HTTP Working Group T. Berners-Lee INTERNET-DRAFT R. T. Fielding H. Frystyk Nielsen Expires September 8, 1995 March 8, 1995 Hypertext Transfer Protocol -- HTTP/1.0 Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this document is unlimited. Please send comments to the HTTP working group at . Discussions of the working group are archived at . General discussions about HTTP and the applications which use HTTP should take place on the mailing list. Abstract The Hypertext Transfer Protocol (HTTP) is an application-level protocol with the lightness and speed necessary 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 (commands). 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 reflects preferred usage of the protocol referred to as "HTTP/1.0", and is compatible with the most commonly used HTTP server and client programs implemented prior to November 1994. Table of Contents 1. Introduction 1.1 Purpose 1.2 Overall Operation 1.3 Terminology 2. Notational Conventions and Generic Grammar 2.1 Augmented BNF 2.2 Basic Rules 3. Protocol Parameters 3.1 HTTP Version 3.2 Universal Resource Identifiers 3.3 Date/Time Formats 3.3.1 Full Date 3.3.2 Delta Seconds 4. HTTP Message 4.1 Message Types 4.2 Message Headers 4.3 General Message Header Fields 4.3.1 Date 4.3.2 Forwarded 4.3.3 Message-ID 4.3.4 MIME-Version 5. Request 5.1 Request-Line 5.2 Method 5.2.1 GET 5.2.2 HEAD 5.2.3 POST 5.2.4 PUT 5.2.5 DELETE 5.2.6 LINK 5.2.7 UNLINK 5.3 Request-URI 5.4 Request Header Fields 5.4.1 Accept 5.4.2 Accept-Charset 5.4.3 Accept-Encoding 5.4.4 Accept-Language 5.4.5 Authorization 5.4.6 From 5.4.7 If-Modified-Since 5.4.8 Pragma 5.4.9 Referer 5.4.10 User-Agent 6. Response 6.1 Status-Line 6.2 Status Codes and Reason Phrases 6.2.1 Successful 2xx 6.2.2 Redirection 3xx 6.2.3 Client Error 4xx 6.2.4 Server Errors 5xx 6.3 Response Header Fields 6.3.1 Public 6.3.2 Retry-After 6.3.3 Server 6.3.4 WWW-Authenticate 7. Entity 7.1 Entity Header Fields 7.1.1 Allow 7.1.2 Content-Encoding 7.1.3 Content-Language 7.1.4 Content-Length 7.1.5 Content-Transfer-Encoding 7.1.6 Content-Type 7.1.7 Derived-From 7.1.8 Expires 7.1.9 Last-Modified 7.1.10 Link 7.1.11 Location 7.1.12 Title 7.1.13 URI 7.1.14 Version 7.2 Entity Body 7.2.1 Type 7.2.2 Length 8. Content Parameters 8.1 Media Types 8.1.1 Canonicalization and Text Defaults 8.1.2 Multipart Types 8.2 Language Tags 8.3 Character Sets 8.4 Encoding Mechanisms 8.5 Transfer Encodings 9. Content Negotiation 10. Access Authentication 10.1 Basic Authentication Scheme 11. Security Considerations 11.1 Authentication of Clients 11.2 Idempotent Methods 11.3 Abuse of Server Log Information 12. Acknowledgments 13. References 14. Authors' Addresses Appendix A. Internet Media Type message/http Appendix B. Minimum Compliance B.1 Servers Appendix C. Tolerant Applications C.1 Request-Line, Status-Line, and Header Fields Appendix D. Relationship to MIME D.1 Conversion to Canonical Form D.1.1 Representation of Line Breaks D.1.2 Default Character Set D.2 Default Content-Transfer-Encoding D.3 Introduction of Content-Encoding Appendix E. Example of Version Control 1. Introduction 1.1 Purpose The Hypertext Transfer Protocol (HTTP) is an application-level protocol with the lightness and speed necessary for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification reflects preferred usage of the protocol referred to as "HTTP/1.0". This specification does not necessarily reflect the "current practice" of any single HTTP server or client implementation. It does, however, seek to remain compatible with existing implementations wherever possible, and should be considered the reference for future implementations of HTTP/1.0. Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP/1.0 allows an open-ended set of methods to be used to indicate the purpose of a request. It builds on the discipline of reference provided by the Universal Resource Identifier (URI) [3], as a location (URL) [5] or name (URN), for indicating the resource on which a method is to be applied. Messages are passed in a format similar to that used by Internet Mail [8] and the Multipurpose Internet Mail Extensions (MIME) [6]. HTTP/1.0 is also used for communication between user agents and various gateways, allowing hypermedia access to existing Internet protocols like SMTP [14], NNTP [12], FTP [16], Gopher [2], and WAIS [9]. HTTP/1.0 is designed to allow such gateways, via proxy servers, without any loss of the data conveyed by those earlier protocols. 1.2 Overall Operation The HTTP protocol is based on a request/response paradigm. A requesting program (termed a client) establishes a connection with a receiving program (termed a server) and 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. The server responds with a status line (including its protocol version and a success or error code), followed by a MIME-like message containing server information, entity metainformation, and possible body content. It should be noted that a given program may be capable of being both a client and a server; our use of those terms refers only to the role being performed by the program during a particular connection, rather than to the program's purpose in general. On the Internet, the communication generally takes place over a TCP/IP connection. The default port is TCP 80 [17], but other ports can be used. This does not preclude the HTTP/1.0 protocol from being implemented on top of any other protocol on the Internet, or on other networks. The mapping of the HTTP/1.0 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification. For most implementations, the connection is established by the client prior to each request and closed by the server after sending the response. However, this is not a feature of the protocol and is not required by this specification. Both clients and servers must be capable of handling cases where either party closes the connection prematurely, due to user action, automated time-out, or program failure. In any case, the closing of the connection by either or both parties always terminates the current request, regardless of its status. 