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| author | Erlang/OTP <[email protected]> | 2009-11-20 14:54:40 +0000 |
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| committer | Erlang/OTP <[email protected]> | 2009-11-20 14:54:40 +0000 |
| commit | 84adefa331c4159d432d22840663c38f155cd4c1 (patch) | |
| tree | bff9a9c66adda4df2106dfd0e5c053ab182a12bd /lib/inets/doc/archive/rfc2068.txt | |
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The R13B03 release.OTP_R13B03
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diff --git a/lib/inets/doc/archive/rfc2068.txt b/lib/inets/doc/archive/rfc2068.txt new file mode 100644 index 0000000000..e16e4fdf7d --- /dev/null +++ b/lib/inets/doc/archive/rfc2068.txt @@ -0,0 +1,9075 @@ + + + + + + +Network Working Group R. Fielding +Request for Comments: 2068 UC Irvine +Category: Standards Track J. Gettys + J. Mogul + DEC + H. Frystyk + T. Berners-Lee + MIT/LCS + January 1997 + + + 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. + +Abstract + + The Hypertext Transfer Protocol (HTTP) is an application-level + protocol for distributed, collaborative, hypermedia information + systems. It is a generic, stateless, object-oriented protocol which + can be used for many tasks, such as name servers and distributed + object management systems, through extension of its request methods. + 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". + +Table of Contents + + 1 Introduction.............................................7 + 1.1 Purpose ..............................................7 + 1.2 Requirements .........................................7 + 1.3 Terminology ..........................................8 + 1.4 Overall Operation ...................................11 + 2 Notational Conventions and Generic Grammar..............13 + 2.1 Augmented BNF .......................................13 + 2.2 Basic Rules .........................................15 + 3 Protocol Parameters.....................................17 + 3.1 HTTP Version ........................................17 + + + +Fielding, et. al. Standards Track [Page 1] + +RFC 2068 HTTP/1.1 January 1997 + + + 3.2 Uniform Resource Identifiers ........................18 + 3.2.1 General Syntax ...................................18 + 3.2.2 http URL .........................................19 + 3.2.3 URI Comparison ...................................20 + 3.3 Date/Time Formats ...................................21 + 3.3.1 Full Date ........................................21 + 3.3.2 Delta Seconds ....................................22 + 3.4 Character Sets ......................................22 + 3.5 Content Codings .....................................23 + 3.6 Transfer Codings ....................................24 + 3.7 Media Types .........................................25 + 3.7.1 Canonicalization and Text Defaults ...............26 + 3.7.2 Multipart Types ..................................27 + 3.8 Product Tokens ......................................28 + 3.9 Quality Values ......................................28 + 3.10 Language Tags ......................................28 + 3.11 Entity Tags ........................................29 + 3.12 Range Units ........................................30 + 4 HTTP Message............................................30 + 4.1 Message Types .......................................30 + 4.2 Message Headers .....................................31 + 4.3 Message Body ........................................32 + 4.4 Message Length ......................................32 + 4.5 General Header Fields ...............................34 + 5 Request.................................................34 + 5.1 Request-Line ........................................34 + 5.1.1 Method ...........................................35 + 5.1.2 Request-URI ......................................35 + 5.2 The Resource Identified by a Request ................37 + 5.3 Request Header Fields ...............................37 + 6 Response................................................38 + 6.1 Status-Line .........................................38 + 6.1.1 Status Code and Reason Phrase ....................39 + 6.2 Response Header Fields ..............................41 + 7 Entity..................................................41 + 7.1 Entity Header Fields ................................41 + 7.2 Entity Body .........................................42 + 7.2.1 Type .............................................42 + 7.2.2 Length ...........................................43 + 8 Connections.............................................43 + 8.1 Persistent Connections ..............................43 + 8.1.1 Purpose ..........................................43 + 8.1.2 Overall Operation ................................44 + 8.1.3 Proxy Servers ....................................45 + 8.1.4 Practical Considerations .........................45 + 8.2 Message Transmission Requirements ...................46 + 9 Method Definitions......................................48 + 9.1 Safe and Idempotent Methods .........................48 + + + +Fielding, et. al. Standards Track [Page 2] + +RFC 2068 HTTP/1.1 January 1997 + + + 9.1.1 Safe Methods .....................................48 + 9.1.2 Idempotent Methods ...............................49 + 9.2 OPTIONS .............................................49 + 9.3 GET .................................................50 + 9.4 HEAD ................................................50 + 9.5 POST ................................................51 + 9.6 PUT .................................................52 + 9.7 DELETE ..............................................53 + 9.8 TRACE ...............................................53 + 10 Status Code Definitions................................53 + 10.1 Informational 1xx ..................................54 + 10.1.1 100 Continue ....................................54 + 10.1.2 101 Switching Protocols .........................54 + 10.2 Successful 2xx .....................................54 + 10.2.1 200 OK ..........................................54 + 10.2.2 201 Created .....................................55 + 10.2.3 202 Accepted ....................................55 + 10.2.4 203 Non-Authoritative Information ...............55 + 10.2.5 204 No Content ..................................55 + 10.2.6 205 Reset Content ...............................56 + 10.2.7 206 Partial Content .............................56 + 10.3 Redirection 3xx ....................................56 + 10.3.1 300 Multiple Choices ............................57 + 10.3.2 301 Moved Permanently ...........................57 + 10.3.3 302 Moved Temporarily ...........................58 + 10.3.4 303 See Other ...................................58 + 10.3.5 304 Not Modified ................................58 + 10.3.6 305 Use Proxy ...................................59 + 10.4 Client Error 4xx ...................................59 + 10.4.1 400 Bad Request .................................60 + 10.4.2 401 Unauthorized ................................60 + 10.4.3 402 Payment Required ............................60 + 10.4.4 403 Forbidden ...................................60 + 10.4.5 404 Not Found ...................................60 + 10.4.6 405 Method Not Allowed ..........................61 + 10.4.7 406 Not Acceptable ..............................61 + 10.4.8 407 Proxy Authentication Required ...............61 + 10.4.9 408 Request Timeout .............................62 + 10.4.10 409 Conflict ...................................62 + 10.4.11 410 Gone .......................................62 + 10.4.12 411 Length Required ............................63 + 10.4.13 412 Precondition Failed ........................63 + 10.4.14 413 Request Entity Too Large ...................63 + 10.4.15 414 Request-URI Too Long .......................63 + 10.4.16 415 Unsupported Media Type .....................63 + 10.5 Server Error 5xx ...................................64 + 10.5.1 500 Internal Server Error .......................64 + 10.5.2 501 Not Implemented .............................64 + + + +Fielding, et. al. Standards Track [Page 3] + +RFC 2068 HTTP/1.1 January 1997 + + + 10.5.3 502 Bad Gateway .................................64 + 10.5.4 503 Service Unavailable .........................64 + 10.5.5 504 Gateway Timeout .............................64 + 10.5.6 505 HTTP Version Not Supported ..................65 + 11 Access Authentication..................................65 + 11.1 Basic Authentication Scheme ........................66 + 11.2 Digest Authentication Scheme .......................67 + 12 Content Negotiation....................................67 + 12.1 Server-driven Negotiation ..........................68 + 12.2 Agent-driven Negotiation ...........................69 + 12.3 Transparent Negotiation ............................70 + 13 Caching in HTTP........................................70 + 13.1.1 Cache Correctness ...............................72 + 13.1.2 Warnings ........................................73 + 13.1.3 Cache-control Mechanisms ........................74 + 13.1.4 Explicit User Agent Warnings ....................74 + 13.1.5 Exceptions to the Rules and Warnings ............75 + 13.1.6 Client-controlled Behavior ......................75 + 13.2 Expiration Model ...................................75 + 13.2.1 Server-Specified Expiration .....................75 + 13.2.2 Heuristic Expiration ............................76 + 13.2.3 Age Calculations ................................77 + 13.2.4 Expiration Calculations .........................79 + 13.2.5 Disambiguating Expiration Values ................80 + 13.2.6 Disambiguating Multiple Responses ...............80 + 13.3 Validation Model ...................................81 + 13.3.1 Last-modified Dates .............................82 + 13.3.2 Entity Tag Cache Validators .....................82 + 13.3.3 Weak and Strong Validators ......................82 + 13.3.4 Rules for When to Use Entity Tags and Last- + modified Dates..........................................85 + 13.3.5 Non-validating Conditionals .....................86 + 13.4 Response Cachability ...............................86 + 13.5 Constructing Responses From Caches .................87 + 13.5.1 End-to-end and Hop-by-hop Headers ...............88 + 13.5.2 Non-modifiable Headers ..........................88 + 13.5.3 Combining Headers ...............................89 + 13.5.4 Combining Byte Ranges ...........................90 + 13.6 Caching Negotiated Responses .......................90 + 13.7 Shared and Non-Shared Caches .......................91 + 13.8 Errors or Incomplete Response Cache Behavior .......91 + 13.9 Side Effects of GET and HEAD .......................92 + 13.10 Invalidation After Updates or Deletions ...........92 + 13.11 Write-Through Mandatory ...........................93 + 13.12 Cache Replacement .................................93 + 13.13 History Lists .....................................93 + 14 Header Field Definitions...............................94 + 14.1 Accept .............................................95 + + + +Fielding, et. al. Standards Track [Page 4] + +RFC 2068 HTTP/1.1 January 1997 + + + 14.2 Accept-Charset .....................................97 + 14.3 Accept-Encoding ....................................97 + 14.4 Accept-Language ....................................98 + 14.5 Accept-Ranges ......................................99 + 14.6 Age ................................................99 + 14.7 Allow .............................................100 + 14.8 Authorization .....................................100 + 14.9 Cache-Control .....................................101 + 14.9.1 What is Cachable ...............................103 + 14.9.2 What May be Stored by Caches ...................103 + 14.9.3 Modifications of the Basic Expiration Mechanism 104 + 14.9.4 Cache Revalidation and Reload Controls .........105 + 14.9.5 No-Transform Directive .........................107 + 14.9.6 Cache Control Extensions .......................108 + 14.10 Connection .......................................109 + 14.11 Content-Base .....................................109 + 14.12 Content-Encoding .................................110 + 14.13 Content-Language .................................110 + 14.14 Content-Length ...................................111 + 14.15 Content-Location .................................112 + 14.16 Content-MD5 ......................................113 + 14.17 Content-Range ....................................114 + 14.18 Content-Type .....................................116 + 14.19 Date .............................................116 + 14.20 ETag .............................................117 + 14.21 Expires ..........................................117 + 14.22 From .............................................118 + 14.23 Host .............................................119 + 14.24 If-Modified-Since ................................119 + 14.25 If-Match .........................................121 + 14.26 If-None-Match ....................................122 + 14.27 If-Range .........................................123 + 14.28 If-Unmodified-Since ..............................124 + 14.29 Last-Modified ....................................124 + 14.30 Location .........................................125 + 14.31 Max-Forwards .....................................125 + 14.32 Pragma ...........................................126 + 14.33 Proxy-Authenticate ...............................127 + 14.34 Proxy-Authorization ..............................127 + 14.35 Public ...........................................127 + 14.36 Range ............................................128 + 14.36.1 Byte Ranges ...................................128 + 14.36.2 Range Retrieval Requests ......................130 + 14.37 Referer ..........................................131 + 14.38 Retry-After ......................................131 + 14.39 Server ...........................................132 + 14.40 Transfer-Encoding ................................132 + 14.41 Upgrade ..........................................132 + + + +Fielding, et. al. Standards Track [Page 5] + +RFC 2068 HTTP/1.1 January 1997 + + + 14.42 User-Agent .......................................134 + 14.43 Vary .............................................134 + 14.44 Via ..............................................135 + 14.45 Warning ..........................................137 + 14.46 WWW-Authenticate .................................139 + 15 Security Considerations...............................139 + 15.1 Authentication of Clients .........................139 + 15.2 Offering a Choice of Authentication Schemes .......140 + 15.3 Abuse of Server Log Information ...................141 + 15.4 Transfer of Sensitive Information .................141 + 15.5 Attacks Based On File and Path Names ..............142 + 15.6 Personal Information ..............................143 + 15.7 Privacy Issues Connected to Accept Headers ........143 + 15.8 DNS Spoofing ......................................144 + 15.9 Location Headers and Spoofing .....................144 + 16 Acknowledgments.......................................144 + 17 References............................................146 + 18 Authors' Addresses....................................149 + 19 Appendices............................................150 + 19.1 Internet Media Type message/http ..................150 + 19.2 Internet Media Type multipart/byteranges ..........150 + 19.3 Tolerant Applications .............................151 + 19.4 Differences Between HTTP Entities and + MIME Entities...........................................152 + 19.4.1 Conversion to Canonical Form ...................152 + 19.4.2 Conversion of Date Formats .....................153 + 19.4.3 Introduction of Content-Encoding ...............153 + 19.4.4 No Content-Transfer-Encoding ...................153 + 19.4.5 HTTP Header Fields in Multipart Body-Parts .....153 + 19.4.6 Introduction of Transfer-Encoding ..............154 + 19.4.7 MIME-Version ...................................154 + 19.5 Changes from HTTP/1.0 .............................154 + 19.5.1 Changes to Simplify Multi-homed Web Servers and + Conserve IP Addresses .................................155 + 19.6 Additional Features ...............................156 + 19.6.1 Additional Request Methods .....................156 + 19.6.2 Additional Header Field Definitions ............156 + 19.7 Compatibility with Previous Versions ..............160 + 19.7.1 Compatibility with HTTP/1.0 Persistent + Connections............................................161 + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 6] + +RFC 2068 HTTP/1.1 January 1997 + + +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, and 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 that indicate the purpose of a + request. It builds on the discipline of reference provided by the + Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or + name (URN) , 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 as defined by the Multipurpose Internet Mail Extensions + (MIME). + + 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 + + This specification uses the same words as RFC 1123 [8] for defining + the significance of each particular requirement. These words are: + + MUST + This word or the adjective "required" means that the item is an + absolute requirement of the specification. + + + +Fielding, et. al. Standards Track [Page 7] + +RFC 2068 HTTP/1.1 January 1997 + + + SHOULD + This word or the adjective "recommended" means that there may + exist valid reasons in particular circumstances to ignore this + item, but the full implications should be understood and the case + carefully weighed before choosing a different course. + + MAY + This word or the adjective "optional" means that this item is + truly optional. One vendor may choose to include the item because + a particular marketplace requires it or because it enhances the + product, for example; another vendor may omit the same item. + + An implementation is not compliant if it fails to satisfy one or more + of the MUST requirements for the protocols it implements. An + implementation that satisfies all the MUST and all the SHOULD + requirements for its protocols is said to be "unconditionally + compliant"; one that satisfies all the MUST requirements but not all + the SHOULD 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, + resolutions) or vary in other ways. + + + + + + +Fielding, et. al. Standards Track [Page 8] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 `variant.' 