RFC 2616 (rfc2616) - Hypertext Transfer Protocol -- HTTP/1.1

RFC 2616 (RFC2616)

Internet RFC/STD/FYI/BCP Archives

RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1

+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]. + +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 + + 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 + + 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 + + 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 + + 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 + + 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 + + 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. + + 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. + + 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. + + 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. + + 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 + + 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. + + 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)". + + 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]. + + 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 + + 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. + + 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. + +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. + +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 + /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. + + 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: + + 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 + + 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]. + + 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. + + 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). + + 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. + + 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 + + [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 + + 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 + + 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. + + 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 + + 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 + + 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 + + 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 + + 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 + +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 + + 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. + +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 + + | 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. + + 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 + + | "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 + + | 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. + +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. + +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. + +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. + +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. + + 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. + +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 + + 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. + + - 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. + + 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. + + 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 + + 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. + +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. + + 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, + + 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 + + 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 + + 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; + + 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). + +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 + + - 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 + + 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). + + 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 + + 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. + +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. + +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. + +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 + + 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. + +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. + +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. + +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. + + 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. + + 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. + + 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, + + 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); + + 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: + + 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, + + 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. + + 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. + + 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. + + 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 + + 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 + + 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) + +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. + + 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. + + 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. + + 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, + + - 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 + + 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). + + - 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. + +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). + +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. + + 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). + +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. + +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 + + 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 + + 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. + + 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. + +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. + +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". + + 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 + + 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" ":" + + 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., + + "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 + + 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. + +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. + + 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. + +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 ) ] + + 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 + + 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. + + 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. + + 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). + + 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 + + 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 + + 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 + + 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. + + 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. + +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 + + 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. + + 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. + +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. + + 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 + + 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. + + 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: + + 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). + +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. + + 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.) + + 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, + + 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 + + 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 + + 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 + + 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. + + 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 ) + + 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 + + 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. + +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. + + 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 + + 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. + + 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 + + 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 + + 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. + +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.") + + 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 + + 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. + + 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 + + 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. + + 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 + + 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 + + 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. + + 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. + +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 + + 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 + + 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. + +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. + +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). + + 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: + + 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. + + 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. + + [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. + + [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] + + [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. + +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 + + 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 + +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 + +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. + + 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. + + - 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 + + 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 + + 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 + + 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; + + - 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 + + 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). + + 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) + + 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. + + 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. + +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. + +

+ + +

Comments about this RFC:

RFC 2616: Very basic HTTP Server written in Emacs Lisp: http://www.chez.com/emarsden/downl... by Alex Schr�der (5/12/2004)
RFC 2616: If someone has got sample code for a basic HTTP/1.1 Server please let me know...... by Mike (2/4/2004)
RFC 2616: I am seeking ladies for nothing but sex that is all i want from the lady sorry... by treblav (1/16/2004)
RFC 2616: Hello. + + I have realized that point 8.1.4 of the RFC 2616 is a big threat to... by Doolyo (8/20/2005)
RFC 2616: ��::::::@::@"""@:>c + :::@"$$%^^*^^*((((99( + <<>:"? by �� (12/16/2004)
RFC 2616: Man Such a big and boring doc.. thats why ppl shy away from RFC stuff. Make it... by AnurgaM (6/15/2005)
RFC 2616: igeve uo 100 dolars + + iwant password + + farh51@hotmail.com by farh (7/24/2005)
RFC 2616: I have realized that point 8.1.4 of this RFC is a big threat to the load speed... by Doolyo (8/15/2005)
RFC 2616: Would it possible to extend the + protocol to allow another GET, + HEAD, POST,... by CCaldwell (9/15/2004)
RFC 2616: Wonder if he got his lady? by SinJax (3/26/2004)
RFC 2616: it's for shareaza that the program go fasther by sylle (5/24/2005)
RFC 2616: Only 2 simultaneous connections is a big threat to the load speed of HTML... by Doolyo (9/30/2005)
RFC 2616: yo to the southampton uni coursework massive by anon (4/6/2004)
RFC 2616: If you know everything about HTTP this rfc will be helpful to you (as a recap... by just a developer (1/5/2005)
RFC 2616: Please note that section 3.6.2, referenced in section 3.5 (page 25) + may most... by Patrick Powell (8/12/2004)
RFC 2616: gtu87t 86t8o;koip]0-n df bhvfjkhfgfter6ecvgjlhhjuhjioy uftyei vn bk.jljpoi + =- ... by jkl (10/20/2004)
RFC 2616: I HOPE KNOW MANY OF SOME RFC by MARRY (5/5/2005)

rfc2616.pdf

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