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 virtual circuit established between two parties for the purpose of communication. message A structured sequence of octets transmitted via the connection as the basic component of communication. 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 which can be identified by a URI. entity A particular representation or rendition of a resource that may be enclosed within a request or response message. An entity consists of metainformation (in the form of entity headers) and content (in the form of an entity body). client A program that establishes connections for the purpose of sending requests. user agent The client program which is closest to the user and which initiates requests at their behest. server A program that accepts connections in order to service requests by sending back responses. origin server The server on which a given resource resides or is to be created. proxy An intermediary program which acts as both a server and a client for the purpose of forwarding requests. Proxies are often used to act as a portal through a network firewall. A proxy server accepts requests from other clients and services them either internally or by passing them (with possible translation) on to other servers. A caching proxy is a proxy server with a local cache of server responses -- some requested resources can be serviced from the cache rather than from the origin server. Some proxy servers also act as origin servers. gateway A proxy which services HTTP requests by translation into protocols other than HTTP. The reply sent from the remote server to the gateway is likewise translated into HTTP before being forwarded to the user agent. 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 [8]. Implementors will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs: name = definition The name of a rule is simply the name itself (without any enclosing "<" and ">") and is separated from its definition by the equal character "=". 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, HTAB, 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 ("I") 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 "*element" indicating at least and at most 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: "(element)" is equivalent to "*(element)"; that is, exactly 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 "#element" indicating at least and at most 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 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, zero or more linear whitespace (LWS) can be included between any two words (token or quoted-string) without changing the interpretation of a field. However, applications should attempt to follow "common form" when generating HTTP constructs, since there exist some implementations that fail to accept anything beyond the common forms. 2.2 Basic Rules The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII character set is defined by [18]. OCTET = CHAR = UPALPHA = LOALPHA = ALPHA = UPALPHA | LOALPHA DIGIT = CTL = CR = LF = SP = HTAB = <"> = HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker for all protocol elements except the Entity-Body (see Appendix C for tolerant applications). The end-of-line marker for an Entity-Body is defined by its associated media type, as described in Section 8.1. CRLF = CR LF HTTP/1.0 headers can be folded onto multiple lines if the continuation lines begin with linear whitespace characters. All linear whitespace (including folding) has the same semantics as SP. LWS = [CRLF] 1*( SP | HTAB ) Many HTTP/1.0 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. word = token | quoted-string token = 1* tspecials = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | SP | HTAB A string of text is parsed as a single word if it is quoted using double-quote marks or angle brackets. quoted-string = ( <"> *(qdtext) <"> ) | ( "<" *(qatext) ">" ) qdtext = and CTLs, but including LWS> qatext = ", and CTLs, but including LWS> The text rule is only used for descriptive field contents. Words of *text may contain characters from character sets other than US-ASCII only when encoded according to the rules of RFC 1522 [13]. text = 3. Protocol Parameters 3.1 HTTP Version HTTP uses a "." 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 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 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. If the protocol version is not specified, the recipient must assume that the message is in the simple HTTP/0.9 format. HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT Note that the major and minor numbers should 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 zeroes should be ignored by recipients and never generated by senders. This document defines both the 0.9 and 1.0 versions of the HTTP protocol. Applications sending Full-Request or Full-Response messages, as defined by this specification, must include an HTTP-Version of "HTTP/1.0". HTTP servers are required to be able to recognize the format of the Request-Line for all lower-version requests, understand requests with a format within one major number of their native version (i.e. and ), and