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. + + + + + + + +Fielding, et. al. Standards Track [Page 9] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + 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 cachable 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. + + cachable + A response is cachable if a cache is allowed to store a copy of + the response message for use in answering subsequent requests. The + rules for determining the cachability of HTTP responses are + defined in section 13. Even if a resource is cachable, 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. + + + + + + +Fielding, et. al. Standards Track [Page 10] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + +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. + + + + + + + +Fielding, et. al. Standards Track [Page 11] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + + + + +Fielding, et. al. Standards Track [Page 12] + +RFC 2068 HTTP/1.1 January 1997 + + + request chain ----------> + UA -----v----- A -----v----- B - - - - - - C - - - - - - O + <--------- response chain + + Not all responses are usefully cachable, and some requests may + contain modifiers which place special requirements on cache behavior. + HTTP requirements for cache behavior and cachable 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, 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]. Implementers will need to be familiar with the + notation in order to understand this specification. The augmented BNF + includes the following constructs: + + + + + +Fielding, et. al. Standards Track [Page 13] + +RFC 2068 HTTP/1.1 January 1997 + + +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. Whitespace 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 whitespace (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 + + + +Fielding, et. al. Standards Track [Page 14] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 whitespace (LWS) can be included + between any two adjacent words (token or quoted-string), and + between adjacent tokens and delimiters (tspecials), without + changing the interpretation of a field. At least one delimiter + (tspecials) must exist between any two tokens, 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)> + + + + + + + + + + +Fielding, et. al. Standards Track [Page 15] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 headers 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. + + 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 1522 + [14]. + + TEXT = <any OCTET except CTLs, + but including LWS> + + 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. + + token = 1*<any CHAR except CTLs or tspecials> + + tspecials = "(" | ")" | "<" | ">" | "@" + | "," | ";" | ":" | "\" | <"> + | "/" | "[" | "]" | "?" | "=" + | "{" | "}" | 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 | comment ) ")" + ctext = <any TEXT excluding "(" and ")"> + + + + + +Fielding, et. al. Standards Track [Page 16] + +RFC 2068 HTTP/1.1 January 1997 + + + A string of text is parsed as a single word if it is quoted using + double-quote marks. + + quoted-string = ( <"> *(qdtext) <"> ) + + 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. + + 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. + + Applications sending Request or Response messages, as defined by this + specification, MUST include an HTTP-Version of "HTTP/1.1". Use of + this version number indicates that the sending application is at + least conditionally compliant with this specification. + + The HTTP version of an application is the highest HTTP version for + which the application is at least conditionally compliant. + + + +Fielding, et. al. Standards Track [Page 17] + +RFC 2068 HTTP/1.1 January 1997 + + + Proxy and gateway applications must 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 never 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, respond with an error, or switch to tunnel + behavior. Requests with a version lower than that of the + proxy/gateway's version MAY be upgraded before being forwarded; 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 , and finally the + combination of Uniform Resource Locators (URL) and Names (URN). 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, 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. + + URI = ( absoluteURI | relativeURI ) [ "#" fragment ] + + absoluteURI = scheme ":" *( uchar | reserved ) + + relativeURI = net_path | abs_path | rel_path + + net_path = "//" net_loc [ abs_path ] + abs_path = "/" rel_path + rel_path = [ path ] [ ";" params ] [ "?" query ] + + path = fsegment *( "/" segment ) + fsegment = 1*pchar + segment = *pchar + + params = param *( ";" param ) + param = *( pchar | "/" ) + + + + +Fielding, et. al. Standards Track [Page 18] + +RFC 2068 HTTP/1.1 January 1997 + + + scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." ) + net_loc = *( pchar | ";" | "?" ) + + query = *( uchar | reserved ) + fragment = *( uchar | reserved ) + + pchar = uchar | ":" | "@" | "&" | "=" | "+" + uchar = unreserved | escape + unreserved = ALPHA | DIGIT | safe | extra | national + + escape = "%" HEX HEX + reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" + extra = "!" | "*" | "'" | "(" | ")" | "," + safe = "$" | "-" | "_" | "." + unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">" + national = <any OCTET excluding ALPHA, DIGIT, + reserved, extra, safe, and unsafe> + + For definitive information on URL syntax and semantics, see RFC 1738 + [4] and RFC 1808 [11]. The BNF above includes national characters not + allowed in valid URLs as specified by RFC 1738, since HTTP servers + are not restricted in the set of unreserved characters allowed to + represent the rel_path part of addresses, and HTTP proxies may + receive requests for URIs not defined by RFC 1738. + + 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 should be cautious about depending on URI lengths + above 255 bytes, because some older client or proxy implementations + may 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. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 19] + +RFC 2068 HTTP/1.1 January 1997 + + + http_URL = "http:" "//" host [ ":" port ] [ abs_path ] + + host = <A legal Internet host domain name + or IP address (in dotted-decimal form), + as defined by Section 2.1 of RFC 1123> + + port = *DIGIT + + 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. The use of IP addresses in URL's 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). + +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: + + o A port that is empty or not given is equivalent to the default + port for that URI; + + o Comparisons of host names MUST be case-insensitive; + + o Comparisons of scheme names MUST be case-insensitive; + + o An empty abs_path is equivalent to an abs_path of "/". + + Characters other than those in the "reserved" and "unsafe" sets (see + section 3.2) are equivalent to their ""%" HEX HEX" encodings. + + For example, the following three URIs are equivalent: + + http://abc.com:80/~smith/home.html + http://ABC.com/%7Esmith/home.html + http://ABC.com:/%7esmith/home.html + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 20] + +RFC 2068 HTTP/1.1 January 1997 + + +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 (an update to RFC + 822). 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. + + 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. 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 = 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" + + + +Fielding, et. al. Standards Track [Page 21] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 encodings, 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 22] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 MUST represent the character set defined by + that registry. Applications SHOULD limit their use of character sets + to those defined by the IANA registry. + +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 14.12) 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). + + + + + + + + + +Fielding, et. al. Standards Track [Page 23] + +RFC 2068 HTTP/1.1 January 1997 + + + Note: Use of program names for the identification of encoding + formats is not desirable and should be 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]. + + 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 + + All transfer-coding values are case-insensitive. HTTP/1.1 uses + transfer coding values in the Transfer-Encoding header field (section + 14.40). + + Transfer codings are analogous to the Content-Transfer-Encoding + values of MIME , 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 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 footer 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. + + + + + +Fielding, et. al. Standards Track [Page 24] + +RFC 2068 HTTP/1.1 January 1997 + + + Chunked-Body = *chunk + "0" CRLF + footer + CRLF + + chunk = chunk-size [ chunk-ext ] CRLF + chunk-data CRLF + + hex-no-zero = <HEX excluding "0"> + + chunk-size = hex-no-zero *HEX + chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] ) + chunk-ext-name = token + chunk-ext-val = token | quoted-string + chunk-data = chunk-size(OCTET) + + footer = *entity-header + + The chunked encoding is ended by a zero-sized chunk followed by the + footer, which is terminated by an empty line. The purpose of the + footer is to provide an efficient way to supply information about an + entity that is generated dynamically; applications MUST NOT send + header fields in the footer which are not explicitly defined as being + appropriate for the footer, such as Content-MD5 or future extensions + to HTTP for digital signatures or other facilities. + + 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 transfer coding extensions + they do not understand. 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. + +3.7 Media Types + + HTTP uses Internet Media Types in the Content-Type (section 14.18) + 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. + + + +Fielding, et. al. Standards Track [Page 25] + +RFC 2068 HTTP/1.1 January 1997 + + + parameter = attribute "=" value + attribute = token + value = token | quoted-string + + The type, subtype, and parameter attribute names are case- + insensitive. Parameter values may or may 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. User agents that recognize the media-type + MUST process (or arrange to be processed by any external applications + used to process that type/subtype by the user agent) the parameters + for that MIME type as described by that type/subtype definition to + the and inform the user of any problems discovered. + + Note: 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). The media type registration process is outlined in + RFC 2048 [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. In + general, an entity-body transferred via HTTP messages MUST be + represented in the appropriate canonical form prior to its + transmission; the exception is "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-Encoding, the underlying + data MUST be in a form defined above prior to being encoded. + + + +Fielding, et. al. Standards Track [Page 26] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + 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. + +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 MIME [7], 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 MIME, the + epilogue of any multipart message MUST be empty; HTTP applications + MUST NOT transmit the epilogue (even if the original multipart + contains an epilogue). + + In HTTP, multipart body-parts MAY contain header fields which are + significant to the meaning of that part. A Content-Location header + field (section 14.15) SHOULD be included in the body-part of each + enclosed entity that can be identified by a URL. + + 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]. + + + + + + +Fielding, et. al. Standards Track [Page 27] + +RFC 2068 HTTP/1.1 January 1997 + + +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 whitespace. 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 -- use of them for + advertising or other non-essential information is explicitly + forbidden. 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. + 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. + + + + +Fielding, et. al. Standards Track [Page 28] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 + + Whitespace 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 + 14.20), If-Match (section 14.25), 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 without + implying anything about the equivalence of those entities. + + + +Fielding, et. al. Standards Track [Page 29] + +RFC 2068 HTTP/1.1 January 1997 + + +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 14.36) and Content-Range (section 14.17) + header fields. An entity may 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, one + 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 an optional message-body. + + generic-message = start-line + *message-header + 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. + + + + + + +Fielding, et. al. Standards Track [Page 30] + +RFC 2068 HTTP/1.1 January 1997 + + + Note: certain buggy HTTP/1.0 client implementations generate an + 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 SHOULD follow "common form" 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 ] CRLF + + 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, tspecials, and quoted-string> + + 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 31] + +RFC 2068 HTTP/1.1 January 1997 + + +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 + 14.40). + + 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 can be added or removed by any application along the + request/response chain. + + 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 MAY be included in a + request only when the request method (section 5.1.1) allows an + entity-body. + + 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 + + When a message-body is included with a message, the 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 14.40) is present and + indicates that the "chunked" transfer coding has been applied, then + + + +Fielding, et. al. Standards Track [Page 32] + +RFC 2068 HTTP/1.1 January 1997 + + + the length is defined by the chunked encoding (section 3.6). + + 3. If a Content-Length header field (section 14.14) is present, its + value in bytes represents the length of the message-body. + + 4. If the message uses the media type "multipart/byteranges", which is + self-delimiting, then that defines the length. This media type MUST + NOT be used unless the sender knows that the recipient can parse it; + the presence in a request of a Range header with multiple byte-range + specifiers implies that the client can parse multipart/byteranges + responses. + + 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 the + "chunked" transfer coding. If both are received, 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. + + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 33] + +RFC 2068 HTTP/1.1 January 1997 + + +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 14.19 + | Pragma ; Section 14.32 + | Transfer-Encoding ; Section 14.40 + | Upgrade ; Section 14.41 + | Via ; Section 14.44 + + 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 ) ; Section 7.1 + CRLF + [ message-body ] ; Section 7.2 + +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 are allowed + except in the final CRLF sequence. + + Request-Line = Method SP Request-URI SP HTTP-Version CRLF + + + + + + + + +Fielding, et. al. Standards Track [Page 34] + +RFC 2068 HTTP/1.1 January 1997 + + +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 + | 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. + Servers SHOULD return the status code 405 (Method Not Allowed) if the + method is known by the server but not allowed for the requested + resource, and 501 (Not Implemented) if the method is unrecognized or + not implemented by the server. The list of methods known by a server + can be listed in a Public response-header field (section 14.35). + + 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 + + The three 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 + + + +Fielding, et. al. Standards Track [Page 35] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 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 (net_loc) 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). + + If a proxy receives a request without any path in the Request-URI and + the method specified is capable of supporting the asterisk form of + request, then the last proxy on the request chain MUST forward the + request with "*" as the final Request-URI. For example, the request + + OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1 + + would be forwarded by the proxy as + + OPTIONS * HTTP/1.1 + Host: www.ics.uci.edu:8001 + + after connecting to port 8001 of host "www.ics.uci.edu". + + The Request-URI is transmitted in the format specified in section + 3.2.1. 