage.) If the response includes "s- - maxage=0", the proxy MUST always revalidate it before re-using - it. - - 2. If the response includes the "must-revalidate" cache-control - directive, the cache MAY use that response in replying to a - subsequent request. But if the response is stale, all caches - MUST first revalidate it with the origin server, using the - request-headers from the new request to allow the origin server - to authenticate the new request. - - 3. If the response includes the "public" cache-control directive, - it MAY be returned in reply to any subsequent request. - - - - -Fielding, et al. Standards Track [Page 107] - -RFC 2616 HTTP/1.1 June 1999 - - -14.9 Cache-Control - - The Cache-Control general-header field is used to specify directives - that MUST be obeyed by all caching mechanisms along the - request/response chain. The directives specify behavior intended to - prevent caches from adversely interfering with the request or - response. These directives typically override the default caching - algorithms. Cache directives are unidirectional in that the presence - of a directive in a request does not imply that the same directive is - to be given in the response. - - Note that HTTP/1.0 caches might not implement Cache-Control and - might only implement Pragma: no-cache (see section 14.32). - - Cache directives MUST be passed through by a proxy or gateway - application, regardless of their significance to that application, - since the directives might be applicable to all recipients along the - request/response chain. It is not possible to specify a cache- - directive for a specific cache. - - Cache-Control = "Cache-Control" ":" 1#cache-directive - - cache-directive = cache-request-directive - | cache-response-directive - - cache-request-directive = - "no-cache" ; Section 14.9.1 - | "no-store" ; Section 14.9.2 - | "max-age" "=" delta-seconds ; Section 14.9.3, 14.9.4 - | "max-stale" [ "=" delta-seconds ] ; Section 14.9.3 - | "min-fresh" "=" delta-seconds ; Section 14.9.3 - | "no-transform" ; Section 14.9.5 - | "only-if-cached" ; Section 14.9.4 - | cache-extension ; Section 14.9.6 - - cache-response-directive = - "public" ; Section 14.9.1 - | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1 - | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1 - | "no-store" ; Section 14.9.2 - | "no-transform" ; Section 14.9.5 - | "must-revalidate" ; Section 14.9.4 - | "proxy-revalidate" ; Section 14.9.4 - | "max-age" "=" delta-seconds ; Section 14.9.3 - | "s-maxage" "=" delta-seconds ; Section 14.9.3 - | cache-extension ; Section 14.9.6 - - cache-extension = token [ "=" ( token | quoted-string ) ] - - - -Fielding, et al. Standards Track [Page 108] - -RFC 2616 HTTP/1.1 June 1999 - - - When a directive appears without any 1#field-name parameter, the - directive applies to the entire request or response. When such a - directive appears with a 1#field-name parameter, it applies only to - the named field or fields, and not to the rest of the request or - response. This mechanism supports extensibility; implementations of - future versions of the HTTP protocol might apply these directives to - header fields not defined in HTTP/1.1. - - The cache-control directives can be broken down into these general - categories: - - - Restrictions on what are cacheable; these may only be imposed by - the origin server. - - - Restrictions on what may be stored by a cache; these may be - imposed by either the origin server or the user agent. - - - Modifications of the basic expiration mechanism; these may be - imposed by either the origin server or the user agent. - - - Controls over cache revalidation and reload; these may only be - imposed by a user agent. - - - Control over transformation of entities. - - - Extensions to the caching system. - -14.9.1 What is Cacheable - - By default, a response is cacheable if the requirements of the - request method, request header fields, and the response status - indicate that it is cacheable. Section 13.4 summarizes these defaults - for cacheability. The following Cache-Control response directives - allow an origin server to override the default cacheability of a - response: - - public - Indicates that the response MAY be cached by any cache, even if it - would normally be non-cacheable or cacheable only within a non- - shared cache. (See also Authorization, section 14.8, for - additional details.) - - private - Indicates that all or part of the response message is intended for - a single user and MUST NOT be cached by a shared cache. This - allows an origin server to state that the specified parts of the - - - - - -Fielding, et al. Standards Track [Page 109] - -RFC 2616 HTTP/1.1 June 1999 - - - response are intended for only one user and are not a valid - response for requests by other users. A private (non-shared) cache - MAY cache the response. - - Note: This usage of the word private only controls where the - response may be cached, and cannot ensure the privacy of the - message content. - - no-cache - If the no-cache directive does not specify a field-name, then a - cache MUST NOT use the response to satisfy a subsequent request - without successful revalidation with the origin server. This - allows an origin server to prevent caching even by caches that - have been configured to return stale responses to client requests. - - If the no-cache directive does specify one or more field-names, - then a cache MAY use the response to satisfy a subsequent request, - subject to any other restrictions on caching. However, the - specified field-name(s) MUST NOT be sent in the response to a - subsequent request without successful revalidation with the origin - server. This allows an origin server to prevent the re-use of - certain header fields in a response, while still allowing caching - of the rest of the response. - - Note: Most HTTP/1.0 caches will not recognize or obey this - directive. - -14.9.2 What May be Stored by Caches - - no-store - The purpose of the no-store directive is to prevent the - inadvertent release or retention of sensitive information (for - example, on backup tapes). The no-store directive applies to the - entire message, and MAY be sent either in a response or in a - request. If sent in a request, a cache MUST NOT store any part of - either this request or any response to it. If sent in a response, - a cache MUST NOT store any part of either this response or the - request that elicited it. This directive applies to both non- - shared and shared caches. "MUST NOT store" in this context means - that the cache MUST NOT intentionally store the information in - non-volatile storage, and MUST make a best-effort attempt to - remove the information from volatile storage as promptly as - possible after forwarding it. - - Even when this directive is associated with a response, users - might explicitly store such a response outside of the caching - system (e.g., with a "Save As" dialog). History buffers MAY store - such responses as part of their normal operation. - - - -Fielding, et al. Standards Track [Page 110] - -RFC 2616 HTTP/1.1 June 1999 - - - The purpose of this directive is to meet the stated requirements - of certain users and service authors who are concerned about - accidental releases of information via unanticipated accesses to - cache data structures. While the use of this directive might - improve privacy in some cases, we caution that it is NOT in any - way a reliable or sufficient mechanism for ensuring privacy. In - particular, malicious or compromised caches might not recognize or - obey this directive, and communications networks might be - vulnerable to eavesdropping. - -14.9.3 Modifications of the Basic Expiration Mechanism - - The expiration time of an entity MAY be specified by the origin - server using the Expires header (see section 14.21). Alternatively, - it MAY be specified using the max-age directive in a response. When - the max-age cache-control directive is present in a cached response, - the response is stale if its current age is greater than the age - value given (in seconds) at the time of a new request for that - resource. The max-age directive on a response implies that the - response is cacheable (i.e., "public") unless some other, more - restrictive cache directive is also present. - - If a response includes both an Expires header and a max-age - directive, the max-age directive overrides the Expires header, even - if the Expires header is more restrictive. This rule allows an origin - server to provide, for a given response, a longer expiration time to - an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be - useful if certain HTTP/1.0 caches improperly calculate ages or - expiration times, perhaps due to desynchronized clocks. - - Many HTTP/1.0 cache implementations will treat an Expires value that - is less than or equal to the response Date value as being equivalent - to the Cache-Control response directive "no-cache". If an HTTP/1.1 - cache receives such a response, and the response does not include a - Cache-Control header field, it SHOULD consider the response to be - non-cacheable in order to retain compatibility with HTTP/1.0 servers. - - Note: An origin server might wish to use a relatively new HTTP - cache control feature, such as the "private" directive, on a - network including older caches that do not understand that - feature. The origin server will need to combine the new feature - with an Expires field whose value is less than or equal to the - Date value. This will prevent older caches from improperly - caching the response. - - - - - - - -Fielding, et al. Standards Track [Page 111] - -RFC 2616 HTTP/1.1 June 1999 - - - s-maxage - If a response includes an s-maxage directive, then for a shared - cache (but not for a private cache), the maximum age specified by - this directive overrides the maximum age specified by either the - max-age directive or the Expires header. The s-maxage directive - also implies the semantics of the proxy-revalidate directive (see - section 14.9.4), i.e., that the shared cache must not use the - entry after it becomes stale to respond to a subsequent request - without first revalidating it with the origin server. The s- - maxage directive is always ignored by a private cache. - - Note that most older caches, not compliant with this specification, - do not implement any cache-control directives. An origin server - wishing to use a cache-control directive that restricts, but does not - prevent, caching by an HTTP/1.1-compliant cache MAY exploit the - requirement that the max-age directive overrides the Expires header, - and the fact that pre-HTTP/1.1-compliant caches do not observe the - max-age directive. - - Other directives allow a user agent to modify the basic expiration - mechanism. These directives MAY be specified on a request: - - max-age - Indicates that the client is willing to accept a response whose - age is no greater than the specified time in seconds. Unless max- - stale directive is also included, the client is not willing to - accept a stale response. - - min-fresh - Indicates that the client is willing to accept a response whose - freshness lifetime is no less than its current age plus the - specified time in seconds. That is, the client wants a response - that will still be fresh for at least the specified number of - seconds. - - max-stale - Indicates that the client is willing to accept a response that has - exceeded its expiration time. If max-stale is assigned a value, - then the client is willing to accept a response that has exceeded - its expiration time by no more than the specified number of - seconds. If no value is assigned to max-stale, then the client is - willing to accept a stale response of any age. - - If a cache returns a stale response, either because of a max-stale - directive on a request, or because the cache is configured to - override the expiration time of a response, the cache MUST attach a - Warning header to the stale response, using Warning 110 (Response is - stale). - - - -Fielding, et al. Standards Track [Page 112] - -RFC 2616 HTTP/1.1 June 1999 - - - A cache MAY be configured to return stale responses without - validation, but only if this does not conflict with any "MUST"-level - requirements concerning cache validation (e.g., a "must-revalidate" - cache-control directive). - - If both the new request and the cached entry include "max-age" - directives, then the lesser of the two values is used for determining - the freshness of the cached entry for that request. - -14.9.4 Cache Revalidation and Reload Controls - - Sometimes a user agent might want or need to insist that a cache - revalidate its cache entry with the origin server (and not just with - the next cache along the path to the origin server), or to reload its - cache entry from the origin server. End-to-end revalidation might be - necessary if either the cache or the origin server has overestimated - the expiration time of the cached response. End-to-end reload may be - necessary if the cache entry has become corrupted for some reason. - - End-to-end revalidation may be requested either when the client does - not have its own local cached copy, in which case we call it - "unspecified end-to-end revalidation", or when the client does have a - local cached copy, in which case we call it "specific end-to-end - revalidation." - - The client can specify these three kinds of action using Cache- - Control request directives: - - End-to-end reload - The request includes a "no-cache" cache-control directive or, for - compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field - names MUST NOT be included with the no-cache directive in a - request. The server MUST NOT use a cached copy when responding to - such a request. - - Specific end-to-end revalidation - The request includes a "max-age=0" cache-control directive, which - forces each cache along the path to the origin server to - revalidate its own entry, if any, with the next cache or server. - The initial request includes a cache-validating conditional with - the client's current validator. - - Unspecified end-to-end revalidation - The request includes "max-age=0" cache-control directive, which - forces each cache along the path to the origin server to - revalidate its own entry, if any, with the next cache or server. - The initial request does not include a cache-validating - - - - -Fielding, et al. Standards Track [Page 113] - -RFC 2616 HTTP/1.1 June 1999 - - - conditional; the first cache along the path (if any) that holds a - cache entry for this resource includes a cache-validating - conditional with its current validator. - - max-age - When an intermediate cache is forced, by means of a max-age=0 - directive, to revalidate its own cache entry, and the client has - supplied its own validator in the request, the supplied validator - might differ from the validator currently stored with the cache - entry. In this case, the cache MAY use either validator in making - its own request without affecting semantic transparency. - - However, the choice of validator might affect performance. The - best approach is for the intermediate cache to use its own - validator when making its request. If the server replies with 304 - (Not Modified), then the cache can return its now validated copy - to the client with a 200 (OK) response. If the server replies with - a new entity and cache validator, however, the intermediate cache - can compare the returned validator with the one provided in the - client's request, using the strong comparison function. If the - client's validator is equal to the origin server's, then the - intermediate cache simply returns 304 (Not Modified). Otherwise, - it returns the new entity with a 200 (OK) response. - - If a request includes the no-cache directive, it SHOULD NOT - include min-fresh, max-stale, or max-age. - - only-if-cached - In some cases, such as times of extremely poor network - connectivity, a client may want a cache to return only those - responses that it currently has stored, and not to reload or - revalidate with the origin server. To do this, the client may - include the only-if-cached directive in a request. If it receives - this directive, a cache SHOULD either respond using a cached entry - that is consistent with the other constraints of the request, or - respond with a 504 (Gateway Timeout) status. However, if a group - of caches is being operated as a unified system with good internal - connectivity, such a request MAY be forwarded within that group of - caches. - - must-revalidate - Because a cache MAY be configured to ignore a server's specified - expiration time, and because a client request MAY include a max- - stale directive (which has a similar effect), the protocol also - includes a mechanism for the origin server to require revalidation - of a cache entry on any subsequent use. When the must-revalidate - directive is present in a response received by a cache, that cache - MUST NOT use the entry after it becomes stale to respond to a - - - -Fielding, et al. Standards Track [Page 114] - -RFC 2616 HTTP/1.1 June 1999 - - - subsequent request without first revalidating it with the origin - server. (I.e., the cache MUST do an end-to-end revalidation every - time, if, based solely on the origin server's Expires or max-age - value, the cached response is stale.) - - The must-revalidate directive is necessary to support reliable - operation for certain protocol features. In all circumstances an - HTTP/1.1 cache MUST obey the must-revalidate directive; in - particular, if the cache cannot reach the origin server for any - reason, it MUST generate a 504 (Gateway Timeout) response. - - Servers SHOULD send the must-revalidate directive if and only if - failure to revalidate a request on the entity could result in - incorrect operation, such as a silently unexecuted financial - transaction. Recipients MUST NOT take any automated action that - violates this directive, and MUST NOT automatically provide an - unvalidated copy of the entity if revalidation fails. - - Although this is not recommended, user agents operating under - severe connectivity constraints MAY violate this directive but, if - so, MUST explicitly warn the user that an unvalidated response has - been provided. The warning MUST be provided on each unvalidated - access, and SHOULD require explicit user confirmation. - - proxy-revalidate - The proxy-revalidate directive has the same meaning as the must- - revalidate directive, except that it does not apply to non-shared - user