respond appropriately with a message within the same protocol number (even if the response is simply an error message). HTTP clients are required to be able to recognize the format of the Status-Line for all lower-version responses and understand responses with a format within one major number of their request version. The following hypothetical example illustrates the required behavior. o A valid HTTP/3.5 request is received and the server's native protocol version is o Less than 3.0: it should attempt to understand the request and respond (possibly with an error) in its native format; o Major number of 3: It should understand the request and respond in its native format; o Major number of 4: It should understand the request and respond with a version 3 message; o Major number higher than 4: It should attempt to understand the request and respond (possibly with an error) with a version 3 message; o User agent receives a response to an HTTP/3.5 request, and the response version is o Less than 2.0: It should attempt to understand the response and unobtrusively warn the user of the version mismatch; o 2.0--3.4: It should understand the response and be aware that its request may not have been fully understood by the server; o 3.5 or higher 3: It should understand the response and can assume that the server understood all aspects of the request if the response does not indicate an error; o 4.0 or higher: It should attempt to understand the response and unobtrusively warn the user of the version mismatch. Proxies must be careful in forwarding requests that are received in a format different than that of the proxy's native version. Since the protocol version indicates the protocol capability of the sender, a proxy must never send a message with a version indicator which is greater than its native version; if a higher version request is received, the proxy must either downgrade the request version or respond with an error. Requests with a version lower than that of the proxy's native format may be upgraded by the proxy before being forwarded; the proxy's response to that request must follow the normal server requirements. 3.2 Universal Resource Identifiers This section is To-Be-Specified. It will contain a brief description of URIs as defined by RFC 1630, including mention of the allowed characters and the hex encoding, and provide a full definition of the http URL scheme. URI = All URI protocol elements must be encoded using the escaping scheme described in RFC 1630 [3]. 3.3 Date/Time Formats 3.3.1 Full Date HTTP/1.0 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 [7] (an update to RFC 822 [8]). The second format is in common use, but is based on the obsolete RFC 850 [11] date format and lacks a four- digit year. HTTP/1.0 clients and servers must accept all three formats, though they should never generate the third (asctime) format. Future clients and servers must only generate the RFC 1123 format for representing date/time stamps in HTTP/1.0 requests and responses. All HTTP/1.0 date/time stamps must be represented in Universal Time (UT), also known as 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 should 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" weekday = "Monday" | "Tuesday" | "Wednesday" | "Thursday" | "Friday" | "Saturday" | "Sunday" month = "Jan" | "Feb" | "Mar" | "Apr" | "May" | "Jun" | "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec" Comments and/or extra LWS are not permitted inside an HTTP-date value generated by a conformant application. However, recipients of date values should be robust in accepting date values that may have been generated by non-HTTP applications (as is sometimes the case when retrieving or posting messages via gateways to SMTP or NNTP). Note: HTTP/1.0 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. This format should only be used to represent short time periods or periods that cannot start until receipt of the message. delta-seconds = 1*DIGIT 4. HTTP Message 4.1 Message Types HTTP messages consist of requests from client to server and responses from server to client. HTTP-message = Simple-Request ; HTTP/0.9 messages | Simple-Response | Full-Request ; HTTP/1.0 messages | Full-Response Full-Request and Full-Response use the generic message format of RFC 822 [8] for transferring entities. Both messages may include optional header fields (a.k.a. "headers") and an entity body. The entity body is separated from the headers by a null line (i.e., a line with nothing preceding the CRLF). Full-Request = Request-Line ; Section 5.1 *( General-Header ; Section 4.3 | Request-Header ; Section 5.4 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2 Full-Response = Status-Line ; Section 6.1 *( General-Header ; Section 4.3 | Response-Header ; Section 6.3 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2 Simple-Request and Simple-Response do not allow the use of any header information and are limited to a single request method (GET). Simple-Request = "GET" SP Request-URI CRLF Simple-Response = [ Entity-Body ] Use of the Simple-Request format is discouraged because it prevents the client from using content negotiation and the server from identifying the media type of the returned entity. 4.2 Message Headers HTTP header fields, which include General-Header (Section 4.3), Request-Header (Section 5.4), Response-Header (Section 6.3), and Entity-Header (Section 7.1) fields, follow the same generic format as that given in Section 3.1 of RFC 822 [8]. Each header field consists of a name followed by a colon (":") and the field value. 