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. + + + +Fielding, et. al. Standards Track [Page 36] + +RFC 2068 HTTP/1.1 January 1997 + + + In requests that they forward, proxies MUST NOT rewrite the + "abs_path" part of a Request-URI in any way except as noted above to + replace a null abs_path with "*", no matter what the proxy does in + its internal implementation. + + 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 URL character for a reserved purpose. Implementers + 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 + + HTTP/1.1 origin servers SHOULD be aware that 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. (But see + section 19.5.1 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 + hostnames) 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 + + + +Fielding, et. al. Standards Track [Page 37] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 + | From ; Section 14.22 + | Host ; Section 14.23 + | If-Modified-Since ; Section 14.24 + | If-Match ; 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 14.36 + | Referer ; Section 14.37 + | User-Agent ; Section 14.42 + + 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 ) ; 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. + + + + +Fielding, et. al. Standards Track [Page 38] + +RFC 2068 HTTP/1.1 January 1997 + + + 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: + + o 1xx: Informational - Request received, continuing process + + o 2xx: Success - The action was successfully received, understood, + and accepted + + o 3xx: Redirection - Further action must be taken in order to + complete the request + + o 4xx: Client Error - The request contains bad syntax or cannot be + fulfilled + + o 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 recommended + -- they may be replaced by local equivalents without affecting the + protocol. + + Status-Code = "100" ; Continue + | "101" ; Switching Protocols + | "200" ; OK + | "201" ; Created + | "202" ; Accepted + | "203" ; Non-Authoritative Information + | "204" ; No Content + | "205" ; Reset Content + | "206" ; Partial Content + | "300" ; Multiple Choices + | "301" ; Moved Permanently + | "302" ; Moved Temporarily + + + +Fielding, et. al. Standards Track [Page 39] + +RFC 2068 HTTP/1.1 January 1997 + + + | "303" ; See Other + | "304" ; Not Modified + | "305" ; Use Proxy + | "400" ; Bad Request + | "401" ; Unauthorized + | "402" ; Payment Required + | "403" ; Forbidden + | "404" ; Not Found + | "405" ; Method Not Allowed + | "406" ; Not Acceptable + | "407" ; Proxy Authentication Required + | "408" ; Request Time-out + | "409" ; Conflict + | "410" ; Gone + | "411" ; Length Required + | "412" ; Precondition Failed + | "413" ; Request Entity Too Large + | "414" ; Request-URI Too Large + | "415" ; Unsupported Media Type + | "500" ; Internal Server Error + | "501" ; Not Implemented + | "502" ; Bad Gateway + | "503" ; Service Unavailable + | "504" ; Gateway Time-out + | "505" ; 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. + + + + + + + +Fielding, et. al. Standards Track [Page 40] + +RFC 2068 HTTP/1.1 January 1997 + + +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 = Age ; Section 14.6 + | Location ; Section 14.30 + | Proxy-Authenticate ; Section 14.33 + | Public ; Section 14.35 + | Retry-After ; Section 14.38 + | Server ; Section 14.39 + | Vary ; Section 14.43 + | Warning ; Section 14.45 + | WWW-Authenticate ; Section 14.46 + + 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 optional metainformation about the + entity-body or, if no body is present, about the resource identified + by the request. + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 41] + +RFC 2068 HTTP/1.1 January 1997 + + + entity-header = Allow ; Section 14.7 + | Content-Base ; Section 14.11 + | Content-Encoding ; Section 14.12 + | Content-Language ; Section 14.13 + | Content-Length ; Section 14.14 + | Content-Location ; Section 14.15 + | Content-MD5 ; Section 14.16 + | Content-Range ; Section 14.17 + | Content-Type ; Section 14.18 + | ETag ; Section 14.20 + | 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 forwarded by 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 may 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. + + + + + +Fielding, et. al. Standards Track [Page 42] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 URL used to identify the + resource. If the media type remains unknown, the recipient SHOULD + treat it as type "application/octet-stream". + +7.2.2 Length + + The length of an entity-body is the length of the message-body after + any transfer codings have been removed. Section 4.4 defines how the + 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 requires a client to make multiple + requests of the same server in a short amount of time. Analyses of + these performance problems are available [30][27]; analysis and + results from a prototype implementation are in [26]. + + Persistent HTTP connections have a number of advantages: + + o By opening and closing fewer TCP connections, CPU time is saved, + and memory used for TCP protocol control blocks is also saved. + o 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. + o 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. + o HTTP can evolve more gracefully; since errors can be reported + without the penalty of closing the TCP connection. Clients using + future versions of HTTP might optimistically try a new feature, but + if communicating with an older server, retry with old semantics + after an error is reported. + + HTTP implementations SHOULD implement persistent connections. + + + + + +Fielding, et. al. Standards Track [Page 43] + +RFC 2068 HTTP/1.1 January 1997 + + +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 may + assume that the server will maintain a persistent connection. + + 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. 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 19.7.1 for more information on backwards + 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. + + + +Fielding, et. al. Standards Track [Page 44] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + +8.1.3 Proxy Servers + + It is especially important that proxies correctly implement the + properties of the Connection header field as specified in 14.2.1. + + 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 persistent connection with an + HTTP/1.0 client. + +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 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 MAY 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 request without user + interaction so long as the request method is idempotent (see section + + + +Fielding, et. al. Standards Track [Page 45] + +RFC 2068 HTTP/1.1 January 1997 + + + 9.1.2); other methods MUST NOT be automatically retried, although + user agents MAY offer a human operator the choice of retrying the + request. + + However, this automatic retry SHOULD NOT be repeated if the second + request 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 maintain AT MOST 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 of the Internet or other networks. + +8.2 Message Transmission Requirements + +General requirements: + +o 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. + +o 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 footer 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. + +o An HTTP/1.1 (or later) client MUST be prepared to accept a 100 + (Continue) status followed by a regular response. + +o An HTTP/1.1 (or later) server that receives a request from a + HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue) + response; it SHOULD either wait for the request to be completed + normally (thus avoiding an interrupted request) or close the + connection prematurely. + + + + +Fielding, et. al. Standards Track [Page 46] + +RFC 2068 HTTP/1.1 January 1997 + + + Upon receiving a method subject to these requirements from an + HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either + respond with 100 (Continue) status and continue to read from the + input stream, or respond with an error status. If it responds with an + error status, it MAY close the transport (TCP) connection or it MAY + continue to read and discard the rest of the request. It MUST NOT + perform the requested method if it returns an error status. + + Clients SHOULD remember the version number of at least the most + recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or + later response from the server, and it sees the connection close + before receiving any status from the server, the client SHOULD retry + the request without user interaction so long as the request method is + idempotent (see section 9.1.2); other methods MUST NOT be + automatically retried, although user agents MAY offer a human + operator the choice of retrying the request.. If the client does + retry the request, the client + + o MUST first send the request header fields, and then + + o MUST wait for the server to respond with either a 100 (Continue) + response, in which case the client should continue, or with an + error status. + + If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from + the server, it should assume that the server implements HTTP/1.0 or + older and will not use the 100 (Continue) response. If in this case + the client sees the connection close before receiving any status from + the server, the client SHOULD retry the request. If the client does + retry the request to this HTTP/1.0 server, it should 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) + + + + +Fielding, et. al. Standards Track [Page 47] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + No matter what the server version, if an error status is received, + the client + + o MUST NOT continue and + + o MUST close the connection if it has not completed sending the + message. + + An HTTP/1.1 (or later) client that sees the connection close after + receiving a 100 (Continue) but before receiving any other status + SHOULD retry the request, and need not wait for 100 (Continue) + response (but MAY do so if this simplifies the implementation). + +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 + + Implementers 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 may 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 never have the significance of taking an action + other than retrieval. These methods should 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 + + + +Fielding, et. al. Standards Track [Page 48] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 may 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. + +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. + + Unless the server's response is an error, the response MUST NOT + include entity information other than what can be considered as + communication options (e.g., Allow is appropriate, but Content-Type + is not). Responses to this method are not cachable. + + If the Request-URI is an asterisk ("*"), the OPTIONS request is + intended to apply to the server as a whole. A 200 response SHOULD + include any header fields which indicate optional features + implemented by the server (e.g., Public), including any extensions + not defined by this specification, in addition to any applicable + general or response-header fields. As described in section 5.1.2, an + "OPTIONS *" request can be applied through a proxy by specifying the + destination server in the Request-URI without any path information. + + 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 which + indicate optional features implemented by the server and applicable + to that resource (e.g., Allow), including any extensions not defined + by this specification, in addition to any applicable general or + response-header fields. If the OPTIONS request passes through a + proxy, the proxy MUST edit the response to exclude those options + which apply to a proxy's capabilities and which are known to be + unavailable through that proxy. + + + + + + + +Fielding, et. al. Standards Track [Page 49] + +RFC 2068 HTTP/1.1 January 1997 + + +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 + 14.36. 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 cachable if and only if it meets the + requirements for HTTP caching described in section 13. + +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 cachable 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. + + + + + + +Fielding, et. al. Standards Track [Page 50] + +RFC 2068 HTTP/1.1 January 1997 + + +9.5 POST + + The POST method is used to request that the destination 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: + + o Annotation of existing resources; + + o Posting a message to a bulletin board, newsgroup, mailing list, + or similar group of articles; + + o Providing a block of data, such as the result of submitting a + form, to a data-handling process; + + o 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 cachable, 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 cachable resource. + + POST requests must obey the message transmission requirements set out + in section 8.2. + + + + + + + + + +Fielding, et. al. Standards Track [Page 51] + +RFC 2068 HTTP/1.1 January 1997 + + +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 cachable. + + 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 may 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 may 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 may 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. + + + + +Fielding, et. al. Standards Track [Page 52] + +RFC 2068 HTTP/1.1 January 1997 + + +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 response is OK but 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 cachable. + +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 14.44) 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 successful, 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. + +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. + + + +Fielding, et. al. Standards Track [Page 53] + +RFC 2068 HTTP/1.1 January 1997 + + +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. 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. + +10.1.1 100 Continue + + The client may 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. + +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 14.41), 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 only be switched 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 may 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; + + + + +Fielding, et. al. Standards Track [Page 54] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 URL + for the resource given by a Location 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. + +10.2.3 202 Accepted + + The request has been accepted for processing, but the processing has + not been completed. The request MAY or MAY NOT eventually be acted + upon, as it MAY 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 MAY 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 there is no new information + to send back. If the client is a user agent, it SHOULD NOT change its + document view from that which caused the request to be sent. This + + + +Fielding, et. al. Standards Track [Page 55] + +RFC 2068 HTTP/1.1 January 1997 + + + response is primarily intended to allow input for actions to take + place without causing a change to the user agent's active document + view. The response MAY include new metainformation in the form of + entity-headers, which SHOULD apply 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 14.36) + indicating the desired range. The response MUST include either a + Content-Range header field (section 14.17) indicating the range + included with this response, or a multipart/byteranges Content-Type + including Content-Range fields for each part. If multipart/byteranges + is not used, the Content-Length header field in the response MUST + match the actual number of OCTETs transmitted in the message-body. + + 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 user agent SHOULD NOT automatically redirect a request + more than 5 times, since such redirections usually indicate an + infinite loop. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 56] + +RFC 2068 HTTP/1.1 January 1997 + + +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 URL for that representation in the Location + field; user agents MAY use the Location field value for automatic + redirection. This response is cachable 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 be done using one of the + returned URIs. Clients with link editing capabilities SHOULD + 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 cachable unless indicated otherwise. + + If the new URI is a location, its URL 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. + + + + + + + +Fielding, et. al. Standards Track [Page 57] + +RFC 2068 HTTP/1.1 January 1997 + + +10.3.3 302 Moved Temporarily + + 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 cachable if indicated by a Cache-Control or Expires header + field. + + If the new URI is a location, its URL 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: When automatically redirecting a POST request after receiving + a 302 status code, some existing HTTP/1.0 user agents will + erroneously change it into a GET request. + +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 is not cachable, but the response to the second (redirected) + request MAY be cachable. + + If the new URI is a location, its URL 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). + +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 response MUST NOT contain a + message-body. + + + + + + + + +Fielding, et. al. Standards Track [Page 58] + +RFC 2068 HTTP/1.1 January 1997 + + + The response MUST include the following header fields: + + o Date + + o ETag and/or Content-Location, if the header would have been sent in + a 200 response to the same request + + o 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. + + The 304 response MUST NOT include a message-body, and thus is always + terminated by the first empty line after the header fields. + +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 URL of the proxy. + The recipient is expected to repeat the request via the proxy. + +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. + + Note: 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 + + + +Fielding, et. al. Standards Track [Page 59] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 14.46) 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 MAY include + relevant diagnostic information. HTTP access authentication is + explained in section 11. + +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. 