agent caches. It can be used on a response to an - authenticated request to permit the user's cache to store and - later return the response without needing to revalidate it (since - it has already been authenticated once by that user), while still - requiring proxies that service many users to revalidate each time - (in order to make sure that each user has been authenticated). - Note that such authenticated responses also need the public cache - control directive in order to allow them to be cached at all. - -14.9.5 No-Transform Directive - - no-transform - Implementors of intermediate caches (proxies) have found it useful - to convert the media type of certain entity bodies. A non- - transparent proxy might, for example, convert between image - formats in order to save cache space or to reduce the amount of - traffic on a slow link. - - Serious operational problems occur, however, when these - transformations are applied to entity bodies intended for certain - kinds of applications. For example, applications for medical - - - -Fielding, et al. Standards Track [Page 115] - -RFC 2616 HTTP/1.1 June 1999 - - - imaging, scientific data analysis and those using end-to-end - authentication, all depend on receiving an entity body that is bit - for bit identical to the original entity-body. - - Therefore, if a message includes the no-transform directive, an - intermediate cache or proxy MUST NOT change those headers that are - listed in section 13.5.2 as being subject to the no-transform - directive. This implies that the cache or proxy MUST NOT change - any aspect of the entity-body that is specified by these headers, - including the value of the entity-body itself. - -14.9.6 Cache Control Extensions - - The Cache-Control header field can be extended through the use of one - or more cache-extension tokens, each with an optional assigned value. - Informational extensions (those which do not require a change in - cache behavior) MAY be added without changing the semantics of other - directives. Behavioral extensions are designed to work by acting as - modifiers to the existing base of cache directives. Both the new - directive and the standard directive are supplied, such that - applications which do not understand the new directive will default - to the behavior specified by the standard directive, and those that - understand the new directive will recognize it as modifying the - requirements associated with the standard directive. In this way, - extensions to the cache-control directives can be made without - requiring changes to the base protocol. - - This extension mechanism depends on an HTTP cache obeying all of the - cache-control directives defined for its native HTTP-version, obeying - certain extensions, and ignoring all directives that it does not - understand. - - For example, consider a hypothetical new response directive called - community which acts as a modifier to the private directive. We - define this new directive to mean that, in addition to any non-shared - cache, any cache which is shared only by members of the community - named within its value may cache the response. An origin server - wishing to allow the UCI community to use an otherwise private - response in their shared cache(s) could do so by including - - Cache-Control: private, community="UCI" - - A cache seeing this header field will act correctly even if the cache - does not understand the community cache-extension, since it will also - see and understand the private directive and thus default to the safe - behavior. - - - - - -Fielding, et al. Standards Track [Page 116] - -RFC 2616 HTTP/1.1 June 1999 - - - Unrecognized cache-directives MUST be ignored; it is assumed that any - cache-directive likely to be unrecognized by an HTTP/1.1 cache will - be combined with standard directives (or the response's default - cacheability) such that the cache behavior will remain minimally - correct even if the cache does not understand the extension(s). - -14.10 Connection - - The Connection general-header field allows the sender to specify - options that are desired for that particular connection and MUST NOT - be communicated by proxies over further connections. - - The Connection header has the following grammar: - - Connection = "Connection" ":" 1#(connection-token) - connection-token = token - - HTTP/1.1 proxies MUST parse the Connection header field before a - message is forwarded and, for each connection-token in this field, - remove any header field(s) from the message with the same name as the - connection-token. Connection options are signaled by the presence of - a connection-token in the Connection header field, not by any - corresponding additional header field(s), since the additional header - field may not be sent if there are no parameters associated with that - connection option. - - Message headers listed in the Connection header MUST NOT include - end-to-end headers, such as Cache-Control. - - HTTP/1.1 defines the "close" connection option for the sender to - signal that the 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 connection SHOULD NOT be considered `persistent' (section 8.1) - after the current request/response is complete. - - HTTP/1.1 applications that do not support persistent connections MUST - include the "close" connection option in every message. - - A system receiving an HTTP/1.0 (or lower-version) message that - includes a Connection header MUST, for each connection-token in this - field, remove and ignore any header field(s) from the message with - the same name as the connection-token. This protects against mistaken - forwarding of such header fields by pre-HTTP/1.1 proxies. See section - 19.6.2. - - - -Fielding, et al. Standards Track [Page 117] - -RFC 2616 HTTP/1.1 June 1999 - - -14.11 Content-Encoding - - The Content-Encoding entity-header field is used as a modifier to the - media-type. When present, its value indicates what additional content - codings have been applied to the entity-body, and thus what decoding - mechanisms must be applied in order to obtain the media-type - referenced by the Content-Type header field. Content-Encoding is - primarily used to allow a document to be compressed without losing - the identity of its underlying media type. - - Content-Encoding = "Content-Encoding" ":" 1#content-coding - - Content codings are defined in section 3.5. An example of its use is - - Content-Encoding: gzip - - The content-coding is a characteristic of the entity identified by - the Request-URI. Typically, the entity-body is stored with this - encoding and is only decoded before rendering or analogous usage. - However, a non-transparent proxy MAY modify the content-coding if the - new coding is known to be acceptable to the recipient, unless the - "no-transform" cache-control directive is present in the message. - - If the content-coding of an entity is not "identity", then the - response MUST include a Content-Encoding entity-header (section - 14.11) that lists the non-identity content-coding(s) used. - - If the content-coding of an entity in a request message is not - acceptable to the origin server, the server SHOULD respond with a - status code of 415 (Unsupported Media Type). - - If multiple encodings have been applied to an entity, the content - codings MUST be listed in the order in which they were applied. - Additional information about the encoding parameters MAY be provided - by other entity-header fields not defined by this specification. - -14.12 Content-Language - - The Content-Language entity-header field describes the natural - language(s) of the intended audience for the enclosed entity. Note - that this might not be equivalent to all the languages used within - the entity-body. - - Content-Language = "Content-Language" ":" 1#language-tag - - - - - - - -Fielding, et al. Standards Track [Page 118] - -RFC 2616 HTTP/1.1 June 1999 - - - Language tags are defined in section 3.10. The primary purpose of - Content-Language is to allow a user to identify and differentiate - entities according to the user's own preferred language. Thus, if the - body content is intended only for a Danish-literate audience, the - appropriate field is - - Content-Language: da - - If no Content-Language is specified, the default is that the content - is intended for all language audiences. This might mean that the - sender does not consider it to be specific to any natural language, - or that the sender does not know for which language it is intended. - - Multiple languages MAY be listed for content that is intended for - multiple audiences. For example, a rendition of the "Treaty of - Waitangi," presented simultaneously in the original Maori and English - versions, would call for - - Content-Language: mi, en - - However, just because multiple languages are present within an entity - does not mean that it is intended for multiple linguistic audiences. - An example would be a beginner's language primer, such as "A First - Lesson in Latin," which is clearly intended to be used by an - English-literate audience. In this case, the Content-Language would - properly only include "en". - - Content-Language MAY be applied to any media type -- it is not - limited to textual documents. - -14.13 Content-Length - - The Content-Length entity-header field indicates the size of the - entity-body, in decimal number of OCTETs, sent to the recipient or, - in the case of the HEAD method, the size of the entity-body that - would have been sent had the request been a GET. - - Content-Length = "Content-Length" ":" 1*DIGIT - - An example is - - Content-Length: 3495 - - Applications SHOULD use this field to indicate the transfer-length of - the message-body, unless this is prohibited by the rules in section - 4.4. - - - - - -Fielding, et al. Standards Track [Page 119] - -RFC 2616 HTTP/1.1 June 1999 - - - Any Content-Length greater than or equal to zero is a valid value. - Section 4.4 describes how to determine the length of a message-body - if a Content-Length is not given. - - Note that the meaning of this field is significantly different from - the corresponding definition in MIME, where it is an optional field - used within the "message/external-body" content-type. In HTTP, it - SHOULD be sent whenever the message's length can be determined prior - to being transferred, unless this is prohibited by the rules in - section 4.4. - -14.14 Content-Location - - The Content-Location entity-header field MAY be used to supply the - resource location for the entity enclosed in the message when that - entity is accessible from a location separate from the requested - resource's URI. A server SHOULD provide a Content-Location for the - variant corresponding to the response entity; especially in the case - where a resource has multiple entities associated with it, and those - entities actually have separate locations by which they might be - individually accessed, the server SHOULD provide a Content-Location - for the particular variant which is returned. - - Content-Location = "Content-Location" ":" - ( absoluteURI | relativeURI ) - - The value of Content-Location also defines the base URI for the - entity. - - The Content-Location value is not a replacement for the original - requested URI; it is only a statement of the location of the resource - corresponding to this particular entity at the time of the request. - Future requests MAY specify the Content-Location URI as the request- - URI if the desire is to identify the source of that particular - entity. - - A cache cannot assume that an entity with a Content-Location - different from the URI used to retrieve it can be used to respond to - later requests on that Content-Location URI. However, the Content- - Location can be used to differentiate between multiple entities - retrieved from a single requested resource, as described in section - 13.6. - - If the Content-Location is a relative URI, the relative URI is - interpreted relative to the Request-URI. - - The meaning of the Content-Location header in PUT or POST requests is - undefined; servers are free to ignore it in those cases. - - - -Fielding, et al. Standards Track [Page 120] - -RFC 2616 HTTP/1.1 June 1999 - - -14.15 Content-MD5 - - The Content-MD5 entity-header field, as defined in RFC 1864 [23], is - an MD5 digest of the entity-body for the purpose of providing an - end-to-end message integrity check (MIC) of the entity-body. (Note: a - MIC is good for detecting accidental modification of the entity-body - in transit, but is not proof against malicious attacks.) - - Content-MD5 = "Content-MD5" ":" md5-digest - md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864> - - The Content-MD5 header field MAY be generated by an origin server or - client to function as an integrity check of the entity-body. Only - origin servers or clients MAY generate the Content-MD5 header field; - proxies and gateways MUST NOT generate it, as this would defeat its - value as an end-to-end integrity check. Any recipient of the entity- - body, including gateways and proxies, MAY check that the digest value - in this header field matches that of the entity-body as received. - - The MD5 digest is computed based on the content of the entity-body, - including any content-coding that has been applied, but not including - any transfer-encoding applied to the message-body. If the message is - received with a transfer-encoding, that encoding MUST be removed - prior to checking the Content-MD5 value against the received entity. - - This has the result that the digest is computed on the octets of the - entity-body exactly as, and in the order that, they would be sent if - no transfer-encoding were being applied. - - HTTP extends RFC 1864 to permit the digest to be computed for MIME - composite media-types (e.g., multipart/* and message/rfc822), but - this does not change how the digest is computed as defined in the - preceding paragraph. - - There are several consequences of this. The entity-body for composite - types MAY contain many body-parts, each with its own MIME and HTTP - headers (including Content-MD5, Content-Transfer-Encoding, and - Content-Encoding headers). If a body-part has a Content-Transfer- - Encoding or Content-Encoding header, it is assumed that the content - of the body-part has had the encoding applied, and the body-part is - included in the Content-MD5 digest as is -- i.e., after the - application. The Transfer-Encoding header field is not allowed within - body-parts. - - Conversion of all line breaks to CRLF MUST NOT be done before - computing or checking the digest: the line break convention used in - the text actually transmitted MUST be left unaltered when computing - the digest. - - - -Fielding, et al. Standards Track [Page 121] - -RFC 2616 HTTP/1.1 June 1999 - - - Note: while the definition of Content-MD5 is exactly the same for - HTTP as in RFC 1864 for MIME entity-bodies, there are several ways - in which the application of Content-MD5 to HTTP entity-bodies - differs from its application to MIME entity-bodies. One is that - HTTP, unlike MIME, does not use Content-Transfer-Encoding, and - does use Transfer-Encoding and Content-Encoding. Another is that - HTTP more frequently uses binary content types than MIME, so it is - worth noting that, in such cases, the byte order used to compute - the digest is the transmission byte order defined for the type. - Lastly, HTTP allows transmission of text types with any of several - line break conventions and not just the canonical form using CRLF. - -14.16 Content-Range - - The Content-Range entity-header is sent with a partial entity-body to - specify where in the full entity-body the partial body should be - applied. Range units are defined in section 3.12. - - Content-Range = "Content-Range" ":" content-range-spec - - content-range-spec = byte-content-range-spec - byte-content-range-spec = bytes-unit SP - byte-range-resp-spec "/" - ( instance-length | "*" ) - - byte-range-resp-spec = (first-byte-pos "-" last-byte-pos) - | "*" - instance-length = 1*DIGIT - - The header SHOULD indicate the total length of the full entity-body, - unless this length is unknown or difficult to determine. The asterisk - "*" character means that the instance-length is unknown at the time - when the response was generated. - - Unlike byte-ranges-specifier values (see section 14.35.1), a byte- - range-resp-spec MUST only specify one range, and MUST contain - absolute byte positions for both the first and last byte of the - range. - - A byte-content-range-spec with a byte-range-resp-spec whose last- - byte-pos value is less than its first-byte-pos value, or whose - instance-length value is less than or equal to its last-byte-pos - value, is invalid. The recipient of an invalid byte-content-range- - spec MUST ignore it and any content transferred along with it. - - A server sending a response with status code 416 (Requested range not - satisfiable) SHOULD include a Content-Range field with a byte-range- - resp-spec of "*". The instance-length specifies the current length of - - - -Fielding, et al. Standards Track [Page 122] - -RFC 2616 HTTP/1.1 June 1999 - - - the selected resource. A response with status code 206 (Partial - Content) MUST NOT include a Content-Range field with a byte-range- - resp-spec of "*". - - Examples of byte-content-range-spec values, assuming that the entity - contains a total of 1234 bytes: - - . The first 500 bytes: - bytes 0-499/1234 - - . The second 500 bytes: - bytes 500-999/1234 - - . All except for the first 500 bytes: - bytes 500-1233/1234 - - . The last 500 bytes: - bytes 734-1233/1234 - - When an HTTP message includes the content of a single range (for - example, a response to a request for a single range, or to a request - for a set of ranges that overlap without any holes), this content is - transmitted with a Content-Range header, and a Content-Length header - showing the number of bytes actually transferred. For example, - - HTTP/1.1 206 Partial content - Date: Wed, 15 Nov 1995 06:25:24 GMT - Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT - Content-Range: bytes 21010-47021/47022 - Content-Length: 26012 - Content-Type: image/gif - - When an HTTP message includes the content of multiple ranges (for - example, a response to a request for multiple non-overlapping - ranges), these are transmitted as a multipart message. The multipart - media type used for this purpose is "multipart/byteranges" as defined - in appendix 19.2. See appendix 19.6.3 for a compatibility issue. - - A response to a request for a single range