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 LWS. HTTP-header = field-name ":" [ field-value ] CRLF field-name = 1* field-value = *( field-content | comment | LWS ) field-content = The order in which header fields are received is not significant. However, it is considered "good practice" to send General-Header fields first, followed by Request-Header or Response-Header fields prior to the Entity-Header fields. Comments can be included in HTTP header fields by surrounding the comment text with parentheses. comment = "(" *( ctext | comment ) ")" ctext = Note: Use of comments within HTTP headers is generally discouraged, since they are rarely seen by human eyes and hence only increase network traffic. However, they may be useful for messages posted or retrieved via NNTP and SMTP gateways. 4.3 General Message 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 communicating parties or the entity being transferred. Although none of the General-Header fields are required, they are all strongly recommended where their use is appropriate, and should be understood by all future HTTP/1.0 clients and servers. These headers apply only to the message being transmitted. General-Header = Date | Forwarded | Message-ID | MIME-Version | extension-header Although additional general header fields may be implemented via the extension mechanism, applications which do not recognize those fields should treat them as Entity-Header fields. 4.3.1 Date The Date header 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. 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 default 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 should always include a Date header. 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 receiver if and only if the message will be cached by that receiver or gatewayed via a protocol which requires a Date. Only one Date header field is allowed per message. In theory, the date should represent the moment just before the status/request- line is generated (i.e. the time at which the originator made the determination of what the request/response should be). In practice, the date can be generated at any time during the message origination without affecting its semantic value. Note: An earlier version of this document incorrectly specified that this field should contain the creation date of the enclosed Entity-Body. This has been changed to reflect actual (and proper) usage. 4.3.2 Forwarded The Forwarded header is to be used by proxies to indicate the intermediate steps 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 tracing transport problems and avoiding request loops. Forwarded = "Forwarded" ":" "by" URI [ "(" product ")" ] [ "for" FQDN ] FQDN = For example, a message could be sent from a client on ptsun00.cern.ch to a server at www.ics.uci.edu port 80, via an intermediate HTTP proxy at info.cern.ch port 8000. The request received by the server at www.ics.uci.edu would then have the following Forwarded header field: Forwarded: by http://info.cern.ch:8000/ for ptsun00.cern.ch Multiple Forwarded header fields are allowed and should represent each proxy that has forwarded the message. It is strongly recommended that proxies used as a portal through a network firewall do not, by default, send out information about the internal hosts within the firewall region. This information should only be propagated if explicitly enabled. If not enabled, the for token and FQDN should not be included in the field value. 4.3.3 Message-ID The Message-ID field in HTTP is identical to that used by Internet Mail and USENET messages, as defined in [11]. That is, it gives the message a single, unique identifier which can be used for identifying the message (not its contents) for "much longer" than the expected lifetime of that message. Message-ID = "Message-ID" ":" "<" addr-spec ">" addr-spec = Although it is not required, the addr-spec format typically used within a Message-ID consists of a string that is unique at the originator's machine, followed by the required "@" character and the fully-qualified domain name of that machine. An example is Message-ID: <9411151630.4256@info.cern.ch> which is composed using the time, date and process-ID on the host info.cern.ch. However, this is only one of many possible methods for generating a unique Message-ID and recipients of a message should consider the entire value opaque. The Message-ID field is normally not generated by HTTP applications and is never required. It should only be generated by a gateway application when the message is being posted to some other protocol that desires a Message-ID. HTTP responses should only include a Message-ID header field when the entity being transferred already has one assigned to it (as in the case of resources that were originally posted via Internet Mail or USENET). 4.3.4 MIME-Version HTTP is not a MIME-conformant protocol. However, HTTP/1.0 messages may include a single MIME-Version header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field should indicate that the message is in full compliance with the MIME protocol (as defined in [6]). Unfortunately, current versions of HTTP/1.0 clients and servers use this field indiscriminately, and thus receivers must not take it for granted that the message is indeed in full compliance with MIME. Gateways are responsible for ensuring this compliance (where possible) when exporting HTTP messages to strict MIME environments. Future HTTP/1.0 applications must only use MIME-Version when the message is intended to be MIME-conformant. MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT MIME version "1.0" is the default for use in HTTP/1.0. However, HTTP/1.0 message parsing and semantics are defined by this document and not the MIME specification. 