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. + +10.4.5 404 Not Found + + The server has not found anything matching the Request-URI. No + indication is given of whether the condition is temporary or + permanent. + + + + + + + + +Fielding, et. al. Standards Track [Page 60] + +RFC 2068 HTTP/1.1 January 1997 + + + If the server does not wish to make this information available to the + client, the status code 403 (Forbidden) can be used instead. 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. + +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 section 11. + + + + + + +Fielding, et. al. Standards Track [Page 61] + +RFC 2068 HTTP/1.1 January 1997 + + +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 may not be + possible and is not required. + + Conflicts are most likely to occur in response to a PUT request. If + versioning is being used and the entity being PUT includes changes to + a resource which conflict with those made by an earlier (third-party) + request, the server MAY use the 409 response to indicate that it + can't complete the request. In this case, the response entity SHOULD + 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 SHOULD 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 cachable 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. + + + + + + + +Fielding, et. al. Standards Track [Page 62] + +RFC 2068 HTTP/1.1 January 1997 + + +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 URL "black hole" of + redirection (e.g., a redirected URL 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 63] + +RFC 2068 HTTP/1.1 January 1997 + + +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 it accessed in attempting to + complete the request. + + + + + +Fielding, et. al. Standards Track [Page 64] + +RFC 2068 HTTP/1.1 January 1997 + + +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 a simple challenge-response authentication mechanism + which MAY be used by a server to challenge a client request and by a + client to provide authentication information. It uses an extensible, + case-insensitive token to identify the authentication scheme, + followed by a comma-separated list of attribute-value pairs which + carry the parameters necessary for achieving authentication via that + scheme. + + auth-scheme = token + + auth-param = token "=" quoted-string + + The 401 (Unauthorized) response message is used by an origin server + to challenge the authorization of a user agent. This response MUST + include a WWW-Authenticate header field containing at least one + challenge applicable to the requested resource. + + challenge = auth-scheme 1*SP realm *( "," auth-param ) + + realm = "realm" "=" realm-value + realm-value = quoted-string + + The realm attribute (case-insensitive) is required for all + authentication schemes which issue a challenge. The realm value + (case-sensitive), in combination with the canonical root URL (see + section 5.1.2) of the server being accessed, defines the protection + space. These realms allow the protected resources on a server to be + partitioned into a set of protection spaces, each with its own + authentication scheme and/or authorization database. The realm value + is a string, generally assigned by the origin server, which may have + additional semantics specific to the authentication scheme. + + A user agent that wishes to authenticate itself with a server-- + usually, but not necessarily, after receiving a 401 or 411 response- + -MAY do so by including an Authorization header field with the + request. The Authorization field value consists of credentials + + + +Fielding, et. al. Standards Track [Page 65] + +RFC 2068 HTTP/1.1 January 1997 + + + containing the authentication information of the user agent for the + realm of the resource being requested. + + credentials = basic-credentials + | auth-scheme #auth-param + + The domain over which credentials can be automatically applied by a + user agent is determined by the protection space. If a prior request + has been authorized, the same credentials MAY be reused for all other + requests within that protection space for a period of time determined + by the authentication scheme, parameters, and/or user preference. + Unless otherwise defined by the authentication scheme, a single + protection space cannot extend outside the scope of its server. + + If the server does not wish to accept the credentials sent with a + request, it SHOULD return a 401 (Unauthorized) response. The response + MUST include a WWW-Authenticate header field containing the (possibly + new) challenge applicable to the requested resource and an entity + explaining the refusal. + + The HTTP protocol does not restrict applications to this simple + challenge-response mechanism for access authentication. Additional + mechanisms MAY be used, such as encryption at the transport level or + via message encapsulation, and with additional header fields + specifying authentication information. However, these additional + mechanisms are not defined by this specification. + + Proxies MUST be completely transparent regarding user agent + authentication. That is, they MUST forward the WWW-Authenticate and + Authorization headers untouched, and follow the rules found in + section 14.8. + + HTTP/1.1 allows a client to pass authentication information to and + from a proxy via the Proxy-Authenticate and Proxy-Authorization + headers. + +11.1 Basic Authentication Scheme + + The "basic" authentication scheme is based on the model that the user + agent must authenticate itself with a user-ID and a password for each + realm. The realm value should be considered an opaque string which + can only be compared for equality with other realms on that server. + The server will service the request only if it can validate the + user-ID and password for the protection space of the Request-URI. + There are no optional authentication parameters. + + + + + + +Fielding, et. al. Standards Track [Page 66] + +RFC 2068 HTTP/1.1 January 1997 + + + Upon receipt of an unauthorized request for a URI within the + protection space, the server MAY respond with a challenge like the + following: + + WWW-Authenticate: Basic realm="WallyWorld" + + where "WallyWorld" is the string assigned by the server to identify + the protection space of the Request-URI. + + To receive authorization, the client sends the userid and password, + separated by a single colon (":") character, within a base64 encoded + string in the credentials. + + basic-credentials = "Basic" SP basic-cookie + + basic-cookie = <base64 [7] encoding of user-pass, + except not limited to 76 char/line> + + user-pass = userid ":" password + + userid = *<TEXT excluding ":"> + + password = *TEXT + + Userids might be case sensitive. + + If the user agent wishes to send the userid "Aladdin" and password + "open sesame", it would use the following header field: + + Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== + + See section 15 for security considerations associated with Basic + authentication. + +11.2 Digest Authentication Scheme + + A digest authentication for HTTP is specified in RFC 2069 [32]. + +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 + + + +Fielding, et. al. Standards Track [Page 67] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 + + + +Fielding, et. al. Standards Track [Page 68] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 14.42). 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. + + HTTP/1.1 origin servers MUST include an appropriate Vary header field + (section 14.43) in any cachable response based on server-driven + negotiation. The Vary header field describes the dimensions over + which the response might vary (i.e. the dimensions over which the + origin server picks its "best guess" response from multiple + representations). + + HTTP/1.1 public caches MUST recognize the Vary header field when it + is included in a response and obey the requirements described in + section 13.6 that describes the interactions between caching and + content negotiation. + +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 (this specification reserves the field-name Alternates, + as described in appendix 19.6.2.1) 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. + + + +Fielding, et. al. Standards Track [Page 69] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 and used within HTTP/1.1. An HTTP/1.1 + cache performing transparent negotiation MUST include a Vary header + field in the response (defining the dimensions of its variance) if it + is cachable to ensure correct interoperation with all HTTP/1.1 + clients. The agent-driven negotiation information supplied by the + origin server SHOULD be included with the transparently negotiated + response. + +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, + + + +Fielding, et. al. Standards Track [Page 70] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 may be incompatible with certain server applications (such + as those for ordering merchandise), the protocol requires that + transparency be relaxed + + o only by an explicit protocol-level request when relaxed by client + or origin server + + o only with an explicit warning to the end user when relaxed by cache + or client + + + + + + + + + + + + + + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 71] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 implementer may be faced with + design decisions not explicitly discussed in this specification. If + a decision may affect semantic transparency, the implementer 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, server, and cache (see section 14.9); if the origin server + so specifies, it is the freshness requirement of the origin server + alone. + + 3. It includes a warning if the freshness demand of the client or the + origin server is violated (see section 13.1.5 and 14.45). + + 4. 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 + + + +Fielding, et. al. Standards Track [Page 72] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. An 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 response- + header. This 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 always cachable, because they never weaken the + transparency of a response. This means that warnings can be passed to + HTTP/1.0 caches without danger; such caches will simply pass the + warning along as an entity-header in the response. + + Warnings are assigned numbers between 0 and 99. This specification + defines the code numbers and meanings of each currently assigned + warnings, allowing a client or cache to take automated action in some + (but not all) cases. + + 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 may provide the same warning with texts + in both English and Basque. + + When multiple warnings are attached to a response, it may 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. + + + +Fielding, et. al. Standards Track [Page 73] + +RFC 2068 HTTP/1.1 January 1997 + + + The Warning header and the currently defined warnings are described + in section 14.45. + +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 may 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 should be 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 may 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 should have to explicitly + request either non-transparent behavior, or behavior that results in + abnormally ineffective caching. + + 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 visual 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 display an indication (for example, a picture of currency + in flames) so that the user does not inadvertently consume excess + resources or suffer from excessive latency. + + + + +Fielding, et. al. Standards Track [Page 74] + +RFC 2068 HTTP/1.1 January 1997 + + +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 should 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). This + allows the client software to alert the user that there may 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 may 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 may violate the origin server's specified constraints + on semantic transparency, but may 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 + + + +Fielding, et. al. Standards Track [Page 75] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 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 may compromise semantic transparency, they should be used + cautiously, and we encourage origin servers to provide explicit + expiration times as much as possible. + + + + + + + +Fielding, et. al. Standards Track [Page 76] + +RFC 2068 HTTP/1.1 January 1997 + + +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. + + Also note that HTTP/1.1 requires origin servers to send a Date header + with every response, giving the time at which the response was + generated. 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 help convey age information + between caches. The Age header value is the sender's estimate of the + amount of time since the response was generated at the origin server. + In the case of a cached response that has been revalidated with the + origin server, the Age value is based on the time of revalidation, + not of the original response. + + 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- + + + +Fielding, et. al. Standards Track [Page 77] + +RFC 2068 HTTP/1.1 January 1997 + + + HTTP/1.1 paths, one gets a reliable (conservative) result. + + Note that this correction is applied at each HTTP/1.1 cache along the + path, so that if there is an HTTP/1.0 cache in the path, the correct + received age is computed as long as the receiving cache's clock is + nearly in sync. We don't need end-to-end clock synchronization + (although it is good to have), and there is no explicit clock + synchronization step. + + Because of network-imposed delays, some significant interval may pass + from 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); + + + +Fielding, et. al. Standards Track [Page 78] + +RFC 2068 HTTP/1.1 January 1997 + + + 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; + + When a cache sends a response, it must add to the + corrected_initial_age the amount of time that the response was + resident locally. It must then transmit this total age, using the Age + header, to the next recipient cache. + + Note that a client cannot reliably tell that a response is first- + hand, but the presence of an Age header indicates that a response + is definitely not first-hand. Also, if the Date in a response is + earlier than the client's local request time, the response is + probably not first-hand (in the absence of serious clock skew). + +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 14.10. + + 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 neither Expires nor Cache-Control: max-age appears in the + response, and the response does not include other restrictions on + caching, the cache MAY compute a freshness lifetime using a + heuristic. If the value is greater than 24 hours, the cache must + attach Warning 13 to any response whose age is more than 24 hours if + + + +Fielding, et. al. Standards Track [Page 79] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 may 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 may 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 may + 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. However, the + HTTP/1.1 specification requires the transmission of Date headers on + every response, and the Date values are ordered to a granularity of + one second. + + + + + +Fielding, et. al. Standards Track [Page 80] + +RFC 2068 HTTP/1.1 January 1997 + + + 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, 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. + + + + +Fielding, et. al. Standards Track [Page 81] + +RFC 2068 HTTP/1.1 January 1997 + + + Note: the comparison functions used to decide if validators match + are defined in section 13.3.3. + + 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 entity-header field value, an entity tag, provides for an + "opaque" cache validator. This may 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 may 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 14.20, 14.25, 14.26 and 14.43. + +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 may be cases when a server prefers to change the + validator only on semantically significant changes, and not when + + + +Fielding, et. al. Standards Track [Page 82] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 may 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 may end up with an internally + inconsistent entity. + + 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: + + o The strong comparison function: in order to be considered equal, + both validators must be identical in every way, and neither may be + weak. + o 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. + + The weak comparison function MAY be used for simple (non-subrange) + + + +Fielding, et. al. Standards Track [Page 83] + +RFC 2068 HTTP/1.1 January 1997 + + + GET requests. The strong comparison