MUST NOT be sent using the - multipart/byteranges media type. A response to a request for - multiple ranges, whose result is a single range, MAY be sent as a - multipart/byteranges media type with one part. A client that cannot - decode a multipart/byteranges message MUST NOT ask for multiple - byte-ranges in a single request. - - When a client requests multiple byte-ranges in one request, the - server SHOULD return them in the order that they appeared in the - request. - - - -Fielding, et al. Standards Track [Page 123] - -RFC 2616 HTTP/1.1 June 1999 - - - If the server ignores a byte-range-spec because it is syntactically - invalid, the server SHOULD treat the request as if the invalid Range - header field did not exist. (Normally, this means return a 200 - response containing the full entity). - - If the server receives a request (other than one including an If- - Range request-header field) with an unsatisfiable Range request- - header field (that is, all of whose byte-range-spec values have a - first-byte-pos value greater than the current length of the selected - resource), it SHOULD return a response code of 416 (Requested range - not satisfiable) (section 10.4.17). - - Note: clients cannot depend on servers to send a 416 (Requested - range not satisfiable) response instead of a 200 (OK) response for - an unsatisfiable Range request-header, since not all servers - implement this request-header. - -14.17 Content-Type - - The Content-Type entity-header field indicates the media type of the - entity-body sent to the recipient or, in the case of the HEAD method, - the media type that would have been sent had the request been a GET. - - Content-Type = "Content-Type" ":" media-type - - Media types are defined in section 3.7. An example of the field is - - Content-Type: text/html; charset=ISO-8859-4 - - Further discussion of methods for identifying the media type of an - entity is provided in section 7.2.1. - -14.18 Date - - The Date general-header field represents the date and time at which - the message was originated, having the same semantics as orig-date in - RFC 822. The field value is an HTTP-date, as described in section - 3.3.1; it MUST be sent in RFC 1123 [8]-date format. - - Date = "Date" ":" HTTP-date - - An example is - - Date: Tue, 15 Nov 1994 08:12:31 GMT - - Origin servers MUST include a Date header field in all responses, - except in these cases: - - - - -Fielding, et al. Standards Track [Page 124] - -RFC 2616 HTTP/1.1 June 1999 - - - 1. If the response status code is 100 (Continue) or 101 (Switching - Protocols), the response MAY include a Date header field, at - the server's option. - - 2. If the response status code conveys a server error, e.g. 500 - (Internal Server Error) or 503 (Service Unavailable), and it is - inconvenient or impossible to generate a valid Date. - - 3. If the server does not have a clock that can provide a - reasonable approximation of the current time, its responses - MUST NOT include a Date header field. In this case, the rules - in section 14.18.1 MUST be followed. - - A received message that does not have a Date header field MUST be - assigned one by the recipient if the message will be cached by that - recipient or gatewayed via a protocol which requires a Date. An HTTP - implementation without a clock MUST NOT cache responses without - revalidating them on every use. An HTTP cache, especially a shared - cache, SHOULD use a mechanism, such as NTP [28], to synchronize its - clock with a reliable external standard. - - Clients SHOULD only send a Date header field in messages that include - an entity-body, as in the case of the PUT and POST requests, and even - then it is optional. A client without a clock MUST NOT send a Date - header field in a request. - - The HTTP-date sent in a Date header SHOULD NOT represent a date and - time subsequent to the generation of the message. It SHOULD represent - the best available approximation of the date and time of message - generation, unless the implementation has no means of generating a - reasonably accurate date and time. In theory, the date ought to - represent the moment just before the entity is generated. In - practice, the date can be generated at any time during the message - origination without affecting its semantic value. - -14.18.1 Clockless Origin Server Operation - - Some origin server implementations might not have a clock available. - An origin server without a clock MUST NOT assign Expires or Last- - Modified values to a response, unless these values were associated - with the resource by a system or user with a reliable clock. It MAY - assign an Expires value that is known, at or before server - configuration time, to be in the past (this allows "pre-expiration" - of responses without storing separate Expires values for each - resource). - - - - - - -Fielding, et al. Standards Track [Page 125] - -RFC 2616 HTTP/1.1 June 1999 - - -14.19 ETag - - The ETag response-header field provides the current value of the - entity tag for the requested variant. The headers used with entity - tags are described in sections 14.24, 14.26 and 14.44. The entity tag - MAY be used for comparison with other entities from the same resource - (see section 13.3.3). - - ETag = "ETag" ":" entity-tag - - Examples: - - ETag: "xyzzy" - ETag: W/"xyzzy" - ETag: "" - -14.20 Expect - - The Expect request-header field is used to indicate that particular - server behaviors are required by the client. - - Expect = "Expect" ":" 1#expectation - - expectation = "100-continue" | expectation-extension - expectation-extension = token [ "=" ( token | quoted-string ) - *expect-params ] - expect-params = ";" token [ "=" ( token | quoted-string ) ] - - - A server that does not understand or is unable to comply with any of - the expectation values in the Expect field of a request MUST respond - with appropriate error status. The server MUST respond with a 417 - (Expectation Failed) status if any of the expectations cannot be met - or, if there are other problems with the request, some other 4xx - status. - - This header field is defined with extensible syntax to allow for - future extensions. If a server receives a request containing an - Expect field that includes an expectation-extension that it does not - support, it MUST respond with a 417 (Expectation Failed) status. - - Comparison of expectation values is case-insensitive for unquoted - tokens (including the 100-continue token), and is case-sensitive for - quoted-string expectation-extensions. - - - - - - - -Fielding, et al. Standards Track [Page 126] - -RFC 2616 HTTP/1.1 June 1999 - - - The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST - return a 417 (Expectation Failed) status if it receives a request - with an expectation that it cannot meet. However, the Expect - request-header itself is end-to-end; it MUST be forwarded if the - request is forwarded. - - Many older HTTP/1.0 and HTTP/1.1 applications do not understand the - Expect header. - - See section 8.2.3 for the use of the 100 (continue) status. - -14.21 Expires - - The Expires entity-header field gives the date/time after which the - response is considered stale. A stale cache entry may not normally be - returned by a cache (either a proxy cache or a user agent cache) - unless it is first validated with the origin server (or with an - intermediate cache that has a fresh copy of the entity). See section - 13.2 for further discussion of the expiration model. - - The presence of an Expires field does not imply that the original - resource will change or cease to exist at, before, or after that - time. - - The format is an absolute date and time as defined by HTTP-date in - section 3.3.1; it MUST be in RFC 1123 date format: - - Expires = "Expires" ":" HTTP-date - - An example of its use is - - Expires: Thu, 01 Dec 1994 16:00:00 GMT - - Note: if a response includes a Cache-Control field with the max- - age directive (see section 14.9.3), that directive overrides the - Expires field. - - HTTP/1.1 clients and caches MUST treat other invalid date formats, - especially including the value "0", as in the past (i.e., "already - expired"). - - To mark a response as "already expired," an origin server sends an - Expires date that is equal to the Date header value. (See the rules - for expiration calculations in section 13.2.4.) - - - - - - - -Fielding, et al. Standards Track [Page 127] - -RFC 2616 HTTP/1.1 June 1999 - - - To mark a response as "never expires," an origin server sends an - Expires date approximately one year from the time the response is - sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one - year in the future. - - The presence of an Expires header field with a date value of some - time in the future on a response that otherwise would by default be - non-cacheable indicates that the response is cacheable, unless - indicated otherwise by a Cache-Control header field (section 14.9). - -14.22 From - - The From request-header field, if given, SHOULD contain an Internet - e-mail address for the human user who controls the requesting user - agent. The address SHOULD be machine-usable, as defined by "mailbox" - in RFC 822 [9] as updated by RFC 1123 [8]: - - From = "From" ":" mailbox - - An example is: - - From: webmaster@w3.org - - This header field MAY be used for logging purposes and as a means for - identifying the source of invalid or unwanted requests. It SHOULD NOT - be used as an insecure form of access protection. The interpretation - of this field is that the request is being performed on behalf of the - person given, who accepts responsibility for the method performed. In - particular, robot agents SHOULD include this header so that the - person responsible for running the robot can be contacted if problems - occur on the receiving end. - - The Internet e-mail address in this field MAY be separate from the - Internet host which issued the request. For example, when a request - is passed through a proxy the original issuer's address SHOULD be - used. - - The client SHOULD NOT send the From header field without the user's - approval, as it might conflict with the user's privacy interests or - their site's security policy. It is strongly recommended that the - user be able to disable, enable, and modify the value of this field - at any time prior to a request. - -14.23 Host - - The Host request-header field specifies the Internet host and port - number of the resource being requested, as obtained from the original - URI given by the user or referring resource (generally an HTTP URL, - - - -Fielding, et al. Standards Track [Page 128] - -RFC 2616 HTTP/1.1 June 1999 - - - as described in section 3.2.2). The Host field value MUST represent - the naming authority of the origin server or gateway given by the - original URL. This allows the origin server or gateway to - differentiate between internally-ambiguous URLs, such as the root "/" - URL of a server for multiple host names on a single IP address. - - Host = "Host" ":" host [ ":" port ] ; Section 3.2.2 - - A "host" without any trailing port information implies the default - port for the service requested (e.g., "80" for an HTTP URL). For - example, a request on the origin server for - <http://www.w3.org/pub/WWW/> would properly include: - - GET /pub/WWW/ HTTP/1.1 - Host: www.w3.org - - A client MUST include a Host header field in all HTTP/1.1 request - messages . If the requested URI does not include an Internet host - name for the service being requested, then the Host header field MUST - be given with an empty value. An HTTP/1.1 proxy MUST ensure that any - request message it forwards does contain an appropriate Host header - field that identifies the service being requested by the proxy. All - Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request) - status code to any HTTP/1.1 request message which lacks a Host header - field. - - See sections 5.2 and 19.6.1.1 for other requirements relating to - Host. - -14.24 If-Match - - The If-Match request-header field is used with a method to make it - conditional. A client that has one or more entities previously - obtained from the resource can verify that one of those entities is - current by including a list of their associated entity tags in the - If-Match header field. Entity tags are defined in section 3.11. The - purpose of this feature is to allow efficient updates of cached - information with a minimum amount of transaction overhead. It is also - used, on updating requests, to prevent inadvertent modification of - the wrong version of a resource. As a special case, the value "*" - matches any current entity of the resource. - - If-Match = "If-Match" ":" ( "*" | 1#entity-tag ) - - If any of the entity tags match the entity tag of the entity that - would have been returned in the response to a similar GET request - (without the If-Match header) on that resource, or if "*" is given - - - - -Fielding, et al. Standards Track [Page 129] - -RFC 2616 HTTP/1.1 June 1999 - - - and any current entity exists for that resource, then the server MAY - perform the requested method as if the If-Match header field did not - exist. - - A server MUST use the strong comparison function (see section 13.3.3) - to compare the entity tags in If-Match. - - If none of the entity tags match, or if "*" is given and no current - entity exists, the server MUST NOT perform the requested method, and - MUST return a 412 (Precondition Failed) response. This behavior is - most useful when the client wants to prevent an updating method, such - as PUT, from modifying a resource that has changed since the client - last retrieved it. - - If the request would, without the If-Match header field, result in - anything other than a 2xx or 412 status, then the If-Match header - MUST be ignored. - - The meaning of "If-Match: *" is that the method SHOULD be performed - if the representation selected by the origin server (or by a cache, - possibly using the Vary mechanism, see section 14.44) exists, and - MUST NOT be performed if the representation does not exist. - - A request intended to update a resource (e.g., a PUT) MAY include an - If-Match header field to signal that the request method MUST NOT be - applied if the entity corresponding to the If-Match value (a single - entity tag) is no longer a representation of that resource. This - allows the user to indicate that they do not wish the request to be - successful if the resource has been changed without their knowledge. - Examples: - - If-Match: "xyzzy" - If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" - If-Match: * - - The result of a request having both an If-Match header field and - either an If-None-Match or an If-Modified-Since header fields is - undefined by this specification. - -14.25 If-Modified-Since - - The If-Modified-Since request-header field is used with a method to - make it conditional: if the requested variant has not been modified - since the time specified in this field, an entity will not be - returned from the server; instead, a 304 (not modified) response will - be returned without any message-body. - - If-Modified-Since = "If-Modified-Since" ":" HTTP-date - - - -Fielding, et al. Standards Track [Page 130] - -RFC 2616 HTTP/1.1 June 1999 - - - An example of the field is: - - If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT - - A GET method with an If-Modified-Since header and no Range header - requests that the identified entity be transferred only if it has - been modified since the date given by the If-Modified-Since header. - The algorithm for determining this includes the following cases: - - a) If the request would normally result in anything other than a - 200 (OK) status, or if the passed If-Modified-Since date is - invalid, the response is exactly the same as for a normal GET. - A date which is later than the server's current time is - invalid. - - b) If the variant has been modified since the If-Modified-Since - date, the response is exactly the same as for a normal GET. - - c) If the variant has not been modified since a valid If- - Modified-Since date, the server SHOULD return a 304 (Not - Modified) response. - - The purpose of this feature is to allow efficient updates of cached - information with a minimum amount of transaction overhead. - - Note: The Range request-header field modifies the meaning of If- - Modified-Since; see section 14.35 for full details. - - Note: If-Modified-Since times are interpreted by the server, whose - clock might not be synchronized with the client. - - Note: When handling an If-Modified-Since header field, some - servers will use an exact date comparison function, rather than a - less-than function, for deciding whether to send a 304 (Not - Modified) response. To get best results when sending an If- - Modified-Since header field for cache validation, clients are - advised to use the exact date string received in a previous Last- - Modified header field whenever possible. - - Note: If a client uses an arbitrary date in the If-Modified-Since - header instead of a date taken from the Last-Modified header for - the same request, the client should be aware of the fact that this - date is interpreted in the server's understanding of time. The - client should consider unsynchronized clocks and rounding problems - due to the different encodings of time between the client and - server. This includes the possibility of race conditions if the - document has changed between the time it was first requested and - the If-Modified-Since date of a subsequent request, and the - - - -Fielding, et al. Standards Track [Page 131] - -RFC 2616 HTTP/1.1 June 1999 - - - possibility of clock-skew-related problems if the If-Modified- - Since date is derived from the client's clock without correction - to the server's clock. Corrections for different time bases - between client and server are at best approximate due to network - latency. - - The result of a request having both an If-Modified-Since header field - and either an If-Match or an If-Unmodified-Since header fields is - undefined by this specification. - -14.26 If-None-Match - - The If-None-Match request-header field is used with a method to make - it conditional. A client that has one or more entities previously - obtained from the resource can verify that none of those entities is - current by including a list of their associated entity tags in the - If-None-Match header field. The purpose of this feature is to allow - efficient updates of cached information with a minimum amount of - transaction overhead. It is also used to prevent a method (e.g. PUT) - from inadvertently modifying an existing resource when the client - believes that the resource does not exist. - - As a special case, the value "*" matches any current entity of the - resource. - - If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag ) - - If any of the entity tags match the entity tag of the entity that - would have been returned in the response to a similar GET request - (without the If-None-Match header) on that resource, or if "*" is - given and any current entity exists for that resource, then the - server MUST NOT perform the requested method, unless required to do - so because the resource's modification date fails to match that - supplied in an If-Modified-Since header field in the request. - Instead, if the request method was GET or HEAD, the server SHOULD - respond with a 304 (Not Modified) response, including the cache- - related header fields (particularly ETag) of one of the entities that - matched. For all other request methods, the server MUST respond with - a status of 412 (Precondition Failed). - - See section 13.3.3 for rules on how to determine if two entities tags - match. The weak comparison function can only be used with GET or HEAD - requests. - - - - - - - - -Fielding, et al. Standards Track [Page 132] - -RFC 2616 HTTP/1.1 June 1999 - - - If none of the entity tags match, then the server MAY perform the - requested method as if the If-None-Match header field did not exist, - but MUST also ignore any If-Modified-Since header field(s) in the - request. That is, if no entity tags match, then the server MUST NOT - return a 304 (Not Modified) response. - - If the request would, without the If-None-Match header field, result - in anything other than a 2xx or 304 status, then the If-None-Match - header MUST be ignored. (See section 13.3.4 for a discussion of - server behavior when both If-Modified-Since and If-None-Match appear - in the same request.) - - The meaning of "If-None-Match: *" is that the method MUST NOT be - performed if the representation selected by the origin server (or by - a cache, possibly using the Vary mechanism, see section 14.44) - exists, and SHOULD be performed if the representation does not exist. - This feature is intended to be useful in preventing races between PUT - operations. - - Examples: - - If-None-Match: "xyzzy" - If-None-Match: W/"xyzzy" - If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" - If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" - If-None-Match: * - - The result of a request having both an If-None-Match header field and - either an If-Match or an If-Unmodified-Since header fields is - undefined by this specification. - -14.27 If-Range - - If a client has a partial copy of an entity in its cache, and wishes - to have an up-to-date copy of the entire entity in its cache, it - could use the Range request-header with a conditional GET (using - either or both of If-Unmodified-Since and If-Match.) However, if the - condition fails because the entity has been modified, the client - would then have to make a second request to obtain the entire current - entity-body. - - The If-Range header allows a client to "short-circuit" the second - request. Informally, its meaning is `if the entity is unchanged, send - me the part(s) that I am missing; otherwise, send me the entire new - entity'. - - If-Range = "If-Range" ":" ( entity-tag | HTTP-date ) - - - - -Fielding, et al. Standards Track [Page 133] - -RFC 2616 HTTP/1.1 June 1999 - - - If the client has no entity tag for an entity, but does have a Last- - Modified date, it MAY use that date in an If-Range header. (The - server can distinguish between a valid HTTP-date and any form of - entity-tag by examining no more than two characters.) The If-Range - header SHOULD only be used together with a Range header, and MUST be - ignored if the request does not include a Range header, or if the - server does not support the sub-range operation. - - If the entity tag given in the If-Range header matches the current - entity tag for the entity, then the server SHOULD provide the - specified sub-range of the entity using a 206 (Partial content) - response. If the entity tag does not match, then the server SHOULD - return the entire entity using a 200 (OK) response. - -14.28 If-Unmodified-Since - - The If-Unmodified-Since request-header field is used with a method to - make it conditional. If the requested resource has not been modified - since the time specified in this field, the server SHOULD perform the - requested operation as if the If-Unmodified-Since header were not - present. - - If the requested variant has been modified since the specified time, - the server MUST NOT perform the requested operation, and MUST return - a 412 (Precondition Failed). - - If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date - - An example of the field is: - - If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT - - If the request normally (i.e., without the If-Unmodified-Since - header) would result in anything other than a 2xx or 412 status, the - If-Unmodified-Since header SHOULD be ignored. - - If the specified date is invalid, the header is ignored. - - The result of a request having both an If-Unmodified-Since header - field and either an If-None-Match or an If-Modified-Since header - fields is undefined by this specification. - -14.29 Last-Modified - - The Last-Modified entity-header field indicates the date and time at - which the origin server believes the variant was last modified. - - Last-Modified = "Last-Modified" ":" HTTP-date - - - -Fielding, et al. Standards Track [Page 134] - -RFC 2616 HTTP/1.1 June 1999 - - - An example of its use is - - Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT - - The exact meaning of this header field depends on the implementation - of the origin server and the nature of the original resource. For - files, it may be just the file system last-modified time. For - entities with dynamically included parts, it may be the most recent - of the set of last-modify times for its component parts. For database - gateways, it may be the last-update time stamp of the record. For - virtual objects, it may be the last time the internal state changed. - - An origin server MUST NOT send a Last-Modified date which is later - than the server's time of message origination. In such cases, where - the resource's last modification would indicate some time in the - future, the server MUST replace that date with the message - origination date. - - An origin server SHOULD obtain the Last-Modified value of the entity - as close as possible to the time that it generates the Date value of - its response. This allows a recipient to make an accurate assessment - of the entity's modification time, especially if the entity changes - near the time that the response is generated. - - HTTP/1.1 servers SHOULD send Last-Modified whenever feasible. - -14.30 Location - - The Location response-header field is used to redirect the recipient - to a location other than the Request-URI for completion of the - request or identification of a new resource. For 201 (Created) - responses, the Location is that of the new resource which was created - by the request. For 3xx responses, the location SHOULD indicate the - server's preferred URI for automatic redirection to the resource. The - field value consists of a single absolute URI. - - Location = "Location" ":" absoluteURI - - An example is: - - Location: http://www.w3.org/pub/WWW/People.html - - Note: The Content-Location header field (section 14.14) differs - from Location in that the Content-Location identifies the original - location of the entity enclosed in the request. It is therefore - possible for a response to contain header fields for both Location - and Content-Location. Also see section 13.10 for cache - requirements of some methods. - - - -Fielding, et al. Standards Track [Page 135] - -RFC 2616 HTTP/1.1 June 1999 - - -14.31 Max-Forwards - - The Max-Forwards request-header field provides a mechanism with the - TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the - number of proxies or gateways that can forward the request to the - next inbound server. This can be useful when the client is attempting - to trace a request chain which appears to be failing or looping in - mid-chain. - - Max-Forwards = "Max-Forwards" ":" 1*DIGIT - - The Max-Forwards value is a decimal integer indicating the remaining - number of times this request message may be forwarded. - - Each proxy or gateway recipient of a TRACE or OPTIONS request - containing a Max-Forwards header field MUST check and update its - value prior to forwarding the request. If the received value is zero - (0), the recipient MUST NOT forward the request; instead, it MUST - respond as the final recipient. If the received Max-Forwards value is - greater than zero, then the forwarded message MUST contain an updated - Max-Forwards field with a value decremented by one (1). - - The Max-Forwards header field MAY be ignored for all other methods - defined by this specification and for any extension methods for which - it is not explicitly referred to as part of that method definition. - -14.32 Pragma - - The Pragma general-header field is used to include implementation- - specific directives that might apply to any recipient along the - request/response chain. All pragma directives specify optional - behavior from the viewpoint of the protocol; however, some systems - MAY require that behavior be consistent with the directives. - - Pragma = "Pragma" ":" 1#pragma-directive - pragma-directive = "no-cache" | extension-pragma - extension-pragma = token [ "=" ( token | quoted-string ) ] - - When the no-cache directive is present in a request message, an - application SHOULD forward the request toward the origin server even - if it has a cached copy of what is being requested. This pragma - directive has the same semantics as the no-cache cache-directive (see - section 14.9) and is defined here for backward compatibility with - HTTP/1.0. Clients SHOULD include both header fields when a no-cache - request is sent to a server not known to be HTTP/1.1 compliant. - - - - - - -Fielding, et al. Standards Track [Page 136] - -RFC 2616 HTTP/1.1 June 1999 - - - Pragma directives MUST be passed through by a proxy or gateway - application, regardless of their significance to that application, - since the directives might be applicable to all recipients along the - request/response chain. It is not possible to specify a pragma for a - specific recipient; however, any pragma directive not relevant to a - recipient SHOULD be ignored by that recipient. - - HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had - sent "Cache-Control: no-cache". No new Pragma directives will be - defined in HTTP. - - Note: because the meaning of "Pragma: no-cache as a response - header field is not actually specified, it does not provide a - reliable replacement for "Cache-Control: no-cache" in a response - -14.33 Proxy-Authenticate - - The Proxy-Authenticate response-header field MUST be included as part - of a 407 (Proxy Authentication Required) response. The field value - consists of a challenge that indicates the authentication scheme and - parameters applicable to the proxy for this Request-URI. - - Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge - - The HTTP access authentication process is described in "HTTP - Authentication: Basic and Digest Access Authentication" [43]. Unlike - WWW-Authenticate, the Proxy-Authenticate header field applies only to - the current connection and SHOULD NOT be passed on to downstream - clients. However, an intermediate proxy might need to obtain its own - credentials by requesting them from the downstream client, which in - some circumstances will appear as if the proxy is forwarding the - Proxy-Authenticate header field. - -14.34 Proxy-Authorization - - The Proxy-Authorization request-header field allows the client to - identify itself (or its user) to a proxy which requires - authentication. The Proxy-Authorization field value consists of - credentials containing the authentication information of the user - agent for the proxy and/or realm of the resource being requested. - - Proxy-Authorization = "Proxy-Authorization" ":" credentials - - The HTTP access authentication process is described in "HTTP - Authentication: Basic and Digest Access Authentication" [43] . Unlike - Authorization, the Proxy-Authorization header field applies only to - the next outbound proxy that demanded authentication using the Proxy- - Authenticate field. When multiple proxies are used in a chain, the - - - -Fielding, et al. Standards Track [Page 137] - -RFC 2616 HTTP/1.1 June 1999 - - - Proxy-Authorization header field is consumed by the first outbound - proxy that was expecting to receive credentials. A proxy MAY relay - the credentials from the client request to the next proxy if that is - the mechanism by which the proxies cooperatively authenticate a given - request. - -14.35 Range - -14.35.1 Byte Ranges - - Since all HTTP entities are represented in HTTP messages as sequences - of bytes, the concept of a byte range is meaningful for any HTTP - entity. (However, not all clients and servers need to support byte- - range operations.) - - Byte range specifications in HTTP apply to the sequence of bytes in - the entity-body (not necessarily the same as the message-body). - - A byte range operation MAY specify a single range of bytes, or a set - of ranges within a single entity. - - ranges-specifier = byte-ranges-specifier - byte-ranges-specifier = bytes-unit "=" byte-range-set - byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) - byte-range-spec = first-byte-pos "-" [last-byte-pos] - first-byte-pos = 1*DIGIT - last-byte-pos = 1*DIGIT - - The first-byte-pos value in a byte-range-spec gives the byte-offset - of the first byte in a range. The last-byte-pos value gives the - byte-offset of the last byte in the range; that is, the byte - positions specified are inclusive. Byte offsets start at zero. - - If the last-byte-pos value is present, it MUST be greater than or - equal to the first-byte-pos in that byte-range-spec, or the byte- - range-spec is syntactically invalid. The recipient of a byte-range- - set that includes one or more syntactically invalid byte-range-spec - values MUST ignore the header field that includes that byte-range- - set. - - If the last-byte-pos value is absent, or if the value is greater than - or equal to the current length of the entity-body, last-byte-pos is - taken to be equal to one less than the current length of the entity- - body in bytes. - - By its choice of last-byte-pos, a client can limit the number of - bytes retrieved without knowing the size of the entity. - - - - -Fielding, et al. Standards Track [Page 138] - -RFC 2616 HTTP/1.1 June 1999 - - - suffix-byte-range-spec = "-" suffix-length - suffix-length = 1*DIGIT - - A suffix-byte-range-spec is used to specify the suffix of the - entity-body, of a length given by the suffix-length value. (That is, - this form specifies the last N bytes of an entity-body.) If the - entity is shorter than the specified suffix-length, the entire - entity-body is used. - - If a syntactically valid byte-range-set includes at least one byte- - range-spec whose first-byte-pos is less than the current length of - the entity-body, or at least one suffix-byte-range-spec with a non- - zero suffix-length, then the byte-range-set is satisfiable. - Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set - is unsatisfiable, the server SHOULD return a response with a status - of 416 (Requested range not satisfiable). Otherwise, the server - SHOULD return a response with a status of 206 (Partial Content) - containing the satisfiable ranges of the entity-body. - - Examples of byte-ranges-specifier values (assuming an entity-body of - length 10000): - - - The first 500 bytes (byte offsets 0-499, inclusive): bytes=0- - 499 - - - The second 500 bytes (byte offsets 500-999, inclusive): - bytes=500-999 - - - The final 500 bytes (byte offsets 9500-9999, inclusive): - bytes=-500 - - - Or bytes=9500- - - - The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1 - - - Several legal but not canonical specifications of the second 500 - bytes (byte offsets 500-999, inclusive): - bytes=500-600,601-999 - bytes=500-700,601-999 - -14.35.2 Range Retrieval Requests - - HTTP retrieval requests using conditional or unconditional GET - methods MAY request one or more sub-ranges of the entity, instead of - the entire entity, using the Range request header, which applies to - the entity returned as the result of the request: - - Range = "Range" ":" ranges-specifier - - - -Fielding, et al. Standards Track [Page 139] - -RFC 2616 HTTP/1.1 June 1999 - - - A server MAY ignore the Range header. However, HTTP/1.1 origin - servers and intermediate caches ought to support byte ranges when - possible, since Range supports efficient recovery from partially - failed transfers, and supports efficient partial retrieval of large - entities. - - If the server supports the Range header and the specified range or - ranges are appropriate for the entity: - - - The presence of a Range header in an unconditional GET modifies - what is returned if the GET is otherwise successful. In other - words, the response carries a status code of 206 (Partial - Content) instead of 200 (OK). - - - The presence of a Range header in a conditional GET (a request - using one or both of If-Modified-Since and If-None-Match, or - one or both of If-Unmodified-Since and If-Match) modifies what - is returned if the GET is otherwise successful and the - condition is true. It does not affect the 304 (Not Modified) - response returned if the conditional is false. - - In some cases, it might be more appropriate to use the If-Range - header (see section 14.27) in addition to the Range header. - - If a proxy that supports ranges receives a Range request, forwards - the request to an inbound server, and receives an entire entity in - reply, it SHOULD only return the requested range to its client. It - SHOULD store the entire received response in its cache if that is - consistent with its cache allocation policies. - -14.36 Referer - - The Referer[sic] request-header field allows the client to specify, - for the server's benefit, the address (URI) of the resource from - which the Request-URI was obtained (the "referrer", although the - header field is misspelled.) The Referer request-header allows a - server to generate lists of back-links to