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 requested, the identifier of the resource, and the protocol version in use. For backwards compatibility with the more limited HTTP/0.9 protocol, there are two valid formats for an HTTP request: Request = Simple-Request | Full-Request Simple-Request = "GET" SP Request-URI CRLF Full-Request = Request-Line ; Section 5.1 *( General-Header ; Section 4.3 | Request-Header ; Section 5.4 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2 If an HTTP/1.0 server receives a Simple-Request, it must respond with an HTTP/0.9 Simple-Response. An HTTP/1.0 client capable of receiving a Full-Response should never generate a Simple-Request. 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 Note that the difference between a Simple-Request and the Request- Line of a Full-Request is the presence of the HTTP-Version field. 5.2 Method The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case- sensitive and extensible. Method = "GET" | "HEAD" | "PUT" | "POST" | "DELETE" | "LINK" | "UNLINK" | extension-method extension-method = token The list of methods acceptable by a specific resource can be specified in an "Allow" Entity-Header (Section 7.1.1). However, the client is always notified through the return code of the response whether a method is currently allowed on a specific resource, as this 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 unknown or not implemented by the server. The set of common methods for HTTP/1.0 is described below. Although this set can be easily expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers. In order to maintain compatibility, the semantic definition for extension methods should be registered with the IANA [17]. 5.2.1 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 changes to a "conditional GET" if the request message includes an If-Modified-Since header field. A conditional GET method requests that the identified resource 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. b) If the resource has been modified since the If-Modified- Since date, the response is exactly the same as for a normal GET. c) If the resource has not been modified since the If-Modified- Since date, the server shall return a "304 Not Modified" response. The conditional GET method is intended to reduce network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring unnecessary data. 5.2.2 HEAD The HEAD method is identical to GET except that the server must not return any Entity-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 resource identified by the Request-URI without transferring the Entity-Body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification. There is no "conditional HEAD" requests analogous to the conditional GET. If an If-Modified-Since header field is included with a HEAD request, it should be ignored. 5.2.3 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 (usually 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 considered to be 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 client can suggest a URI for identifying the new resource by including a URI-header field in the request. However, the server should treat that URI as advisory only and may store the entity under a different URI or without any URI. The client may apply relationships between the new resource and other existing resources by including Link header fields, as described in Section 7.1.10. The server may use the Link information to perform other operations as a result of the new resource being added. For example, lists and indexes might be updated. However, no mandatory operation is imposed on the origin server. The origin server may also generate its own or additional links to other resources. A successful POST does not require that the entity be created as a resource on the origin server or made accessible for future reference. That is, 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 the allocated URI, all applicable Link header fields, and an entity (preferably of type "text/html") which describes the status of the request and refers to the new resource. 5.2.4 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 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, the "200 OK" response 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 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 as an appendage. 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 requestor of a PUT knows what URI is intended and the receiver must not attempt to apply the request to some other resource. If the receiver desires that the request be applied to a different URI, it must send a "301 Moved Permanently" response; the requestor may then make its own decision regarding whether or not to redirect the request. The client can create or modify relationships between the enclosed entity and other existing resources by including Link header fields, as described in Section 7.1.10. As with POST, the server may use the Link information to perform other operations as a result of the request. However, no mandatory operation is imposed on the origin server. The origin server may generate its own or additional links to other resources. The actual method for determining how the resource is placed, and what happens to its predecessor, is defined entirely by the origin server. If version control is implemented by the origin server, the Version and Derived-From header fields should be used to help identify and control revisions to a resource. 5.2.5 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). 