function MUST be used in all + other cases. + + 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: + + o The validator is being compared by an origin server to the actual + current validator for the entity and, + o That origin server reliably knows that the associated entity did + not change twice during the second covered by the presented + validator. +or + + o 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 + o That cache entry includes a Date value, which gives the time when + the origin server sent the original response, and + o The presented Last-Modified time is at least 60 seconds before the + Date value. +or + + o The validator is being compared by an intermediate cache to the + validator stored in its cache entry for the entity, and + o That cache entry includes a Date value, which gives the time when + the origin server sent the original response, and + o 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. + + + + +Fielding, et. al. Standards Track [Page 84] + +RFC 2068 HTTP/1.1 January 1997 + + + A cache or origin server receiving a cache-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 should be + used, and for what purposes. + + HTTP/1.1 origin servers: + + o SHOULD send an entity tag validator unless it is not feasible to + generate one. + o 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. + o 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 may + persist for arbitrarily long periods, regardless of expiration + times, so it may 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: + + o 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). + + + +Fielding, et. al. Standards Track [Page 85] + +RFC 2068 HTTP/1.1 January 1997 + + + o 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). + o 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. + o 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 cache, upon receiving a request, MUST use the most + restrictive validator when deciding whether the client's cache entry + matches the cache's own cache entry. This is only an issue when the + request contains both an entity tag and a last-modified-date + validator (If-Modified-Since or If-Unmodified-Since). + + A note on rationale: 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 Cachability + + 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 + + + +Fielding, et. al. Standards Track [Page 86] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 that some HTTP/1.0 caches are known to violate this + expectation without providing any Warning. + + However, in some cases it may be inappropriate for a cache to retain + an entity, or to return it in response to a subsequent request. This + may 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, may not 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 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", "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 may have to combine parts of a new + response with what is held in the cache entry. + + + + + + + +Fielding, et. al. Standards Track [Page 87] + +RFC 2068 HTTP/1.1 January 1997 + + +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: + + o End-to-end headers, which must be 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 transmitted in any response formed from a cache entry. + o 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: + + o Connection + o Keep-Alive + o Public + o Proxy-Authenticate + o Transfer-Encoding + o Upgrade + + All other headers defined by HTTP/1.1 are end-to-end headers. + + Hop-by-hop headers introduced in future versions of HTTP MUST be + listed in a Connection header, as described in section 14.10. + +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 + cache or non-caching proxy SHOULD NOT modify an end-to-end header + unless the definition of that header requires or specifically allows + that. + + A cache or non-caching proxy MUST NOT modify any of the following + fields in a request or response, nor may it add any of these fields + if not already present: + + o Content-Location + o ETag + o Expires + o Last-Modified + + + + + + + + +Fielding, et. al. Standards Track [Page 88] + +RFC 2068 HTTP/1.1 January 1997 + + + A cache or non-caching proxy MUST NOT modify or add any of the + following fields in a response that contains the no-transform Cache- + Control directive, or in any request: + + o Content-Encoding + o Content-Length + o Content-Range + o Content-Type + + A cache or non-caching proxy MAY modify or add these fields in a + response that does not include no-transform, but if it does so, it + MUST add a Warning 14 (Transformation applied) if one does not + already appear in the response. + + Warning: unnecessary modification of end-to-end headers may 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. + +13.5.3 Combining Headers + + When a cache makes a validating request to a server, and the server + provides a 304 (Not Modified) response, the cache must construct a + response to send to the requesting client. The cache uses the + entity-body stored in the cache entry as the entity-body of this + outgoing response. The end-to-end headers stored in the cache entry + are used for the constructed response, except that any end-to-end + headers provided in the 304 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. + + 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. The cache may add Warning headers (see + section 14.45) to this set. + + If a header field-name in the incoming response matches more than one + header in the cache entry, all such old headers are replaced. + + Note: this rule allows an origin server to use a 304 (Not Modified) + response to update any header associated with a previous response + for the same entity, 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) response to entirely delete a header that it + had provided with a previous response. + + + + +Fielding, et. al. Standards Track [Page 89] + +RFC 2068 HTTP/1.1 January 1997 + + +13.5.4 Combining Byte Ranges + + A response may 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 may 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: + + o Both the incoming response and the cache entry must have a cache + validator. + o The two cache validators must match using the strong comparison + function (see section 13.3.3). + + If either requirement is not meant, 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), 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. + + A server MUST use the Vary header field (section 14.43) to inform a + cache of what header field dimensions are used to select among + multiple representations of a cachable response. A cache may use the + selected representation (the entity included with that particular + response) for replying to subsequent requests on that resource only + when the subsequent requests have the same or equivalent values for + all header fields specified in the Vary response-header. Requests + with a different value for one or more of those header fields would + be forwarded toward the origin server. + + If an entity tag was assigned to the 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 Request- + URI. 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 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 + + + +Fielding, et. al. Standards Track [Page 90] + +RFC 2068 HTTP/1.1 January 1997 + + + new response SHOULD be used to update the header fields of the + existing entry, and the result MUST be returned to the client. + + The Vary header field may also inform the cache that the + representation was selected using criteria not limited to the + request-headers; in this case, a cache MUST NOT use the response in a + reply to a subsequent request unless the cache 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 which entity should be used. + + 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 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, + + + +Fielding, et. al. Standards Track [Page 91] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 URLs 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 at the origin server may cause one or + more existing cache entries to become non-transparently invalid. That + is, although they may continue to be "fresh," they do not accurately + reflect what the origin server would return for a new request. + + 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 may 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 should either remove all instances of that entity from its + storage, or should mark these as "invalid" and in need of a mandatory + revalidation before they can be returned in response to a subsequent + request. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 92] + +RFC 2068 HTTP/1.1 January 1997 + + + Some HTTP methods may invalidate an entity. This is either the entity + referred to by the Request-URI, or by the Location or Content- + Location response-headers (if present). These methods are: + + o PUT + o DELETE + o 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. + +13.11 Write-Through Mandatory + + All methods that may 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 cache from sending a 100 (Continue) response before the inbound + server has replied. + + 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 cachable (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 it + should follow the rules in section 13.5.3. + + Note: a new response that has an older Date header value than + existing cached responses is not cachable. + +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. + + + + +Fielding, et. al. Standards Track [Page 93] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 should not be construed to prohibit the history mechanism from + telling the user that a view may 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. + + + + + + + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 94] + +RFC 2068 HTTP/1.1 January 1997 + + +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 should be 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." + + + +Fielding, et. al. Standards Track [Page 95] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 may 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, + + + +Fielding, et. al. Standards Track [Page 96] + +RFC 2068 HTTP/1.1 January 1997 + + + this default set should 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. The ISO- + 8859-1 character set can be assumed to be acceptable to all user + agents. + + 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 + + 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-coding values (section 14.12) which are + acceptable in the response. + + Accept-Encoding = "Accept-Encoding" ":" + #( content-coding ) + + An example of its use is + + Accept-Encoding: compress, gzip + + If no Accept-Encoding header is present in a request, the server MAY + assume that the client will accept any content coding. If an Accept- + Encoding header is present, 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. + + + + +Fielding, et. al. Standards Track [Page 97] + +RFC 2068 HTTP/1.1 January 1997 + + + An empty Accept-Encoding value indicates none are acceptable. + +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. + + 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 may 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 + + + +Fielding, et. al. Standards Track [Page 98] + +RFC 2068 HTTP/1.1 January 1997 + + + section 15.7. + + Note: 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. + +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. + + 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. + + + + + +Fielding, et. al. Standards Track [Page 99] + +RFC 2068 HTTP/1.1 January 1997 + + + 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). + HTTP/1.1 caches MUST send an Age header in every response. 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" ":" 1#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 MAY have + other means of communicating with the origin server. + + The Allow header field does not indicate what methods are implemented + at the server level. Servers MAY use the Public response-header field + (section 14.35) to describe what methods are implemented on the + server as a whole. + +14.8 Authorization + + A user agent that wishes to authenticate itself with a server-- + usually, but not necessarily, after receiving a 401 response--MAY do + 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. + + + +Fielding, et. al. Standards Track [Page 100] + +RFC 2068 HTTP/1.1 January 1997 + + + Authorization = "Authorization" ":" credentials + + HTTP access authentication is described in section 11. If a request + is authenticated and a realm specified, the same credentials SHOULD + be valid for all other requests within this realm. + + 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 "proxy-revalidate" Cache-Control + directive, the cache MAY use that response in replying to a + subsequent request, but 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. + 2. If the response includes the "must-revalidate" Cache-Control + directive, the cache MAY use that response in replying to a + subsequent request, but 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 + should be given in the response. + + Note that HTTP/1.0 caches may not implement Cache-Control and may + 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 may 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 + + + +Fielding, et. al. Standards Track [Page 101] + +RFC 2068 HTTP/1.1 January 1997 + + + cache-request-directive = + "no-cache" [ "=" <"> 1#field-name <"> ] + | "no-store" + | "max-age" "=" delta-seconds + | "max-stale" [ "=" delta-seconds ] + | "min-fresh" "=" delta-seconds + | "only-if-cached" + | cache-extension + + cache-response-directive = + "public" + | "private" [ "=" <"> 1#field-name <"> ] + | "no-cache" [ "=" <"> 1#field-name <"> ] + | "no-store" + | "no-transform" + | "must-revalidate" + | "proxy-revalidate" + | "max-age" "=" delta-seconds + | cache-extension + + 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 may apply these directives to + header fields not defined in HTTP/1.1. + + The cache-control directives can be broken down into these general + categories: + + o Restrictions on what is cachable; these may only be imposed by the + origin server. + o Restrictions on what may be stored by a cache; these may be imposed + by either the origin server or the user agent. + o Modifications of the basic expiration mechanism; these may be + imposed by either the origin server or the user agent. + o Controls over cache revalidation and reload; these may only be + imposed by a user agent. + o Control over transformation of entities. + o Extensions to the caching system. + + + + + + + + +Fielding, et. al. Standards Track [Page 102] + +RFC 2068 HTTP/1.1 January 1997 + + +14.9.1 What is Cachable + + By default, a response is cachable if the requirements of the request + method, request header fields, and the response status indicate that + it is cachable. Section 13.4 summarizes these defaults for + cachability. The following Cache-Control response directives allow an + origin server to override the default cachability of a response: + +public + Indicates that the response is cachable by any cache, even if it + would normally be non-cachable or cachable 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 + Indicates that all or part of the response message MUST NOT be cached + anywhere. This allows an origin server to prevent caching even by + caches that have been configured to return stale responses to client + requests. + + Note: Most HTTP/1.0 caches will not recognize or obey this + directive. + +14.9.2 What May be Stored by Caches + + 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. + + + +Fielding, et. al. Standards Track [Page 103] + +RFC 2068 HTTP/1.1 January 1997 + + + Even when this directive is associated with a response, users may + 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 may 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 may not recognize or obey this directive; and + communications networks may 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. + + 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 may be + useful if certain HTTP/1.0 caches improperly calculate ages or + expiration times, perhaps due to desynchronized clocks. + + Note: 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 non-HTTP/1.1-compliant caches do not + observe the max-age directive. + + Other directives allow an 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 + + + +Fielding, et. al. Standards Track [Page 104] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 10 (Response is + stale). + +14.9.4 Cache Revalidation and Reload Controls + + Sometimes an user agent may 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 may 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". No field + names may 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 + + + +Fielding, et. al. Standards Track [Page 105] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + 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 may + 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 may 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 should 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 should 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. + + 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. + + 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 + + + +Fielding, et. al. Standards Track [Page 106] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + 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 + + Implementers of intermediate caches (proxies) have found it useful to + convert the media type of certain entity bodies. A 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. HTTP has to date + been silent on these transformations. + + + + + +Fielding, et. al. Standards Track [Page 107] + +RFC 2068 HTTP/1.1 January 1997 + + + Serious operational problems have already occurred, however, when + these transformations have been 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 response 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. + +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 a 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) may do so by including + + Cache-Control: private, community="UCI" + + A cache seeing this header field will act correctly even if the cache + does not understand the "community" cache-extension, since it will + also see and understand the "private" directive and thus default to + the safe behavior. + + + +Fielding, et. al. Standards Track [Page 108] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 + cachability) 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-header = "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. 