resources for interest, - logging, optimized caching, etc. It also allows obsolete or mistyped - links to be traced for maintenance. The Referer field MUST NOT be - sent if the Request-URI was obtained from a source that does not have - its own URI, such as input from the user keyboard. - - Referer = "Referer" ":" ( absoluteURI | relativeURI ) - - Example: - - Referer: http://www.w3.org/hypertext/DataSources/Overview.html - - - - -Fielding, et al. Standards Track [Page 140] - -RFC 2616 HTTP/1.1 June 1999 - - - If the field value is a relative URI, it SHOULD be interpreted - relative to the Request-URI. The URI MUST NOT include a fragment. See - section 15.1.3 for security considerations. - -14.37 Retry-After - - The Retry-After response-header field can be used with a 503 (Service - Unavailable) response to indicate how long the service is expected to - be unavailable to the requesting client. This field MAY also be used - with any 3xx (Redirection) response to indicate the minimum time the - user-agent is asked wait before issuing the redirected request. The - value of this field can be either an HTTP-date or an integer number - of seconds (in decimal) after the time of the response. - - Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) - - Two examples of its use are - - Retry-After: Fri, 31 Dec 1999 23:59:59 GMT - Retry-After: 120 - - In the latter example, the delay is 2 minutes. - -14.38 Server - - The Server response-header field contains information about the - software used by the origin server to handle the request. The field - can contain multiple product tokens (section 3.8) and comments - identifying the server and any significant subproducts. The product - tokens are listed in order of their significance for identifying the - application. - - Server = "Server" ":" 1*( product | comment ) - - Example: - - Server: CERN/3.0 libwww/2.17 - - If the response is being forwarded through a proxy, the proxy - application MUST NOT modify the Server response-header. Instead, it - SHOULD include a Via field (as described in section 14.45). - - Note: Revealing the specific software version of the server might - allow the server machine to become more vulnerable to attacks - against software that is known to contain security holes. Server - implementors are encouraged to make this field a configurable - option. - - - - -Fielding, et al. Standards Track [Page 141] - -RFC 2616 HTTP/1.1 June 1999 - - -14.39 TE - - The TE request-header field indicates what extension transfer-codings - it is willing to accept in the response and whether or not it is - willing to accept trailer fields in a chunked transfer-coding. Its - value may consist of the keyword "trailers" and/or a comma-separated - list of extension transfer-coding names with optional accept - parameters (as described in section 3.6). - - TE = "TE" ":" #( t-codings ) - t-codings = "trailers" | ( transfer-extension [ accept-params ] ) - - The presence of the keyword "trailers" indicates that the client is - willing to accept trailer fields in a chunked transfer-coding, as - defined in section 3.6.1. This keyword is reserved for use with - transfer-coding values even though it does not itself represent a - transfer-coding. - - Examples of its use are: - - TE: deflate - TE: - TE: trailers, deflate;q=0.5 - - The TE header field only applies to the immediate connection. - Therefore, the keyword MUST be supplied within a Connection header - field (section 14.10) whenever TE is present in an HTTP/1.1 message. - - A server tests whether a transfer-coding is acceptable, according to - a TE field, using these rules: - - 1. The "chunked" transfer-coding is always acceptable. If the - keyword "trailers" is listed, the client indicates that it is - willing to accept trailer fields in the chunked response on - behalf of itself and any downstream clients. The implication is - that, if given, the client is stating that either all - downstream clients are willing to accept trailer fields in the - forwarded response, or that it will attempt to buffer the - response on behalf of downstream recipients. - - Note: HTTP/1.1 does not define any means to limit the size of a - chunked response such that a client can be assured of buffering - the entire response. - - 2. If the transfer-coding being tested is one of the transfer- - codings listed in the TE field, then it is acceptable unless it - is accompanied by a qvalue of 0. (As defined in section 3.9, a - qvalue of 0 means "not acceptable.") - - - -Fielding, et al. Standards Track [Page 142] - -RFC 2616 HTTP/1.1 June 1999 - - - 3. If multiple transfer-codings are acceptable, then the - acceptable transfer-coding with the highest non-zero qvalue is - preferred. The "chunked" transfer-coding always has a qvalue - of 1. - - If the TE field-value is empty or if no TE field is present, the only - transfer-coding is "chunked". A message with no transfer-coding is - always acceptable. - -14.40 Trailer - - The Trailer general field value indicates that the given set of - header fields is present in the trailer of a message encoded with - chunked transfer-coding. - - Trailer = "Trailer" ":" 1#field-name - - An HTTP/1.1 message SHOULD include a Trailer header field in a - message using chunked transfer-coding with a non-empty trailer. Doing - so allows the recipient to know which header fields to expect in the - trailer. - - If no Trailer header field is present, the trailer SHOULD NOT include - any header fields. See section 3.6.1 for restrictions on the use of - trailer fields in a "chunked" transfer-coding. - - Message header fields listed in the Trailer header field MUST NOT - include the following header fields: - - . Transfer-Encoding - - . Content-Length - - . Trailer - -14.41 Transfer-Encoding - - The Transfer-Encoding general-header field indicates what (if any) - type of transformation has been applied to the message body in order - to safely transfer it between the sender and the recipient. This - differs from the content-coding in that the transfer-coding is a - property of the message, not of the entity. - - Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding - - Transfer-codings are defined in section 3.6. An example is: - - Transfer-Encoding: chunked - - - -Fielding, et al. Standards Track [Page 143] - -RFC 2616 HTTP/1.1 June 1999 - - - If multiple encodings have been applied to an entity, the transfer- - codings MUST be listed in the order in which they were applied. - Additional information about the encoding parameters MAY be provided - by other entity-header fields not defined by this specification. - - Many older HTTP/1.0 applications do not understand the Transfer- - Encoding header. - -14.42 Upgrade - - The Upgrade general-header allows the client to specify what - additional communication protocols it supports and would like to use - if the server finds it appropriate to switch protocols. The server - MUST use the Upgrade header field within a 101 (Switching Protocols) - response to indicate which protocol(s) are being switched. - - Upgrade = "Upgrade" ":" 1#product - - For example, - - Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 - - The Upgrade header field is intended to provide a simple mechanism - for transition from HTTP/1.1 to some other, incompatible protocol. It - does so by allowing the client to advertise its desire to use another - protocol, such as a later version of HTTP with a higher major version - number, even though the current request has been made using HTTP/1.1. - This eases the difficult transition between incompatible protocols by - allowing the client to initiate a request in the more commonly - supported protocol while indicating to the server that it would like - to use a "better" protocol if available (where "better" is determined - by the server, possibly according to the nature of the method and/or - resource being requested). - - The Upgrade header field only applies to switching application-layer - protocols upon the existing transport-layer connection. Upgrade - cannot be used to insist on a protocol change; its acceptance and use - by the server is optional. The capabilities and nature of the - application-layer communication after the protocol change is entirely - dependent upon the new protocol chosen, although the first action - after changing the protocol MUST be a response to the initial HTTP - request containing the Upgrade header field. - - The Upgrade header field only applies to the immediate connection. - Therefore, the upgrade keyword MUST be supplied within a Connection - header field (section 14.10) whenever Upgrade is present in an - HTTP/1.1 message. - - - - -Fielding, et al. Standards Track [Page 144] - -RFC 2616 HTTP/1.1 June 1999 - - - The Upgrade header field cannot be used to indicate a switch to a - protocol on a different connection. For that purpose, it is more - appropriate to use a 301, 302, 303, or 305 redirection response. - - This specification only defines the protocol name "HTTP" for use by - the family of Hypertext Transfer Protocols, as defined by the HTTP - version rules of section 3.1 and future updates to this - specification. Any token can be used as a protocol name; however, it - will only be useful if both the client and server associate the name - with the same protocol. - -14.43 User-Agent - - The User-Agent request-header field contains information about the - user agent originating the request. This is for statistical purposes, - the tracing of protocol violations, and automated recognition of user - agents for the sake of tailoring responses to avoid particular user - agent limitations. User agents SHOULD include this field with - requests. The field can contain multiple product tokens (section 3.8) - and comments identifying the agent and any subproducts which form a - significant part of the user agent. By convention, the product tokens - are listed in order of their significance for identifying the - application. - - User-Agent = "User-Agent" ":" 1*( product | comment ) - - Example: - - User-Agent: CERN-LineMode/2.15 libwww/2.17b3 - -14.44 Vary - - The Vary field value indicates the set of request-header fields that - fully determines, while the response is fresh, whether a cache is - permitted to use the response to reply to a subsequent request - without revalidation. For uncacheable or stale responses, the Vary - field value advises the user agent about the criteria that were used - to select the representation. A Vary field value of "*" implies that - a cache cannot determine from the request headers of a subsequent - request whether this response is the appropriate representation. See - section 13.6 for use of the Vary header field by caches. - - Vary = "Vary" ":" ( "*" | 1#field-name ) - - An HTTP/1.1 server SHOULD include a Vary header field with any - cacheable response that is subject to server-driven negotiation. - Doing so allows a cache to properly interpret future requests on that - resource and informs the user agent about the presence of negotiation - - - -Fielding, et al. Standards Track [Page 145] - -RFC 2616 HTTP/1.1 June 1999 - - - on that resource. A server MAY include a Vary header field with a - non-cacheable response that is subject to server-driven negotiation, - since this might provide the user agent with useful information about - the dimensions over which the response varies at the time of the - response. - - A Vary field value consisting of a list of field-names signals that - the representation selected for the response is based on a selection - algorithm which considers ONLY the listed request-header field values - in selecting the most appropriate representation. A cache MAY assume - that the same selection will be made for future requests with the - same values for the listed field names, for the duration of time for - which the response is fresh. - - The field-names given are not limited to the set of standard - request-header fields defined by this specification. Field names are - case-insensitive. - - A Vary field value of "*" signals that unspecified parameters not - limited to the request-headers (e.g., the network address of the - client), play a role in the selection of the response representation. - The "*" value MUST NOT be generated by a proxy server; it may only be - generated by an origin server. - -14.45 Via - - The Via general-header field MUST be used by gateways and proxies to - indicate the intermediate protocols and recipients between the user - agent and the server on requests, and between the origin server and - the client on responses. It is analogous to the "Received" field of - RFC 822 [9] and is intended to be used for tracking message forwards, - avoiding request loops, and identifying the protocol capabilities of - all senders along the request/response chain. - - Via = "Via" ":" 1#( received-protocol received-by [ comment ] ) - received-protocol = [ protocol-name "/" ] protocol-version - protocol-name = token - protocol-version = token - received-by = ( host [ ":" port ] ) | pseudonym - pseudonym = token - - The received-protocol indicates the protocol version of the message - received by the server or client along each segment of the - request/response chain. The received-protocol version is appended to - the Via field value when the message is forwarded so that information - about the protocol capabilities of upstream applications remains - visible to all recipients. - - - - -Fielding, et al. Standards Track [Page 146] - -RFC 2616 HTTP/1.1 June 1999 - - - The protocol-name is optional if and only if it would be "HTTP". The - received-by field is normally the host and optional port number of a - recipient server or client that subsequently forwarded the message. - However, if the real host is considered to be sensitive information, - it MAY be replaced by a pseudonym. If the port is not given, it MAY - be assumed to be the default port of the received-protocol. - - Multiple Via field values represents each proxy or gateway that has - forwarded the message. Each recipient MUST append its information - such that the end result is ordered according to the sequence of - forwarding applications. - - Comments MAY be used in the Via header field to identify the software - of the recipient proxy or gateway, analogous to the User-Agent and - Server header fields. However, all comments in the Via field are - optional and MAY be removed by any recipient prior to forwarding the - message. - - For example, a request message could be sent from an HTTP/1.0 user - agent to an internal proxy code-named "fred", which uses HTTP/1.1 to - forward the request to a public proxy at nowhere.com, which completes - the request by forwarding it to the origin server at www.ics.uci.edu. - The request received by www.ics.uci.edu would then have the following - Via header field: - - Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1) - - Proxies and gateways used as a portal through a network firewall - SHOULD NOT, by default, forward the names and ports of hosts within - the firewall region. This information SHOULD only be propagated if - explicitly enabled. If not enabled, the received-by host of any host - behind the firewall SHOULD be replaced by an appropriate pseudonym - for that host. - - For organizations that have strong privacy requirements for hiding - internal structures, a proxy MAY combine an ordered subsequence of - Via header field entries with identical received-protocol values into - a single such entry. For example, - - Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy - - could be collapsed to - - Via: 1.0 ricky, 1.1 mertz, 1.0 lucy - - - - - - - -Fielding, et al. Standards Track [Page 147] - -RFC 2616 HTTP/1.1 June 1999 - - - Applications SHOULD NOT combine multiple entries unless they are all - under the same organizational control and the hosts have already been - replaced by pseudonyms. Applications MUST NOT combine entries which - have different received-protocol values. - -14.46 Warning - - The Warning general-header field is used to carry additional - information about the status or transformation of a message which - might not be reflected in the message. This information is typically - used to warn about a possible lack of semantic transparency from - caching operations or transformations applied to the entity body of - the message. - - Warning headers are sent with responses using: - - Warning = "Warning" ":" 1#warning-value - - warning-value = warn-code SP warn-agent SP warn-text - [SP warn-date] - - warn-code = 3DIGIT - warn-agent = ( host [ ":" port ] ) | pseudonym - ; the name or pseudonym of the server adding - ; the Warning header, for use in debugging - warn-text = quoted-string - warn-date = <"> HTTP-date <"> - - A response MAY carry more than one Warning header. - - The warn-text SHOULD be in a natural language and character set that - is most likely to be intelligible to the human user receiving the - response. This decision MAY be based on any available knowledge, such - as the location of the cache or user, the Accept-Language field in a - request, the Content-Language field in a response, etc. The default - language is English and the default character set is ISO-8859-1. - - If a character set other than ISO-8859-1 is used, it MUST be encoded - in the warn-text using the method described in RFC 2047 [14]. - - Warning headers can in general be applied to any message, however - some specific warn-codes are specific to caches and can only be - applied to response messages. New Warning headers SHOULD be added - after any existing Warning headers. A cache MUST NOT delete any - Warning header that it received with a message. However, if a cache - successfully validates a cache entry, it SHOULD remove any Warning - headers previously attached to that entry except as specified for - - - - -Fielding, et al. Standards Track [Page 148] - -RFC 2616 HTTP/1.1 June 1999 - - - specific Warning codes. It MUST then add any Warning headers received - in the validating response. In other words, Warning headers are those - that would be attached to the most recent relevant response. - - When multiple Warning headers are attached to a response, the user - agent ought to inform the user of as many of them as possible, in the - order that they appear in the response. If it is not possible to - inform the user of all of the warnings, the user agent SHOULD follow - these heuristics: - - - Warnings that appear early in the response take priority over - those appearing later in the response. - - - Warnings in the user's preferred