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. + +14.11 Content-Base + + The Content-Base entity-header field may be used to specify the base + URI for resolving relative URLs within the entity. This header field + is described as Base in RFC 1808, which is expected to be revised. + + Content-Base = "Content-Base" ":" absoluteURI + + If no Content-Base field is present, the base URI of an entity is + defined either by its Content-Location (if that Content-Location URI + is an absolute URI) or the URI used to initiate the request, in that + + + +Fielding, et. al. Standards Track [Page 109] + +RFC 2068 HTTP/1.1 January 1997 + + + order of precedence. Note, however, that the base URI of the contents + within the entity-body may be redefined within that entity-body. + +14.12 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-Encoding 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. + + 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. + +14.13 Content-Language + + The Content-Language entity-header field describes the natural + language(s) of the intended audience for the enclosed entity. Note + that this may 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 may mean that the sender + + + +Fielding, et. al. Standards Track [Page 110] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 should + only include "en". + + Content-Language may be applied to any media type -- it is not + limited to textual documents. + +14.14 Content-Length + + The Content-Length entity-header field indicates the size of the + message-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 size of the + message-body to be transferred, regardless of the media type of the + entity. It must be possible for the recipient to reliably determine + the end of HTTP/1.1 requests containing an entity-body, e.g., because + the request has a valid Content-Length field, uses Transfer-Encoding: + chunked or a multipart body. + + 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 111] + +RFC 2068 HTTP/1.1 January 1997 + + + Note: 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. + +14.15 Content-Location + + The Content-Location entity-header field may be used to supply the + resource location for the entity enclosed in the message. 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. In addition, a server + SHOULD provide a Content-Location for the resource corresponding to + the response entity. + + Content-Location = "Content-Location" ":" + ( absoluteURI | relativeURI ) + + If no Content-Base header field is present, the value of Content- + Location also defines the base URL for the entity (see section + 14.11). + + 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 use the Content-Location 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 URI is interpreted + relative to any Content-Base URI provided in the response. If no + Content-Base is provided, the relative URI is interpreted relative to + the Request-URI. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 112] + +RFC 2068 HTTP/1.1 January 1997 + + +14.16 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 to + function as an integrity check of the entity-body. Only origin + servers 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-Encoding that has been applied, but not + including any Transfer-Encoding that may have been 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. + + Note: 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. + + 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 + + + +Fielding, et. al. Standards Track [Page 113] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + Conversion of all line breaks to CRLF should not be done before + computing or checking the digest: the line break convention used in + the text actually transmitted should be left unaltered when + computing the digest. + +14.17 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 + inserted. It also indicates the total size of the full entity-body. + When a server returns a partial response to a client, it must + describe both the extent of the range covered by the response, and + the length of the entire entity-body. + + Content-Range = "Content-Range" ":" content-range-spec + + content-range-spec = byte-content-range-spec + + byte-content-range-spec = bytes-unit SP first-byte-pos "-" + last-byte-pos "/" entity-length + + entity-length = 1*DIGIT + + Unlike byte-ranges-specifier values, a byte-content-range-spec may + 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 whose last-byte-pos value is less than its + first-byte-pos value, or whose entity-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. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 114] + +RFC 2068 HTTP/1.1 January 1997 + + + Examples of byte-content-range-spec values, assuming that the entity + contains a total of 1234 bytes: + + o The first 500 bytes: + + bytes 0-499/1234 + + o The second 500 bytes: + + bytes 500-999/1234 + + o All except for the first 500 bytes: + + bytes 500-1233/1234 + + o 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 MIME message. The + multipart MIME content-type used for this purpose is defined in this + specification to be "multipart/byteranges". See appendix 19.2 for its + definition. + + A client that cannot decode a MIME multipart/byteranges message + should 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 invalid, the + server should treat the request as if the invalid Range header field + + + +Fielding, et. al. Standards Track [Page 115] + +RFC 2068 HTTP/1.1 January 1997 + + + did not exist. (Normally, this means return a 200 response containing + the full entity). The reason is that the only time a client will make + such an invalid request is when the entity is smaller than the entity + retrieved by a prior request. + +14.18 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. + +14.19 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. + + Date = "Date" ":" HTTP-date + + An example is + + Date: Tue, 15 Nov 1994 08:12:31 GMT + + If a message is received via direct connection with the user agent + (in the case of requests) or the origin server (in the case of + responses), then the date can be assumed to be the current date at + the receiving end. However, since the date--as it is believed by the + origin--is important for evaluating cached responses, origin servers + MUST include a Date header field in all responses. 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 received message which does not have a Date header + field SHOULD be assigned one by the recipient if the message will be + cached by that recipient or gatewayed via a protocol which requires a + Date. + + + + + + + +Fielding, et. al. Standards Track [Page 116] + +RFC 2068 HTTP/1.1 January 1997 + + + In theory, the date SHOULD 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. + + The format of the Date is an absolute date and time as defined by + HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format. + +14.20 ETag + + The ETag entity-header field defines the entity tag for the + associated entity. The headers used with entity tags are described in + sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used + for comparison with other entities from the same resource (see + section 13.3.2). + + ETag = "ETag" ":" entity-tag + + Examples: + + ETag: "xyzzy" + ETag: W/"xyzzy" + ETag: "" + +14.21 Expires + + The Expires entity-header field gives the date/time after which the + response should be considered stale. A stale cache entry may not + normally be returned by a cache (either a proxy cache or an 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; it MUST be in RFC1123-date format: + + Expires = "Expires" ":" HTTP-date + + + + + + + + + + +Fielding, et. al. Standards Track [Page 117] + +RFC 2068 HTTP/1.1 January 1997 + + + 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, 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 should use + 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 should use 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 an response that otherwise would by default be + non-cacheable indicates that the response is cachable, 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 (as updated by RFC 1123 ): + + From = "From" ":" mailbox + + An example is: + + From: [email protected] + + 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. + + + + + +Fielding, et. al. Standards Track [Page 118] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + Note: The client SHOULD not send the From header field without the + user's approval, as it may 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 + URL 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 network location 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/> MUST 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 on the Internet (i.e., on any message corresponding to a + request for a URL which includes an Internet host address for the + service being requested). If the Host field is not already present, + an HTTP/1.1 proxy MUST add a Host field to the request message prior + to forwarding it on the Internet. All Internet-based HTTP/1.1 servers + MUST respond with a 400 status code to any HTTP/1.1 request message + which lacks a Host header field. + + See sections 5.2 and 19.5.1 for other requirements relating to Host. + +14.24 If-Modified-Since + + The If-Modified-Since request-header field is used with the GET + method to make it conditional: if the requested variant has not been + modified since the time specified in this field, an entity will not + + + +Fielding, et. al. Standards Track [Page 119] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 MUST 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 that the Range request-header field modifies the meaning of + If-Modified-Since; see section 14.36 for full details. + + Note that If-Modified-Since times are interpreted by the server, + whose clock may not be synchronized with the client. + + Note that 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. + + + + +Fielding, et. al. Standards Track [Page 120] + +RFC 2068 HTTP/1.1 January 1997 + + +14.25 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. 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 3.11) + 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 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 14.43) exists, and + MUST NOT be performed if the representation does not exist. + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 121] + +RFC 2068 HTTP/1.1 January 1997 + + + 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: * + +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, on updating requests, to + prevent inadvertent modification of a resource which was not known to + 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. Instead, if the request + method was GET or HEAD, the server SHOULD respond with a 304 (Not + Modified) response, including the cache-related entity-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 entity tags + match. The weak comparison function can only be used with GET or HEAD + requests. + + If none of the entity tags match, or if "*" is given and no current + entity exists, then the server MAY perform the requested method as if + the If-None-Match header field did not exist. + + + + +Fielding, et. al. Standards Track [Page 122] + +RFC 2068 HTTP/1.1 January 1997 + + + If the request would, without the If-None-Match header field, result + in anything other than a 2xx status, then the If-None-Match header + MUST be ignored. + + 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 14.43) + exists, and SHOULD be performed if the representation does not exist. + This feature may 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: * + +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 a 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 123] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 status, the If- + Unmodified-Since header should be ignored. + + If the specified date is invalid, the header is ignored. + +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. + + + +Fielding, et. al. Standards Track [Page 124] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 URL for automatic redirection to the resource. The + field value consists of a single absolute URL. + + Location = "Location" ":" absoluteURI + + An example is + + Location: http://www.w3.org/pub/WWW/People.html + + Note: The Content-Location header field (section 14.15) 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 may be used with the TRACE + method (section 14.31) 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 + + + + + +Fielding, et. al. Standards Track [Page 125] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 request containing a Max- + Forwards header field SHOULD check and update its value prior to + forwarding the request. If the received value is zero (0), the + recipient SHOULD NOT forward the request; instead, it SHOULD respond + as the final recipient with a 200 (OK) response containing the + received request message as the response entity-body (as described in + section 9.8). If the received Max-Forwards value is greater than + zero, then the forwarded message SHOULD contain an updated Max- + Forwards field with a value decremented by one (1). + + The Max-Forwards header field SHOULD 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 may 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 backwards 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 may 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. + + + + + + + +Fielding, et. al. Standards Track [Page 126] + +RFC 2068 HTTP/1.1 January 1997 + + + HTTP/1.1 clients SHOULD NOT send the Pragma request-header. 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. + +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" ":" challenge + + The HTTP access authentication process is described in section 11. + 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 may 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 section 11. + 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 Public + + The Public response-header field lists the set of methods supported + by the server. The purpose of this field is strictly to inform the + recipient of the capabilities of the server regarding unusual + methods. The methods listed may or may not be applicable to the + + + +Fielding, et. al. Standards Track [Page 127] + +RFC 2068 HTTP/1.1 January 1997 + + + Request-URI; the Allow header field (section 14.7) MAY be used to + indicate methods allowed for a particular URI. + + Public = "Public" ":" 1#method + + Example of use: + + Public: OPTIONS, MGET, MHEAD, GET, HEAD + + This header field applies only to the server directly connected to + the client (i.e., the nearest neighbor in a chain of connections). If + the response passes through a proxy, the proxy MUST either remove the + Public header field or replace it with one applicable to its own + capabilities. + +14.36 Range + +14.36.1 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. + + + + + +Fielding, et. al. Standards Track [Page 128] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 invalid. The recipient of an invalid byte-range-spec + must ignore it. + + 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. + + Examples of byte-ranges-specifier values (assuming an entity-body of + length 10000): + + o The first 500 bytes (byte offsets 0-499, inclusive): + + bytes=0-499 + + o The second 500 bytes (byte offsets 500-999, inclusive): + + bytes=500-999 + + o The final 500 bytes (byte offsets 9500-9999, inclusive): + + bytes=-500 + + o Or + + bytes=9500- + + o The first and last bytes only (bytes 0 and 9999): + + bytes=0-0,-1 + + + + + + +Fielding, et. al. Standards Track [Page 129] + +RFC 2068 HTTP/1.1 January 1997 + + + o 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 + +14.36.2 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 SHOULD 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: + + o 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). + + o 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 may 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. + + + + + + +Fielding, et. al. Standards Track [Page 130] + +RFC 2068 HTTP/1.1 January 1997 + + +14.37 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 partial URI, it SHOULD be interpreted + relative to the Request-URI. The URI MUST NOT include a fragment. + + Note: Because the source of a link may be private information or + may 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. + +14.38 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. 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. + + + + + + +Fielding, et. al. Standards Track [Page 131] + +RFC 2068 HTTP/1.1 January 1997 + + +14.39 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 14.44). + + Note: Revealing the specific software version of the server may + allow the server machine to become more vulnerable to attacks + against software that is known to contain security holes. Server + implementers are encouraged to make this field a configurable + option. + +14.40 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-Encoding 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 + + Many older HTTP/1.0 applications do not understand the Transfer- + Encoding header. + +14.41 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 + + + +Fielding, et. al. Standards Track [Page 132] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + + + + + + +Fielding, et. al. Standards Track [Page 133] + +RFC 2068 HTTP/1.1 January 1997 + + +14.42 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 + +14.43 Vary + + The Vary response-header field is used by a server to signal that the + response entity was selected from the available representations of + the response using server-driven negotiation (section 12). Field- + names listed in Vary headers are those of request-headers. The Vary + field value indicates either that the given set of header fields + encompass the dimensions over which the representation might vary, or + that the dimensions of variance are unspecified ("*") and thus may + vary over any aspect of future requests. + + Vary = "Vary" ":" ( "*" | 1#field-name ) + + An HTTP/1.1 server MUST include an appropriate Vary header field with + any cachable 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 SHOULD include an appropriate Vary header + field with a non-cachable 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 might vary. + + 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, the + cache MUST NOT use such a cache entry to construct a response to the + new request unless all of the headers named in the cached Vary header + + + +Fielding, et. al. Standards Track [Page 134] + +RFC 2068 HTTP/1.1 January 1997 + + + are present in the new request, and all of the stored selecting + request-headers from the previous request match the corresponding + headers in the new 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 whitespace (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 field value of "*" signals that unspecified parameters, + possibly other than the contents of request-header fields (e.g., the + network address of the client), play a role in the selection of the + response representation. Subsequent requests on that resource can + only be properly interpreted by the origin server, and thus a cache + MUST forward a (possibly conditional) request even when it has a + fresh response cached for the resource. See section 13.6 for use of + the Vary header by caches. + + 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 in + 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. + +14.44 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 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. + + + + + + + + + +Fielding, et. al. Standards Track [Page 135] + +RFC 2068 HTTP/1.1 January 1997 + + + 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 represent 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. + + + +Fielding, et. al. Standards Track [Page 136] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + +14.45 Warning + + The Warning response-header field is used to carry additional + information about the status of a response which may not be reflected + by the response status code. This information is typically, though + not exclusively, used to warn about a possible lack of semantic + transparency from caching operations. + + Warning headers are sent with responses using: + + Warning = "Warning" ":" 1#warning-value + + warning-value = warn-code SP warn-agent SP warn-text + warn-code = 2DIGIT + warn-agent = ( host [ ":" port ] ) | pseudonym + ; the name or pseudonym of the server adding + ; the Warning header, for use in debugging + warn-text = quoted-string + + 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 1522 [14]. + + + + +Fielding, et. al. Standards Track [Page 137] + +RFC 2068 HTTP/1.1 January 1997 + + + Any server or cache may add Warning headers to a response. New + Warning headers should be added after any existing Warning headers. A + cache MUST NOT delete any Warning header that it received with a + response. 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 SHOULD display as many of them as possible, in the order that + they appear in the response. If it is not possible to display all of + the warnings, the user agent should follow these heuristics: + + o Warnings that appear early in the response take priority over those + appearing later in the response. + o 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. + + This is a list of the currently-defined warn-codes, each with a + recommended warn-text in English, and a description of its meaning. + +10 Response is stale + MUST be included whenever the returned response is stale. A cache may + add this warning to any response, but may never remove it until the + response is known to be fresh. + +11 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. A cache may add this warning to any response, but + may never remove it until the response is successfully revalidated. + +12 Disconnected operation + SHOULD be included if the cache is intentionally disconnected from + the rest of the network for a period of time. + +13 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. + + + + + +Fielding, et. al. Standards Track [Page 138] + +RFC 2068 HTTP/1.1 January 1997 + + +14 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, unless this Warning code + already appears in the response. MUST NOT be deleted from a response + even after revalidation. + +99 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. + +14.46 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 section 11. + User agents MUST take special care in parsing the WWW-Authenticate + field value if it contains more than one challenge, or if more than + one WWW-Authenticate header field is provided, since the contents of + a challenge may itself 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 Authentication of Clients + + The Basic authentication scheme is not a secure method of user + authentication, nor does it in any way protect the entity, which is + transmitted in clear text across the physical network used as the + carrier. HTTP does not prevent additional authentication schemes and + encryption mechanisms from being employed to increase security or the + addition of enhancements (such as schemes to use one-time passwords) + to Basic authentication. + + + + + +Fielding, et. al. Standards Track [Page 139] + +RFC 2068 HTTP/1.1 January 1997 + + + The most serious flaw in Basic authentication is that it results in + the essentially clear text transmission of the user's password over + the physical network. It is this problem which Digest Authentication + attempts to address. + + Because Basic authentication involves the clear text transmission of + passwords it SHOULD never be used (without enhancements) to protect + sensitive or valuable information. + + A common use of Basic authentication is for identification purposes + -- requiring the user to provide a user name and password as a means + of identification, for example, for purposes of gathering accurate + usage statistics on a server. When used in this way it is tempting to + think that there is no danger in its use if illicit access to the + protected documents is not a major concern. This is only correct if + the server issues both user name and password to the users and in + particular does not allow the user to choose his or her own password. + The danger arises because naive users frequently reuse a single + password to avoid the task of maintaining multiple passwords. + + If a server permits users to select their own passwords, then the + threat is not only illicit access to documents on the server but also + illicit access to the accounts of all users who have chosen to use + their account password. If users are allowed to choose their own + password that also means the server must maintain files containing + the (presumably encrypted) passwords. Many of these may be the + account passwords of users perhaps at distant sites. The owner or + administrator of such a system could conceivably incur liability if + this information is not maintained in a secure fashion. + + Basic Authentication is also vulnerable to spoofing by counterfeit + servers. If a user can be led to believe that he is connecting to a + host containing information protected by basic authentication when in + fact he is connecting to a hostile server or gateway then the + attacker can request a password, store it for later use, and feign an + error. This type of attack is not possible with Digest Authentication + [32]. Server implementers SHOULD guard against the possibility of + this sort of counterfeiting by gateways or CGI scripts. In particular + it is very dangerous for a server to simply turn over a connection to + a gateway since that gateway can then use the persistent connection + mechanism to engage in multiple transactions with the client while + impersonating the original server in a way that is not detectable by + the client. + +15.2 Offering a Choice of Authentication Schemes + + An HTTP/1.1 server may return multiple challenges with a 401 + (Authenticate) response, and each challenge may use a different + + + +Fielding, et. al. Standards Track [Page 140] + +RFC 2068 HTTP/1.1 January 1997 + + + scheme. The order of the challenges returned to the user agent is in + the order that the server would prefer they be chosen. The server + should order its challenges with the "most secure" authentication + scheme first. A user agent should choose as the challenge to be made + to the user the first one that the user agent understands. + + When the server offers choices of authentication schemes using the + WWW-Authenticate header, the "security" of the authentication is only + as malicious user could capture the set of challenges and try to + authenticate him/herself using the weakest of the authentication + schemes. Thus, the ordering serves more to protect the user's + credentials than the server's information. + + A possible man-in-the-middle (MITM) attack would be to add a weak + authentication scheme to the set of choices, hoping that the client + will use one that exposes the user's credentials (e.g. password). For + this reason, the client should always use the strongest scheme that + it understands from the choices accepted. + + An even better MITM attack would be to remove all offered choices, + and to insert a challenge that requests Basic authentication. For + this reason, user agents that are concerned about this kind of attack + could remember the strongest authentication scheme ever requested by + a server and produce a warning message that requires user + confirmation before using a weaker one. A particularly insidious way + to mount such a MITM attack would be to offer a "free" proxy caching + service to gullible users. + +15.3 Abuse of Server Log Information + + A server is in the position to save personal data about a user's + requests which may identify their reading patterns or subjects of + interest. This information is clearly confidential in nature and its + handling may 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. + +15.4 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. + + + + +Fielding, et. al. Standards Track [Page 141] + +RFC 2068 HTTP/1.1 January 1997 + + + Revealing the specific software version of the server may allow the + server machine to become more vulnerable to attacks against software + that is known to contain security holes. Implementers 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 field 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 field may 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. + +15.5 Attacks Based On File and Path Names + + Implementations of HTTP origin servers SHOULD be careful to restrict + the documents returned by HTTP requests to be only those that were + intended by the server administrators. If an HTTP server translates + HTTP URIs directly into file system calls, the server MUST take + special care not to serve files that were not intended to be + delivered to HTTP clients. For example, UNIX, Microsoft Windows, and + other operating systems use ".." as a path component to indicate a + directory level above the current one. On such a system, an HTTP + server MUST disallow any such construct in the Request-URI if it + would otherwise allow access to a resource outside those intended to + be accessible via the HTTP server. Similarly, files intended for + reference only internally to the server (such as access control + files, configuration files, and script code) MUST be protected from + inappropriate retrieval, since they might contain sensitive + information. Experience has shown that minor bugs in such HTTP server + implementations have turned into security risks. + + + + +Fielding, et. al. Standards Track [Page 142] + +RFC 2068 HTTP/1.1 January 1997 + + +15.6 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 implementers be particularly careful in this area. + History shows that errors in this area are often both serious + security and/or privacy problems, and often generate highly adverse + publicity for the implementer's company. + +15.7 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 it should 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 should 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. + + + + +Fielding, et. al. Standards Track [Page 143] + +RFC 2068 HTTP/1.1 January 1997 + + +15.8 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. These lookups should 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, 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. + +15.9 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. + +16 Acknowledgments + + This specification makes heavy use of the augmented BNF and generic + constructs defined by David H. Crocker for RFC 822. Similarly, it + reuses many of the definitions provided by Nathaniel Borenstein and + Ned Freed for MIME. We hope that their inclusion in this + specification will help reduce past confusion over the relationship + between HTTP and Internet mail message formats. + + + +Fielding, et. al. Standards Track [Page 144] + +RFC 2068 HTTP/1.1 January 1997 + + + The HTTP protocol has evolved considerably over the past four 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 Albert Lunde + Harald Tveit Alvestrand John C. Mallery + Keith Ball Jean-Philippe Martin-Flatin + Brian Behlendorf Larry Masinter + 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 + + 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. + + + + + + +Fielding, et. al. Standards Track [Page 145] + +RFC 2068 HTTP/1.1 January 1997 + + + 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, Alex Hopmann, and Larry Masinter for their + help. + +17 References + + [1] Alvestrand, H., "Tags for the identification of languages", RFC + 1766, UNINETT, 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, University of + Minnesota, March 1993. + + [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", A + Unifying Syntax for the Expression of Names and Addresses of Objects + on the Network as used in the World-Wide Web", RFC 1630, CERN, June + 1994. + + [4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource + Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota, + December 1994. + + [5] Berners-Lee, T., and D. Connolly, "HyperText Markup Language + Specification - 2.0", RFC 1866, MIT/LCS, November 1995. + + [6] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext + Transfer Protocol -- HTTP/1.0.", RFC 1945 MIT/LCS, UC Irvine, May + 1996. + + [7] Freed, N., and N. Borenstein, "Multipurpose Internet Mail + Extensions (MIME) Part One: Format of Internet Message Bodies", RFC + 2045, Innosoft, First Virtual, November 1996. + + [8] Braden, R., "Requirements for Internet hosts - application and + support", STD 3, RFC 1123, IETF, October 1989. + + [9] Crocker, D., "Standard for the Format of ARPA Internet Text + Messages", STD 11, RFC 822, UDEL, August 1982. + + + + +Fielding, et. al. Standards Track [Page 146] + +RFC 2068 HTTP/1.1 January 1997 + + + [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, UC + Irvine, June 1995. + + [12] Horton, M., and R. Adams. "Standard for interchange of USENET + messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic + Studies, December 1987. + + [13] Kantor, B., and P. Lapsley. "Network News Transfer Protocol." A + Proposed Standard for the Stream-Based Transmission of News", RFC + 977, UC San Diego, UC Berkeley, February 1986. + + [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part + Three: Message Header Extensions for Non-ASCII Text", RFC 2047, + University of Tennessee, November 1996. + + [15] Nebel, E., and L. Masinter. "Form-based File Upload in HTML", + RFC 1867, Xerox Corporation, November 1995. + + [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, + USC/ISI, August 1982. + + [17] Postel, J., "Media Type Registration Procedure", RFC 2048, + USC/ISI, November 1996. + + [18] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD + 9, RFC 959, USC/ISI, October 1985. + + [19] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC + 1700, USC/ISI, October 1994. + + [20] Sollins, K., and L. Masinter, "Functional Requirements for + Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation, + 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. + + + +Fielding, et. al. Standards Track [Page 147] + +RFC 2068 HTTP/1.1 January 1997 + + + 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, Carnegie Mellon, Dover Beach Consulting, October, 