character set take priority - over warnings in other character sets but with identical warn- - codes and warn-agents. - - Systems that generate multiple Warning headers SHOULD order them with - this user agent behavior in mind. - - Requirements for the behavior of caches with respect to Warnings are - stated in section 13.1.2. - - This is a list of the currently-defined warn-codes, each with a - recommended warn-text in English, and a description of its meaning. - - 110 Response is stale - MUST be included whenever the returned response is stale. - - 111 Revalidation failed - MUST be included if a cache returns a stale response because an - attempt to revalidate the response failed, due to an inability to - reach the server. - - 112 Disconnected operation - SHOULD be included if the cache is intentionally disconnected from - the rest of the network for a period of time. - - 113 Heuristic expiration - MUST be included if the cache heuristically chose a freshness - lifetime greater than 24 hours and the response's age is greater - than 24 hours. - - 199 Miscellaneous warning - The warning text MAY include arbitrary information to be presented - to a human user, or logged. A system receiving this warning MUST - NOT take any automated action, besides presenting the warning to - the user. - - - -Fielding, et al. Standards Track [Page 149] - -RFC 2616 HTTP/1.1 June 1999 - - - 214 Transformation applied - MUST be added by an intermediate cache or proxy if it applies any - transformation changing the content-coding (as specified in the - Content-Encoding header) or media-type (as specified in the - Content-Type header) of the response, or the entity-body of the - response, unless this Warning code already appears in the response. - - 299 Miscellaneous persistent warning - The warning text MAY include arbitrary information to be presented - to a human user, or logged. A system receiving this warning MUST - NOT take any automated action. - - If an implementation sends a message with one or more Warning headers - whose version is HTTP/1.0 or lower, then the sender MUST include in - each warning-value a warn-date that matches the date in the response. - - If an implementation receives a message with a warning-value that - includes a warn-date, and that warn-date is different from the Date - value in the response, then that warning-value MUST be deleted from - the message before storing, forwarding, or using it. (This prevents - bad consequences of naive caching of Warning header fields.) If all - of the warning-values are deleted for this reason, the Warning header - MUST be deleted as well. - -14.47 WWW-Authenticate - - The WWW-Authenticate response-header field MUST be included in 401 - (Unauthorized) response messages. The field value consists of at - least one challenge that indicates the authentication scheme(s) and - parameters applicable to the Request-URI. - - WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge - - The HTTP access authentication process is described in "HTTP - Authentication: Basic and Digest Access Authentication" [43]. User - agents are advised to take special care in parsing the WWW- - Authenticate field value as it might contain more than one challenge, - or if more than one WWW-Authenticate header field is provided, the - contents of a challenge itself can contain a comma-separated list of - authentication parameters. - -15 Security Considerations - - This section is meant to inform application developers, information - providers, and users of the security limitations in HTTP/1.1 as - described by this document. The discussion does not include - definitive solutions to the problems revealed, though it does make - some suggestions for reducing security risks. - - - -Fielding, et al. Standards Track [Page 150] - -RFC 2616 HTTP/1.1 June 1999 - - -15.1 Personal Information - - HTTP clients are often privy to large amounts of personal information - (e.g. the user's name, location, mail address, passwords, encryption - keys, etc.), and SHOULD be very careful to prevent unintentional - leakage of this information via the HTTP protocol to other sources. - We very strongly recommend that a convenient interface be provided - for the user to control dissemination of such information, and that - designers and implementors be particularly careful in this area. - History shows that errors in this area often create serious security - and/or privacy problems and generate highly adverse publicity for the - implementor's company. - -15.1.1 Abuse of Server Log Information - - A server is in the position to save personal data about a user's - requests which might identify their reading patterns or subjects of - interest. This information is clearly confidential in nature and its - handling can be constrained by law in certain countries. People using - the HTTP protocol to provide data are responsible for ensuring that - such material is not distributed without the permission of any - individuals that are identifiable by the published results. - -15.1.2 Transfer of Sensitive Information - - Like any generic data transfer protocol, HTTP cannot regulate the - content of the data that is transferred, nor is there any a priori - method of determining the sensitivity of any particular piece of - information within the context of any given request. Therefore, - applications SHOULD supply as much control over this information as - possible to the provider of that information. Four header fields are - worth special mention in this context: Server, Via, Referer and From. - - Revealing the specific software version of the server might allow the - server machine to become more vulnerable to attacks against software - that is known to contain security holes. Implementors SHOULD make the - Server header field a configurable option. - - Proxies which serve as a portal through a network firewall SHOULD - take special precautions regarding the transfer of header information - that identifies the hosts behind the firewall. In particular, they - SHOULD remove, or replace with sanitized versions, any Via fields - generated behind the firewall. - - The Referer header allows reading patterns to be studied and reverse - links drawn. Although it can be very useful, its power can be abused - if user details are not separated from the information contained in - - - - -Fielding, et al. Standards Track [Page 151] - -RFC 2616 HTTP/1.1 June 1999 - - - the Referer. Even when the personal information has been removed, the - Referer header might indicate a private document's URI whose - publication would be inappropriate. - - The information sent in the From field might conflict with the user's - privacy interests or their site's security policy, and hence it - SHOULD NOT be transmitted without the user being able to disable, - enable, and modify the contents of the field. The user MUST be able - to set the contents of this field within a user preference or - application defaults configuration. - - We suggest, though do not require, that a convenient toggle interface - be provided for the user to enable or disable the sending of From and - Referer information. - - The User-Agent (section 14.43) or Server (section 14.38) header - fields can sometimes be used to determine that a specific client or - server have a particular security hole which might be exploited. - Unfortunately, this same information is often used for other valuable - purposes for which HTTP currently has no better mechanism. - -15.1.3 Encoding Sensitive Information in URI's - - Because the source of a link might be private information or might - reveal an otherwise private information source, it is strongly - recommended that the user be able to select whether or not the - Referer field is sent. For example, a browser client could have a - toggle switch for browsing openly/anonymously, which would - respectively enable/disable the sending of Referer and From - information. - - Clients SHOULD NOT include a Referer header field in a (non-secure) - HTTP request if the referring page was transferred with a secure - protocol. - - Authors of services which use the HTTP protocol SHOULD NOT use GET - based forms for the submission of sensitive data, because this will - cause this data to be encoded in the Request-URI. Many existing - servers, proxies, and user agents will log the request URI in some - place where it might be visible to third parties. Servers can use - POST-based form submission instead - -15.1.4 Privacy Issues Connected to Accept Headers - - Accept request-headers can reveal information about the user to all - servers which are accessed. The Accept-Language header in particular - can reveal information the user would consider to be of a private - nature, because the understanding of particular languages is often - - - -Fielding, et al. Standards Track [Page 152] - -RFC 2616 HTTP/1.1 June 1999 - - - strongly correlated to the membership of a particular ethnic group. - User agents which offer the option to configure the contents of an - Accept-Language header to be sent in every request are strongly - encouraged to let the configuration process include a message which - makes the user aware of the loss of privacy involved. - - An approach that limits the loss of privacy would be for a user agent - to omit the sending of Accept-Language headers by default, and to ask - the user whether or not to start sending Accept-Language headers to a - server if it detects, by looking for any Vary response-header fields - generated by the server, that such sending could improve the quality - of service. - - Elaborate user-customized accept header fields sent in every request, - in particular if these include quality values, can be used by servers - as relatively reliable and long-lived user identifiers. Such user - identifiers would allow content providers to do click-trail tracking, - and would allow collaborating content providers to match cross-server - click-trails or form submissions of individual users. Note that for - many users not behind a proxy, the network address of the host - running the user agent will also serve as a long-lived user - identifier. In environments where proxies are used to enhance - privacy, user agents ought to be conservative in offering accept - header configuration options to end users. As an extreme privacy - measure, proxies could filter the accept headers in relayed requests. - General purpose user agents which provide a high degree of header - configurability SHOULD warn users about the loss of privacy which can - be involved. - -15.2 Attacks Based On File and Path Names - - Implementations of HTTP origin servers SHOULD be careful to restrict - the documents returned by HTTP requests to be only those that were - intended by the server administrators. If an HTTP server translates - HTTP URIs directly into file system calls, the server MUST take - special care not to serve files that were not intended to be - delivered to HTTP clients. For example, UNIX, Microsoft Windows, and - other operating systems use ".." as a path component to indicate a - directory level above the current one. On such a system, an HTTP - server MUST disallow any such construct in the Request-URI if it - would otherwise allow access to a resource outside those intended to - be accessible via the HTTP server. Similarly, files intended for - reference only internally to the server (such as access control - files, configuration files, and script code) MUST be protected from - inappropriate retrieval, since they might contain sensitive - information. Experience has shown that minor bugs in such HTTP server - implementations have turned into security risks. - - - - -Fielding, et al. Standards Track [Page 153] - -RFC 2616 HTTP/1.1 June 1999 - - -15.3 DNS Spoofing - - Clients using HTTP rely heavily on the Domain Name Service, and are - thus generally prone to security attacks based on the deliberate - mis-association of IP addresses and DNS names. Clients need to be - cautious in assuming the continuing validity of an IP number/DNS name - association. - - In particular, HTTP clients SHOULD rely on their name resolver for - confirmation of an IP number/DNS name association, rather than - caching the result of previous host name lookups. Many platforms - already can cache host name lookups locally when appropriate, and - they SHOULD be configured to do so. It is proper for these lookups to - be cached, however, only when the TTL (Time To Live) information - reported by the name server makes it likely that the cached - information will remain useful. - - If HTTP clients cache the results of host name lookups in order to - achieve a performance improvement, they MUST observe the TTL - information reported by DNS. - - If HTTP clients do not observe this rule, they could be spoofed when - a previously-accessed server's IP address changes. As network - renumbering is expected to become increasingly common [24], the - possibility of this form of attack will grow. Observing this - requirement thus reduces this potential security vulnerability. - - This requirement also improves the load-balancing behavior of clients - for replicated servers using the same DNS name and reduces the - likelihood of a user's experiencing failure in accessing sites which - use that strategy. - -15.4 Location Headers and Spoofing - - If a single server supports multiple organizations that do not trust - one another, then it MUST check the values of Location and Content- - Location headers in responses that are generated under control of - said organizations to make sure that they do not attempt to - invalidate resources over which they have no authority. - -15.5 Content-Disposition Issues - - RFC 1806 [35], from which the often implemented Content-Disposition - (see section 19.5.1) header in HTTP is derived, has a number of very - serious security considerations. Content-Disposition is not part of - the HTTP standard, but since it is widely implemented, we are - documenting its use and risks for implementors. See RFC 2183 [49] - (which updates RFC 1806) for details. - - - -Fielding, et al. Standards Track [Page 154] - -RFC 2616 HTTP/1.1 June 1999 - - -15.6 Authentication Credentials and Idle Clients - - Existing HTTP clients and user agents typically retain authentication - information indefinitely. HTTP/1.1. does not provide a method for a - server to direct clients to discard these cached credentials. This is - a significant defect that requires further extensions to HTTP. - Circumstances under which credential caching can interfere with the - application's security model include but are not limited to: - - - Clients which have been idle for an extended period following - which the server might wish to cause the client to reprompt the - user for credentials. - - - Applications which include a session termination indication - (such as a `logout' or `commit' button on a page) after which - the server side of the application `knows' that there is no - further reason for the client to retain the credentials. - - This is currently under separate study. There are a number of work- - arounds to parts of this problem, and we encourage the use of - password protection in screen savers, idle time-outs, and other - methods which mitigate the security problems inherent in this - problem. In particular, user agents which cache credentials are - encouraged to provide a readily accessible mechanism for discarding - cached credentials under user control. - -15.7 Proxies and Caching - - By their very nature, HTTP proxies are men-in-the-middle, and - represent an opportunity for man-in-the-middle attacks. Compromise of - the systems on which the proxies run can result in serious security - and privacy problems. Proxies have access to security-related - information, personal information about individual users and - organizations, and proprietary information belonging to users and - content providers. A compromised proxy, or a proxy implemented or - configured without regard to security and privacy considerations, - might be used in the commission of a wide range of potential attacks. - - Proxy operators should protect the systems on which proxies run as - they would protect any system that contains or transports sensitive - information. In particular, log information gathered at proxies often - contains highly sensitive personal information, and/or information - about organizations. Log information should be carefully guarded, and - appropriate guidelines for use developed and followed. (Section - 15.1.1). - - - - - - -Fielding, et al. Standards Track [Page 155] - -RFC 2616 HTTP/1.1 June 1999 - - - Caching proxies provide additional potential vulnerabilities, since - the contents of the cache represent an attractive target for - malicious exploitation. Because cache contents persist after an HTTP - request is complete, an attack on the cache can reveal information - long after a user believes that the information has been removed from - the network. Therefore, cache contents should be protected as - sensitive information. - - Proxy implementors should consider the privacy and security - implications of their design and coding decisions, and of the - configuration options they provide to proxy operators (especially the - default configuration). - - Users of a proxy need to be aware that they are no trustworthier than - the people who run the proxy; HTTP itself cannot solve this problem. - - The judicious use of cryptography, when appropriate, may suffice to - protect against a broad range of security and privacy attacks. Such - cryptography is beyond the scope of the HTTP/1.1 specification. - -15.7.1 Denial of Service Attacks on Proxies - - They exist. They are hard to defend against. Research continues. - Beware. - -16 Acknowledgments - - This specification makes heavy use of the augmented BNF and generic - constructs defined by David H. Crocker for RFC 822 [9]. Similarly, it - reuses many of the definitions provided by Nathaniel Borenstein and - Ned Freed for MIME [7]. We hope that their inclusion in this - specification will help reduce past confusion over the relationship - between HTTP and Internet mail message formats. - - The HTTP protocol has evolved considerably over the years. It has - benefited from a large and active developer community--the many - people who have participated on the www-talk mailing list--and it is - that community which has been most responsible for the success of - HTTP and of the World-Wide Web in general. Marc Andreessen, Robert - Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois - Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob - McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc - VanHeyningen deserve special recognition for their efforts in - defining early aspects of the protocol. - - This document has benefited greatly from the comments of all those - participating in the HTTP-WG. In addition to those already mentioned, - the following individuals have contributed to this specification: - - - -Fielding, et al. Standards Track [Page 156] - -RFC 2616 HTTP/1.1 June 1999 - - - Gary Adams Ross Patterson - Harald Tveit Alvestrand Albert Lunde - Keith Ball John C. Mallery - Brian Behlendorf Jean-Philippe Martin-Flatin - Paul Burchard Mitra - Maurizio Codogno David Morris - Mike Cowlishaw Gavin Nicol - Roman Czyborra Bill Perry - Michael A. Dolan Jeffrey Perry - David J. Fiander Scott Powers - Alan Freier Owen Rees - Marc Hedlund Luigi Rizzo - Greg Herlihy David Robinson - Koen Holtman Marc Salomon - Alex Hopmann Rich Salz - Bob Jernigan Allan M. Schiffman - Shel Kaphan Jim Seidman - Rohit Khare Chuck Shotton - John Klensin Eric W. Sink - Martijn Koster Simon E. Spero - Alexei Kosut Richard N. Taylor - David M. Kristol Robert S. Thau - Daniel LaLiberte Bill (BearHeart) Weinman - Ben Laurie Francois Yergeau - Paul J. Leach Mary Ellen Zurko - Daniel DuBois Josh Cohen - - - Much of the content and presentation of the caching design is due to - suggestions and comments from individuals including: Shel Kaphan, - Paul Leach, Koen Holtman, David Morris, and Larry Masinter. - - Most of the specification of ranges is based on work originally done - by Ari Luotonen and John Franks, with additional input from Steve - Zilles. - - Thanks to the "cave men" of Palo Alto. You know who you are. - - Jim Gettys (the current editor of this document) wishes particularly - to thank Roy Fielding, the previous editor of this document, along - with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen - Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and - Larry Masinter for their help. And thanks go particularly to Jeff - Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit. - - - - - - - -Fielding, et al. Standards Track [Page 157] - -RFC 2616 HTTP/1.1 June 1999 - - - The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik - Frystyk implemented RFC 2068 early, and we wish to thank them for the - discovery of many of the problems that this document attempts to - rectify. - -17 References - - [1] Alvestrand, H., "Tags for the Identification of Languages", RFC - 1766, March 1995. - - [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey, - D. and B. Alberti, "The Internet Gopher Protocol (a distributed - document search and retrieval protocol)", RFC 1436, March 1993. - - [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC - 1630, June 1994. - - [4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource - Locators (URL)", RFC 1738, December 1994. - - [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language - - 2.0", RFC 1866, November 1995. - - [6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer - Protocol -- HTTP/1.0", RFC 1945, May 1996. - - [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail - Extensions (MIME) Part One: Format of Internet Message Bodies", - RFC 2045, November 1996. - - [8] Braden, R., "Requirements for Internet Hosts -- Communication - Layers", STD 3, RFC 1123, October 1989. - - [9] Crocker, D., "Standard for The Format of ARPA Internet Text - Messages", STD 11, RFC 822, August 1982. - - [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., - Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype - Functional Specification," (v1.5), Thinking Machines - Corporation, April 1990. - - [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, - June 1995. - - [12] Horton, M. and R. Adams, "Standard for Interchange of USENET - Messages", RFC 1036, December 1987. - - - - - -Fielding, et al. Standards Track [Page 158] - -RFC 2616 HTTP/1.1 June 1999 - - - [13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC - 977, February 1986. - - [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part - Three: Message Header Extensions for Non-ASCII Text", RFC 2047, - November 1996. - - [15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC - 1867, November 1995. - - [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, - August 1982. - - [17] Postel, J., "Media Type Registration Procedure", RFC 1590, - November 1996. - - [18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC - 959, October 1985. - - [19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, - October 1994. - - [20] Sollins, K. and L. Masinter, "Functional Requirements for - Uniform Resource Names", RFC 1737, December 1994. - - [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for - Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986. - - [22] ISO-8859. International Standard -- Information Processing -- - 8-bit Single-Byte Coded Graphic Character Sets -- - Part 1: Latin alphabet No. 1, ISO-8859-1:1987. - Part 2: Latin alphabet No. 2, ISO-8859-2, 1987. - Part 3: Latin alphabet No. 3, ISO-8859-3, 1988. - Part 4: Latin alphabet No. 4, ISO-8859-4, 1988. - Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988. - Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987. - Part 7: Latin/Greek alphabet, ISO-8859-7, 1987. - Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988. - Part 9: Latin alphabet No. 5, ISO-8859-9, 1990. - - [23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC - 1864, October 1995. - - [24] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC - 1900, February 1996. - - [25] Deutsch, P., "GZIP file format specification version 4.3", RFC - 1952, May 1996. - - - -Fielding, et al. Standards Track [Page 159] - -RFC 2616 HTTP/1.1 June 1999 - - - [26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP - Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35, - Dec. 1995. Slightly revised version of paper in Proc. 2nd - International WWW Conference '94: Mosaic and the Web, Oct. 1994, - which is available at - http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat - ency.html. - - [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP - Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>, - ISI Research Report ISI/RR-98-463, (original report dated Aug. - 1996), USC/Information Sciences Institute, August 1998. - - [28] Mills, D., "Network Time Protocol (Version 3) Specification, - Implementation and Analysis", RFC 1305, March 1992. - - [29] Deutsch, P., "DEFLATE Compressed Data Format Specification - version 1.3", RFC 1951, May 1996. - - [30] S. Spero, "Analysis of HTTP Performance Problems," - http://sunsite.unc.edu/mdma-release/http-prob.html. - - [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format - Specification version 3.3", RFC 1950, May 1996. - - [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., - Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP: - Digest Access Authentication", RFC 2069, January 1997. - - [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T. - Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC - 2068, January 1997. - - [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement - Levels", BCP 14, RFC 2119, March 1997. - - [35] Troost, R. and Dorner, S., "Communicating Presentation - Information in Internet Messages: The Content-Disposition - Header", RFC 1806, June 1995. - - [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and - Interpretation of HTTP Version Numbers", RFC 2145, May 1997. - [jg639] - - [37] Palme, J., "Common Internet Message Headers", RFC 2076, February - 1997. [jg640] - - - - - -Fielding, et al. Standards Track [Page 160] - -RFC 2616 HTTP/1.1 June 1999 - - - [38] Yergeau, F., "UTF-8, a transformation format of Unicode and - ISO-10646", RFC 2279, January 1998. [jg641] - - [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E., - Lie, H., and C. Lilley. "Network Performance Effects of - HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes - France, September 1997.[jg642] - - [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail - Extensions (MIME) Part Two: Media Types", RFC 2046, November - 1996. [jg643] - - [41] Alvestrand, H., "IETF Policy on Character Sets and Languages", - BCP 18, RFC 2277, January 1998. [jg644] - - [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource - Identifiers (URI): Generic Syntax and Semantics", RFC 2396, - August 1998. [jg645] - - [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., - Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP - Authentication: Basic and Digest Access Authentication", RFC - 2617, June 1999. [jg646] - - [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy - servers," Work in Progress. [jg647] - - [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of - Aggregate Documents, such as HTML (MHTML)", RFC 2110, March - 1997. - - [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP - 9, RFC 2026, October 1996. - - [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol - (HTCPCP/1.0)", RFC 2324, 1 April 1998. - - [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail - Extensions (MIME) Part Five: Conformance Criteria and Examples", - RFC 2049, November 1996. - - [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation - Information in Internet Messages: The Content-Disposition Header - Field", RFC 2183, August 1997. - - - - - - - -Fielding, et al. Standards Track [Page 161] - -RFC 2616 HTTP/1.1 June 1999 - - -18 Authors' Addresses - - Roy T. Fielding - Information and Computer Science - University of California, Irvine - Irvine, CA 92697-3425, USA - - Fax: +1 (949) 824-1715 - EMail: fielding@ics.uci.edu - - - James Gettys - World Wide Web Consortium - MIT Laboratory for Computer Science - 545 Technology Square - Cambridge, MA 02139, USA - - Fax: +1 (617) 258 8682 - EMail: jg@w3.org - - - Jeffrey C. Mogul - Western Research Laboratory - Compaq Computer Corporation - 250 University Avenue - Palo Alto, California, 94305, USA - - EMail: mogul@wrl.dec.com - - - Henrik Frystyk Nielsen - World Wide Web Consortium - MIT Laboratory for Computer Science - 545 Technology Square - Cambridge, MA 02139, USA - - Fax: +1 (617) 258 8682 - EMail: frystyk@w3.org - - - Larry Masinter - Xerox Corporation - 3333 Coyote Hill Road - Palo Alto, CA 94034, USA - - EMail: masinter@parc.xerox.com - - - - - -Fielding, et al. Standards Track [Page 162] - -RFC 2616 HTTP/1.1 June 1999 - - - Paul J. Leach - Microsoft Corporation - 1 Microsoft Way - Redmond, WA 98052, USA - - EMail: paulle@microsoft.com - - - Tim Berners-Lee - Director, World Wide Web Consortium - MIT Laboratory for Computer Science - 545 Technology Square - Cambridge, MA 02139, USA - - Fax: +1 (617) 258 8682 - EMail: timbl@w3.org - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Fielding, et al. Standards Track [Page 163] - -RFC 2616 HTTP/1.1 June 1999 - - -19 Appendices - -19.1 Internet Media Type message/http and application/http - - In addition to defining the HTTP/1.1 protocol, this document serves - as the specification for the Internet media type "message/http" and - "application/http". The message/http type can be used to enclose a - single HTTP request or response message, provided that it obeys the - MIME restrictions for all "message" types regarding line length and - encodings. The application/http type can be used to enclose a - pipeline of one or more HTTP request or response messages (not - intermixed). The following is to be registered with IANA [17]. - - Media Type name: message - Media subtype name: http - Required parameters: none - Optional parameters: version, msgtype - version: The HTTP-Version number of the enclosed message - (e.g., "1.1"). If not present, the version can be - determined from the first line of the body. - msgtype: The message type -- "request" or "response". If not - present, the type can be determined from the first - line of the body. - Encoding considerations: only "7bit", "8bit", or "binary" are - permitted - Security considerations: none - - Media Type name: application - Media subtype name: http - Required parameters: none - Optional parameters: version, msgtype - version: The HTTP-Version number of the enclosed messages - (e.g., "1.1"). If not present, the version can be - determined from the first line of the body. - msgtype: The message type -- "request" or "response". If not - present, the type can be determined from the first - line of the body. - Encoding considerations: HTTP messages enclosed by this type - are in "binary" format; use of an appropriate - Content-Transfer-Encoding is required when - transmitted via E-mail. - Security considerations: none - - - - - - - - - -Fielding, et al. Standards Track [Page 164] - -RFC 2616 HTTP/1.1 June 1999 - - -19.2 Internet Media Type multipart/byteranges - - When an HTTP 206 (Partial Content) response message includes the - content of multiple ranges (a response to a request for multiple - non-overlapping ranges), these are transmitted as a multipart - message-body. The media type for this purpose is called - "multipart/byteranges". - - The multipart/byteranges media type includes two or more parts, each - with its own Content-Type and Content-Range fields. The required - boundary parameter specifies the boundary string used to separate - each body-part. - - Media Type name: multipart - Media subtype name: byteranges - Required parameters: boundary - Optional parameters: none - Encoding considerations: only "7bit", "8bit", or "binary" are - permitted - Security considerations: none - - - For example: - - HTTP/1.1 206 Partial Content - Date: Wed, 15 Nov 1995 06:25:24 GMT - Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT - Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES - - --THIS_STRING_SEPARATES - Content-type: application/pdf - Content-range: bytes 500-999/8000 - - ...the first range... - --THIS_STRING_SEPARATES - Content-type: application/pdf - Content-range: bytes 7000-7999/8000 - - ...the second range - --THIS_STRING_SEPARATES-- - - Notes: - - 1) Additional CRLFs may precede the first boundary string in the - entity. - - - - - - -Fielding, et al. Standards Track [Page 165] - -RFC 2616 HTTP/1.1 June 1999 - - - 2) Although RFC 2046 [40] permits the boundary string to be - quoted, some existing implementations handle a quoted boundary - string incorrectly. - - 3) A number of browsers and servers were coded to an early draft - of the byteranges specification to use a media type of - multipart/x-byteranges, which is almost, but not quite - compatible with the version documented in HTTP/1.1. - -19.3 Tolerant Applications - - Although this document specifies the requirements for the generation - of HTTP/1.1 messages, not all applications will be correct in their - implementation. We therefore recommend that operational applications - be tolerant of deviations whenever those deviations can be - interpreted unambiguously. - - Clients SHOULD be tolerant in parsing the Status-Line and servers - tolerant when parsing the Request-Line. In particular, they SHOULD - accept any amount of SP or HT characters between fields, even though - only a single SP is required. - - The line terminator for message-header fields is the sequence CRLF. - However, we recommend that applications, when parsing such headers, - recognize a single LF as a line terminator and ignore the leading CR. - - The character set of an entity-body SHOULD be labeled as the lowest - common denominator of the character codes used within that body, with - the exception that not labeling the entity is preferred over labeling - the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1 - and 3.4.1. - - Additional rules for requirements on parsing and encoding of dates - and other potential problems with date encodings include: - - - HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date - which appears to be more than 50 years in the future is in fact - in the past (this helps solve the "year 2000" problem). - - - An HTTP/1.1 implementation MAY internally represent a parsed - Expires date as earlier than the proper value, but MUST NOT - internally represent a parsed Expires date as later than the - proper value. - - - All expiration-related calculations MUST be done in GMT. The - local time zone MUST NOT influence the calculation or comparison - of an age or expiration time. - - - - -Fielding, et al. Standards Track [Page 166] - -RFC 2616 HTTP/1.1 June 1999 - - - - If an HTTP header incorrectly carries a date value with a time - zone other than GMT, it MUST be converted into GMT using the - most conservative possible conversion. - -19.4 Differences Between HTTP Entities and RFC 2045 Entities - - HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC - 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to - allow entities to be transmitted in an open variety of - representations and with extensible mechanisms. However, RFC 2045 - discusses mail, and HTTP has a few features that are different from - those described in RFC 2045. These differences were carefully chosen - to optimize performance over binary connections, to allow greater - freedom in the use of new media types, to make date comparisons - easier, and to acknowledge the practice of some early HTTP servers - and clients. - - This appendix describes specific areas where HTTP differs from RFC - 2045. Proxies and gateways to strict MIME environments SHOULD be - aware of these differences and provide the appropriate conversions - where necessary. Proxies and gateways from MIME environments to HTTP - also need to be aware of the differences because some conversions - might be required. - -19.4.1 MIME-Version - - HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY - include a single MIME-Version general-header field to indicate what - version of the MIME protocol was used to construct the message. Use - of the MIME-Version header field indicates that the message is in - full compliance with the MIME protocol (as defined in RFC 2045[7]). - Proxies/gateways are responsible for ensuring full compliance (where - possible) when exporting HTTP messages to strict MIME environments. - - MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT - - MIME version "1.0" is the default for use in HTTP/1.1. However, - HTTP/1.1 message parsing and semantics are defined by this document - and not the MIME specification. - -19.4.2 Conversion to Canonical Form - - RFC 2045 [7] requires that an Internet mail entity be converted to - canonical form prior to being