1995. + + [24] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC + 1900, IAB, February 1996. + + [25] Deutsch, P., "GZIP file format specification version 4.3." RFC + 1952, Aladdin Enterprises, 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 Conf. '94: Mosaic and the Web, Oct. 1994, which is available at + http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/ + HTTPLatency.html. + + [27] Joe Touch, John Heidemann, and Katia Obraczka, "Analysis of HTTP + Performance", <URL: http://www.isi.edu/lsam/ib/http-perf/>, + USC/Information Sciences Institute, June 1996 + + [28] Mills, D., "Network Time Protocol, Version 3, Specification, + Implementation and Analysis", RFC 1305, University of Delaware, March + 1992. + + [29] Deutsch, P., "DEFLATE Compressed Data Format Specification + version 1.3." RFC 1951, Aladdin Enterprises, May 1996. + + [30] Spero, S., "Analysis of HTTP Performance Problems" + <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>. + + [31] Deutsch, P., and J-L. Gailly, "ZLIB Compressed Data Format + Specification version 3.3", RFC 1950, Aladdin Enterprises, Info-ZIP, + 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. + + + + + + + + +Fielding, et. al. Standards Track [Page 148] + +RFC 2068 HTTP/1.1 January 1997 + + +18 Authors' Addresses + + Roy T. Fielding + Department of Information and Computer Science + University of California + Irvine, CA 92717-3425, USA + + Fax: +1 (714) 824-4056 + EMail: [email protected] + + + Jim Gettys + MIT Laboratory for Computer Science + 545 Technology Square + Cambridge, MA 02139, USA + + Fax: +1 (617) 258 8682 + EMail: [email protected] + + + Jeffrey C. Mogul + Western Research Laboratory + Digital Equipment Corporation + 250 University Avenue + Palo Alto, California, 94305, USA + + EMail: [email protected] + + + Henrik Frystyk Nielsen + W3 Consortium + MIT Laboratory for Computer Science + 545 Technology Square + Cambridge, MA 02139, USA + + Fax: +1 (617) 258 8682 + EMail: [email protected] + + + Tim Berners-Lee + Director, W3 Consortium + MIT Laboratory for Computer Science + 545 Technology Square + Cambridge, MA 02139, USA + + Fax: +1 (617) 258 8682 + EMail: [email protected] + + + + +Fielding, et. al. Standards Track [Page 149] + +RFC 2068 HTTP/1.1 January 1997 + + +19 Appendices + +19.1 Internet Media Type message/http + + In addition to defining the HTTP/1.1 protocol, this document serves + as the specification for the Internet media type "message/http". The + following is to be registered with IANA. + + 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 + +19.2 Internet Media Type multipart/byteranges + + 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 MIME message. The + multipart 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 parts are + separated using a MIME boundary parameter. + + 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 + + + + +Fielding, et. al. Standards Track [Page 150] + +RFC 2068 HTTP/1.1 January 1997 + + +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-- + +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 no label is preferred over the labels US-ASCII or + ISO-8859-1. + + Additional rules for requirements on parsing and encoding of dates + and other potential problems with date encodings include: + + o 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). + + + + +Fielding, et. al. Standards Track [Page 151] + +RFC 2068 HTTP/1.1 January 1997 + + + o 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. + + o 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. + + o 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 MIME Entities + + HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC + 822) and the Multipurpose Internet Mail Extensions (MIME ) to allow + entities to be transmitted in an open variety of representations and + with extensible mechanisms. However, MIME [7] discusses mail, and + HTTP has a few features that are different from those described in + MIME. 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 MIME. + 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 may be + required. + +19.4.1 Conversion to Canonical Form + + MIME requires that an Internet mail entity be converted to canonical + form prior to being transferred. Section 3.7.1 of this document + describes the forms allowed for subtypes of the "text" media type + when transmitted over HTTP. MIME 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 MIME + canonical form of CRLF. Note, however, that this may be complicated + by the presence of a Content-Encoding and by the fact that HTTP + + + +Fielding, et. al. Standards Track [Page 152] + +RFC 2068 HTTP/1.1 January 1997 + + + 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. + +19.4.2 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. + +19.4.3 Introduction of Content-Encoding + + MIME 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 an equivalent + function as Content-Encoding. However, this parameter is not part of + MIME.) + +19.4.4 No Content-Transfer-Encoding + + HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME. + 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. + +19.4.5 HTTP Header Fields in Multipart Body-Parts + + In MIME, most header fields in multipart body-parts are generally + ignored unless the field name begins with "Content-". In HTTP/1.1, + multipart body-parts may contain any HTTP header fields which are + significant to the meaning of that part. + + + + + + +Fielding, et. al. Standards Track [Page 153] + +RFC 2068 HTTP/1.1 January 1997 + + +19.4.6 Introduction of Transfer-Encoding + + HTTP/1.1 introduces the Transfer-Encoding header field (section + 14.40). 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-ext (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 MIME-Version + + HTTP is not a MIME-compliant protocol (see appendix 19.4). 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. + 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. + +19.5 Changes from HTTP/1.0 + + This section summarizes major differences between versions HTTP/1.0 + and HTTP/1.1. + + + + + + +Fielding, et. al. Standards Track [Page 154] + +RFC 2068 HTTP/1.1 January 1997 + + +19.5.1 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: + + o Both clients and servers MUST support the Host request-header. + + o Host request-headers are required in HTTP/1.1 requests. + + o Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 + request does not include a Host request-header. + + o Servers MUST accept absolute URIs. + + + + + + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 155] + +RFC 2068 HTTP/1.1 January 1997 + + +19.6 Additional Features + + This appendix documents protocol elements used by some existing HTTP + implementations, but not consistently and correctly across most + HTTP/1.1 applications. Implementers should 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. + +19.6.1 Additional Request Methods + +19.6.1.1 PATCH + + The PATCH method is similar to PUT except that the entity contains a + list of differences between the original version of the resource + identified by the Request-URI and the desired content of the resource + after the PATCH action has been applied. The list of differences is + in a format defined by the media type of the entity (e.g., + "application/diff") and MUST include sufficient information to allow + the server to recreate the changes necessary to convert the original + version of the resource to the desired version. + + If the request passes through a cache and the Request-URI identifies + a currently cached entity, that entity MUST be removed from the + cache. Responses to this method are not cachable. + + The actual method for determining how the patched resource is placed, + and what happens to its predecessor, is defined entirely by the + origin server. If the original version of the resource being patched + included a Content-Version header field, the request entity MUST + include a Derived-From header field corresponding to the value of the + original Content-Version header field. Applications are encouraged to + use these fields for constructing versioning relationships and + resolving version conflicts. + + PATCH requests must obey the message transmission requirements set + out in section 8.2. + + Caches that implement PATCH should invalidate cached responses as + defined in section 13.10 for PUT. + +19.6.1.2 LINK + + The LINK method establishes one or more Link relationships between + the existing resource identified by the Request-URI and other + existing resources. The difference between LINK and other methods + + + +Fielding, et. al. Standards Track [Page 156] + +RFC 2068 HTTP/1.1 January 1997 + + + allowing links to be established between resources is that the LINK + method does not allow any message-body to be sent in the request and + does not directly result in the creation of new resources. + + If the request passes through a cache and the Request-URI identifies + a currently cached entity, that entity MUST be removed from the + cache. Responses to this method are not cachable. + + Caches that implement LINK should invalidate cached responses as + defined in section 13.10 for PUT. + +19.6.1.3 UNLINK + + The UNLINK method removes one or more Link relationships from the + existing resource identified by the Request-URI. These relationships + may have been established using the LINK method or by any other + method supporting the Link header. The removal of a link to a + resource does not imply that the resource ceases to exist or becomes + inaccessible for future references. + + If the request passes through a cache and the Request-URI identifies + a currently cached entity, that entity MUST be removed from the + cache. Responses to this method are not cachable. + + Caches that implement UNLINK should invalidate cached responses as + defined in section 13.10 for PUT. + +19.6.2 Additional Header Field Definitions + +19.6.2.1 Alternates + + The Alternates response-header field has been proposed as a means for + the origin server to inform the client about other available + representations of the requested resource, along with their + distinguishing attributes, and thus providing a more reliable means + for a user agent to perform subsequent selection of another + representation which better fits the desires of its user (described + as agent-driven negotiation in section 12). + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 157] + +RFC 2068 HTTP/1.1 January 1997 + + + The Alternates header field is orthogonal to the Vary header field in + that both may coexist in a message without affecting the + interpretation of the response or the available representations. It + is expected that Alternates will provide a significant improvement + over the server-driven negotiation provided by the Vary field for + those resources that vary over common dimensions like type and + language. + + The Alternates header field will be defined in a future + specification. + +19.6.2.2 Content-Version + + The Content-Version entity-header field defines the version tag + associated with a rendition of an evolving entity. Together with the + Derived-From field described in section 19.6.2.3, it allows a group + of people to work simultaneously on the creation of a work as an + iterative process. The field should be used to allow evolution of a + particular work along a single path rather than derived works or + renditions in different representations. + + Content-Version = "Content-Version" ":" quoted-string + + Examples of the Content-Version field include: + + Content-Version: "2.1.2" + Content-Version: "Fred 19950116-12:26:48" + Content-Version: "2.5a4-omega7" + +19.6.2.3 Derived-From + + The Derived-From entity-header field can be used to indicate the + version tag of the resource from which the enclosed entity was + derived before modifications were made by the sender. This field is + used to help manage the process of merging successive changes to a + resource, particularly when such changes are being made in parallel + and from multiple sources. + + Derived-From = "Derived-From" ":" quoted-string + + An example use of the field is: + + Derived-From: "2.1.1" + + The Derived-From field is required for PUT and PATCH requests if the + entity being sent was previously retrieved from the same URI and a + Content-Version header was included with the entity when it was last + retrieved. + + + +Fielding, et. al. Standards Track [Page 158] + +RFC 2068 HTTP/1.1 January 1997 + + +19.6.2.4 Link + + The Link entity-header field provides a means for describing a + relationship between two resources, generally between the requested + resource and some other resource. An entity MAY include multiple Link + values. Links at the metainformation level typically indicate + relationships like hierarchical structure and navigation paths. The + Link field is semantically equivalent to the <LINK> element in + HTML.[5] + + Link = "Link" ":" #("<" URI ">" *( ";" link-param ) + + link-param = ( ( "rel" "=" relationship ) + | ( "rev" "=" relationship ) + | ( "title" "=" quoted-string ) + | ( "anchor" "=" <"> URI <"> ) + | ( link-extension ) ) + + link-extension = token [ "=" ( token | quoted-string ) ] + + relationship = sgml-name + | ( <"> sgml-name *( SP sgml-name) <"> ) + + sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" ) + + Relationship values are case-insensitive and MAY be extended within + the constraints of the sgml-name syntax. The title parameter MAY be + used to label the destination of a link such that it can be used as + identification within a human-readable menu. The anchor parameter MAY + be used to indicate a source anchor other than the entire current + resource, such as a fragment of this resource or a third resource. + + Examples of usage include: + + Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous" + + Link: <mailto:[email protected]>; rev="Made"; title="Tim Berners-Lee" + + The first example indicates that chapter2 is previous to this + resource in a logical navigation path. The second indicates that the + person responsible for making the resource available is identified by + the given e-mail address. + +19.6.2.5 URI + + The URI header field has, in past versions of this specification, + been used as a combination of the existing Location, Content- + Location, and Vary header fields as well as the future Alternates + + + +Fielding, et. al. Standards Track [Page 159] + +RFC 2068 HTTP/1.1 January 1997 + + + field (above). Its primary purpose has been to include a list of + additional URIs for the resource, including names and mirror + locations. However, it has become clear that the combination of many + different functions within this single field has been a barrier to + consistently and correctly implementing any of those functions. + Furthermore, we believe that the identification of names and mirror + locations would be better performed via the Link header field. The + URI header field is therefore deprecated in favor of those other + fields. + + URI-header = "URI" ":" 1#( "<" URI ">" ) + +19.7 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, we + would expect commercial HTTP/1.1 servers to: + + o recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 + requests; + + o understand any valid request in the format of HTTP/0.9, 1.0, or + 1.1; + + o respond appropriately with a message in the same major version used + by the client. + + And we would expect HTTP/1.1 clients to: + + o recognize the format of the Status-Line for HTTP/1.0 and 1.1 + responses; + + o 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. A few implementations implement the Keep-Alive + version of persistent connections described in section 19.7.1.1. + + + + + + + + + + +Fielding, et. al. Standards Track [Page 160] + +RFC 2068 HTTP/1.1 January 1997 + + +19.7.1 Compatibility with HTTP/1.0 Persistent Connections + + Some clients and servers may wish to be compatible with some previous + implementations of persistent connections in HTTP/1.0 clients and + servers. Persistent connections in HTTP/1.0 must be 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. + + The following describes the original HTTP/1.0 form of persistent + connections. + + When it connects to an origin server, an HTTP client MAY send the + Keep-Alive connection-token in addition to the Persist connection- + token: + + Connection: Keep-Alive + + An HTTP/1.0 server would then respond with the Keep-Alive connection + token and the client may proceed with an HTTP/1.0 (or Keep-Alive) + persistent connection. + + An HTTP/1.1 server may also establish persistent connections with + HTTP/1.0 clients upon receipt of a Keep-Alive connection token. + However, a persistent connection with an HTTP/1.0 client cannot make + use of the chunked transfer-coding, and therefore MUST use a + Content-Length for marking the ending boundary of each message. + + A client MUST NOT send the Keep-Alive connection token to a proxy + server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 + for parsing the Connection header field. + + + + + +Fielding, et. al. Standards Track [Page 161] + +RFC 2068 HTTP/1.1 January 1997 + + +19.7.1.1 The Keep-Alive Header + + When the Keep-Alive connection-token has been transmitted with a + request or a response, a Keep-Alive header field MAY also be + included. The Keep-Alive header field takes the following form: + + Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param + + keepalive-param = param-name "=" value + + The Keep-Alive header itself is optional, and is used only if a + parameter is being sent. HTTP/1.1 does not define any parameters. + + If the Keep-Alive header is sent, the corresponding connection token + MUST be transmitted. The Keep-Alive header MUST be ignored if + received without the connection token. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Fielding, et. al. Standards Track [Page 162] + |