transferred, as described in section 4 - of RFC 2049 [48]. Section 3.7.1 of this document describes the forms - allowed for subtypes of the "text" media type when transmitted over - HTTP. RFC 2046 requires that content with a type of "text" represent - line breaks as CRLF and forbids the use of CR or LF outside of line - - - -Fielding, et al. Standards Track [Page 167] - -RFC 2616 HTTP/1.1 June 1999 - - - break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a - line break within text content when a message is transmitted over - HTTP. - - Where it is possible, a proxy or gateway from HTTP to a strict MIME - environment SHOULD translate all line breaks within the text media - types described in section 3.7.1 of this document to the RFC 2049 - canonical form of CRLF. Note, however, that this might be complicated - by the presence of a Content-Encoding and by the fact that HTTP - allows the use of some character sets which do not use octets 13 and - 10 to represent CR and LF, as is the case for some multi-byte - character sets. - - Implementors should note that conversion will break any cryptographic - checksums applied to the original content unless the original content - is already in canonical form. Therefore, the canonical form is - recommended for any content that uses such checksums in HTTP. - -19.4.3 Conversion of Date Formats - - HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to - simplify the process of date comparison. Proxies and gateways from - other protocols SHOULD ensure that any Date header field present in a - message conforms to one of the HTTP/1.1 formats and rewrite the date - if necessary. - -19.4.4 Introduction of Content-Encoding - - RFC 2045 does not include any concept equivalent to HTTP/1.1's - Content-Encoding header field. Since this acts as a modifier on the - media type, proxies and gateways from HTTP to MIME-compliant - protocols MUST either change the value of the Content-Type header - field or decode the entity-body before forwarding the message. (Some - experimental applications of Content-Type for Internet mail have used - a media-type parameter of ";conversions=<content-coding>" to perform - a function equivalent to Content-Encoding. However, this parameter is - not part of RFC 2045.) - -19.4.5 No Content-Transfer-Encoding - - HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC - 2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST - remove any non-identity CTE ("quoted-printable" or "base64") encoding - prior to delivering the response message to an HTTP client. - - Proxies and gateways from HTTP to MIME-compliant protocols are - responsible for ensuring that the message is in the correct format - and encoding for safe transport on that protocol, where "safe - - - -Fielding, et al. Standards Track [Page 168] - -RFC 2616 HTTP/1.1 June 1999 - - - transport" is defined by the limitations of the protocol being used. - Such a proxy or gateway SHOULD label the data with an appropriate - Content-Transfer-Encoding if doing so will improve the likelihood of - safe transport over the destination protocol. - -19.4.6 Introduction of Transfer-Encoding - - HTTP/1.1 introduces the Transfer-Encoding header field (section - 14.41). Proxies/gateways MUST remove any transfer-coding prior to - forwarding a message via a MIME-compliant protocol. - - A process for decoding the "chunked" transfer-coding (section 3.6) - can be represented in pseudo-code as: - - length := 0 - read chunk-size, chunk-extension (if any) and CRLF - while (chunk-size > 0) { - read chunk-data and CRLF - append chunk-data to entity-body - length := length + chunk-size - read chunk-size and CRLF - } - read entity-header - while (entity-header not empty) { - append entity-header to existing header fields - read entity-header - } - Content-Length := length - Remove "chunked" from Transfer-Encoding - -19.4.7 MHTML and Line Length Limitations - - HTTP implementations which share code with MHTML [45] implementations - need to be aware of MIME line length limitations. Since HTTP does not - have this limitation, HTTP does not fold long lines. MHTML messages - being transported by HTTP follow all conventions of MHTML, including - line length limitations and folding, canonicalization, etc., since - HTTP transports all message-bodies as payload (see section 3.7.2) and - does not interpret the content or any MIME header lines that might be - contained therein. - -19.5 Additional Features - - RFC 1945 and RFC 2068 document protocol elements used by some - existing HTTP implementations, but not consistently and correctly - across most HTTP/1.1 applications. Implementors are advised to be - aware of these features, but cannot rely upon their presence in, or - interoperability with, other HTTP/1.1 applications. Some of these - - - -Fielding, et al. Standards Track [Page 169] - -RFC 2616 HTTP/1.1 June 1999 - - - describe proposed experimental features, and some describe features - that experimental deployment found lacking that are now addressed in - the base HTTP/1.1 specification. - - A number of other headers, such as Content-Disposition and Title, - from SMTP and MIME are also often implemented (see RFC 2076 [37]). - -19.5.1 Content-Disposition - - The Content-Disposition response-header field has been proposed as a - means for the origin server to suggest a default filename if the user - requests that the content is saved to a file. This usage is derived - from the definition of Content-Disposition in RFC 1806 [35]. - - content-disposition = "Content-Disposition" ":" - disposition-type *( ";" disposition-parm ) - disposition-type = "attachment" | disp-extension-token - disposition-parm = filename-parm | disp-extension-parm - filename-parm = "filename" "=" quoted-string - disp-extension-token = token - disp-extension-parm = token "=" ( token | quoted-string ) - - An example is - - Content-Disposition: attachment; filename="fname.ext" - - The receiving user agent SHOULD NOT respect any directory path - information present in the filename-parm parameter, which is the only - parameter believed to apply to HTTP implementations at this time. The - filename SHOULD be treated as a terminal component only. - - If this header is used in a response with the application/octet- - stream content-type, the implied suggestion is that the user agent - should not display the response, but directly enter a `save response - as...' dialog. - - See section 15.5 for Content-Disposition security issues. - -19.6 Compatibility with Previous Versions - - It is beyond the scope of a protocol specification to mandate - compliance with previous versions. HTTP/1.1 was deliberately - designed, however, to make supporting previous versions easy. It is - worth noting that, at the time of composing this specification - (1996), we would expect commercial HTTP/1.1 servers to: - - - recognize the format of the Request-Line for HTTP/0.9, 1.0, and - 1.1 requests; - - - -Fielding, et al. Standards Track [Page 170] - -RFC 2616 HTTP/1.1 June 1999 - - - - understand any valid request in the format of HTTP/0.9, 1.0, or - 1.1; - - - respond appropriately with a message in the same major version - used by the client. - - And we would expect HTTP/1.1 clients to: - - - recognize the format of the Status-Line for HTTP/1.0 and 1.1 - responses; - - - understand any valid response in the format of HTTP/0.9, 1.0, or - 1.1. - - For most implementations of HTTP/1.0, each connection is established - by the client prior to the request and closed by the server after - sending the response. Some implementations implement the Keep-Alive - version of persistent connections described in section 19.7.1 of RFC - 2068 [33]. - -19.6.1 Changes from HTTP/1.0 - - This section summarizes major differences between versions HTTP/1.0 - and HTTP/1.1. - -19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP - Addresses - - The requirements that clients and servers support the Host request- - header, report an error if the Host request-header (section 14.23) is - missing from an HTTP/1.1 request, and accept absolute URIs (section - 5.1.2) are among the most important changes defined by this - specification. - - Older HTTP/1.0 clients assumed a one-to-one relationship of IP - addresses and servers; there was no other established mechanism for - distinguishing the intended server of a request than the IP address - to which that request was directed. The changes outlined above will - allow the Internet, once older HTTP clients are no longer common, to - support multiple Web sites from a single IP address, greatly - simplifying large operational Web servers, where allocation of many - IP addresses to a single host has created serious problems. The - Internet will also be able to recover the IP addresses that have been - allocated for the sole purpose of allowing special-purpose domain - names to be used in root-level HTTP URLs. Given the rate of growth of - the Web, and the number of servers already deployed, it is extremely - - - - - -Fielding, et al. Standards Track [Page 171] - -RFC 2616 HTTP/1.1 June 1999 - - - important that all implementations of HTTP (including updates to - existing HTTP/1.0 applications) correctly implement these - requirements: - - - Both clients and servers MUST support the Host request-header. - - - A client that sends an HTTP/1.1 request MUST send a Host header. - - - Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 - request does not include a Host request-header. - - - Servers MUST accept absolute URIs. - -19.6.2 Compatibility with HTTP/1.0 Persistent Connections - - Some clients and servers might wish to be compatible with some - previous implementations of persistent connections in HTTP/1.0 - clients and servers. Persistent connections in HTTP/1.0 are - explicitly negotiated as they are not the default behavior. HTTP/1.0 - experimental implementations of persistent connections are faulty, - and the new facilities in HTTP/1.1 are designed to rectify these - problems. The problem was that some existing 1.0 clients may be - sending Keep-Alive to a proxy server that doesn't understand - Connection, which would then erroneously forward it to the next - inbound server, which would establish the Keep-Alive connection and - result in a hung HTTP/1.0 proxy waiting for the close on the - response. The result is that HTTP/1.0 clients must be prevented from - using Keep-Alive when talking to proxies. - - However, talking to proxies is the most important use of persistent - connections, so that prohibition is clearly unacceptable. Therefore, - we need some other mechanism for indicating a persistent connection - is desired, which is safe to use even when talking to an old proxy - that ignores Connection. Persistent connections are the default for - HTTP/1.1 messages; we introduce a new keyword (Connection: close) for - declaring non-persistence. See section 14.10. - - The original HTTP/1.0 form of persistent connections (the Connection: - Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33] - -19.6.3 Changes from RFC 2068 - - This specification has been carefully audited to correct and - disambiguate key word usage; RFC 2068 had many problems in respect to - the conventions laid out in RFC 2119 [34]. - - Clarified which error code should be used for inbound server failures - (e.g. DNS failures). (Section 10.5.5). - - - -Fielding, et al. Standards Track [Page 172] - -RFC 2616 HTTP/1.1 June 1999 - - - CREATE had a race that required an Etag be sent when a resource is - first created. (Section 10.2.2). - - Content-Base was deleted from the specification: it was not - implemented widely, and there is no simple, safe way to introduce it - without a robust extension mechanism. In addition, it is used in a - similar, but not identical fashion in MHTML [45]. - - Transfer-coding and message lengths all interact in ways that - required fixing exactly when chunked encoding is used (to allow for - transfer encoding that may not be self delimiting); it was important - to straighten out exactly how message lengths are computed. (Sections - 3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16) - - A content-coding of "identity" was introduced, to solve problems - discovered in caching. (section 3.5) - - Quality Values of zero should indicate that "I don't want something" - to allow clients to refuse a representation. (Section 3.9) - - The use and interpretation of HTTP version numbers has been clarified - by RFC 2145. Require proxies to upgrade requests to highest protocol - version they support to deal with problems discovered in HTTP/1.0 - implementations (Section 3.1) - - Charset wildcarding is introduced to avoid explosion of character set - names in accept headers. (Section 14.2) - - A case was missed in the Cache-Control model of HTTP/1.1; s-maxage - was introduced to add this missing case. (Sections 13.4, 14.8, 14.9, - 14.9.3) - - The Cache-Control: max-age directive was not properly defined for - responses. (Section 14.9.3) - - There are situations where a server (especially a proxy) does not - know the full length of a response but is capable of serving a - byterange request. We therefore need a mechanism to allow byteranges - with a content-range not indicating the full length of the message. - (Section 14.16) - - Range request responses would become very verbose if all meta-data - were always returned; by allowing the server to only send needed - headers in a 206 response, this problem can be avoided. (Section - 10.2.7, 13.5.3, and 14.27) - - - - - - -Fielding, et al. Standards Track [Page 173] - -RFC 2616 HTTP/1.1 June 1999 - - - Fix problem with unsatisfiable range requests; there are two cases: - syntactic problems, and range doesn't exist in the document. The 416 - status code was needed to resolve this ambiguity needed to indicate - an error for a byte range request that falls outside of the actual - contents of a document. (Section 10.4.17, 14.16) - - Rewrite of message transmission requirements to make it much harder - for implementors to get it wrong, as the consequences of errors here - can have significant impact on the Internet, and to deal with the - following problems: - - 1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where - this was incorrectly placing a requirement on the behavior of - an implementation of a future version of HTTP/1.x - - 2. Made it clear that user-agents should retry requests, not - "clients" in general. - - 3. Converted requirements for clients to ignore unexpected 100 - (Continue) responses, and for proxies to forward 100 responses, - into a general requirement for 1xx responses. - - 4. Modified some TCP-specific language, to make it clearer that - non-TCP transports are possible for HTTP. - - 5. Require that the origin server MUST NOT wait for the request - body before it sends a required 100 (Continue) response. - - 6. Allow, rather than require, a server to omit 100 (Continue) if - it has already seen some of the request body. - - 7. Allow servers to defend against denial-of-service attacks and - broken clients. - - This change adds the Expect header and 417 status code. The message - transmission requirements fixes are in sections 8.2, 10.4.18, - 8.1.2.2, 13.11, and 14.20. - - Proxies should be able to add Content-Length when appropriate. - (Section 13.5.2) - - Clean up confusion between 403 and 404 responses. (Section 10.4.4, - 10.4.5, and 10.4.11) - - Warnings could be cached incorrectly, or not updated appropriately. - (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning - also needed to be a general header, as PUT or other methods may have - need for it in requests. - - - -Fielding, et al. Standards Track [Page 174] - -RFC 2616 HTTP/1.1 June 1999 - - - Transfer-coding had significant problems, particularly with - interactions with chunked encoding. The solution is that transfer- - codings become as full fledged as content-codings. This involves - adding an IANA registry for transfer-codings (separate from content - codings), a new header field (TE) and enabling trailer headers in the - future. Transfer encoding is a major performance benefit, so it was - worth fixing [39]. TE also solves another, obscure, downward - interoperability problem that could have occurred due to interactions - between authentication trailers, chunked encoding and HTTP/1.0 - clients.(Section 3.6, 3.6.1, and 14.39) - - The PATCH, LINK, UNLINK methods were defined but not commonly - implemented in previous versions of this specification. See RFC 2068 - [33]. - - The Alternates, Content-Version, Derived-From, Link, URI, Public and - Content-Base header fields were defined in previous versions of this - specification, but not commonly implemented. See RFC 2068 [33]. - -20 Index - - Please see the PostScript version of this RFC for the INDEX. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Fielding, et al. Standards Track [Page 175] - -RFC 2616 HTTP/1.1 June 1999 - - -21. Full Copyright Statement - - Copyright (C) The Internet Society (1999). All Rights Reserved. - - This document and translations of it may be copied and furnished to - others, and derivative works that comment on or otherwise explain it - or assist in its implementation may be prepared, copied, published - and distributed, in whole or in part, without restriction of any - kind, provided that the above copyright notice and this paragraph are - included on all such copies and derivative works. However, this - document itself may not be modified in any way, such as by removing - the copyright notice or references to the Internet Society or other - Internet organizations, except as needed for the purpose of - developing Internet standards in which case the procedures for - copyrights defined in the Internet Standards process must be - followed, or as required to translate it into languages other than - English. - - The limited permissions granted above are perpetual and will not be - revoked by the Internet Society or its successors or assigns. - - This document and the information contained herein is provided on an - "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING - TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING - BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION - HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF - MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. - -Acknowledgement - - Funding for the RFC Editor function is currently provided by the - Internet Society. - - - - - - - - - - - - - - - - - - - -Fielding, et al. 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