5.2.6 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 allowing links to be established between resources is that the LINK method does not allow any Entity-Body to be sent in the request and does not result in the creation of new resources. 5.2.7 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. 5.3 Request-URI The Request-URI is a Universal Resource Identifier (Section 3.2) and identifies the resource upon which to apply the request. Request-URI = URI Unless the server is being used as a proxy, a partial URI shall be given with the assumptions of the scheme (http) and hostname:port (the server's address) being obvious. That is, if the full URI looks like http://info.cern.ch/hypertext/WWW/TheProject.html then the corresponding partial URI in the Simple-Request or Full-Request is /hypertext/WWW/TheProject.html If the client is sending the request through a proxy, the absolute form of the URI (including scheme and server address) must be used. A proxy must be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. 5.4 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. All header fields are optional and conform to the generic HTTP-header syntax. Request-Header = Accept | Accept-Charset | Accept-Encoding | Accept-Language | Authorization | From | If-Modified-Since | Pragma | Referer | User-Agent | extension-header Although additional request header fields may be implemented via the extension mechanism, applications which do not recognize those fields should treat them as Entity-Header fields. 5.4.1 Accept The Accept header field can be used to indicate a list of media ranges which are acceptable as a response to the request. 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 set of ranges given by the client should represent what types are acceptable given the context of the request. The Accept field should only be used when the request is specifically limited to a set of desired types (as in the case of a request for an in-line image), to indicate qualitative preferences for specific media types, or to indicate acceptance of unusual media types. The field may be folded onto several lines and more than one occurrence of the field is allowed (with the semantics being the same as if all the entries had been in one field value). Accept = "Accept" ":" 1#( media-range [ ";" "q" "=" ( "0" | "1" | float ) ] [ ";" "mxb" "=" 1*DIGIT ] ) media-range = ( "*/*" | ( type "/" "*" ) | ( type "/" subtype ) ) *( ";" parameter ) float = < ANSI-C floating point text representation, where (0.0 < float < 1.0) > The parameter q is used to indicate the quality factor, which represents the user's preference for that range of media types, and the parameter mxb gives the maximum acceptable size of the Entity- Body (in decimal number of octets) for that range of media types. Section 9 describes the content negotiation algorithm which makes use of these values. If at least one Accept header is present, a quality factor of 0 is equivalent to not sending an Accept header field containing that media-type or set of media-types. The default values are: q=1 and mxb=undefined (i.e. infinity). The example Accept: audio/*; q=0.2, audio/basic should verbally be interpreted as "if you have audio/basic, send it; otherwise send me any audio type." If no Accept header is present, then it is assumed that the client accepts all media types with quality factor 1. This is equivalent to the client sending the following accept header field: Accept: */*; q=1 or Accept: */* A more elaborate example is Accept: text/plain; q=0.5, text/html, text/x-dvi; q=0.8; mxb=100000, 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 Entity-Body in text/x-dvi if the entity is less than 100000 bytes, otherwise send text/plain". Note: In earlier versions of this document, the mxs parameter defined the maximum acceptable delay in seconds before the response would arrive. This has been removed as the server has no means of obtaining a useful reference value. However, this does not prevent the client from internally measuring the response time and optimizing the Accept header field accordingly. It must be emphasized that this field should only be used when it is necessary to restrict the response media types to a subset of those possible, to indicate the acceptance of unusual media types, or when the user has been permitted to specify qualitative values for ranges of media types. If no quality factors have been set by the user, and the context of the request is such that the user agent is capable of saving the entity to a file if the received media type is unknown, then the only appropriate value for Accept is "*/*" and the list of unusual types. Whether or not a particular media type is deemed "unusual" should be a configurable aspect of the user agent. 5.4.2 Accept-Charset The Accept-Charset header field can be used to indicate a list of preferred character sets other than the default US-ASCII and ISO-8859-1. This field allows clients capable of understanding more comprehensive or special-purpose character sets to signal that capability to a server which is capable of representing documents in those character sets. Accept-Charset = "Accept-Charset" ":" 1#charset Character set values are described in Section 8.3. An example is Accept-Charset: iso-8859-5, unicode-1-1 The value of this field should not include "US-ASCII" or "ISO-8859-1", since those values are always assumed by default. If a resource is only available in a character set other than the defaults, and that character set is not listed in the Accept- Charset field, it is only acceptable for the server to send the entity if the character set can be identified by an appropriate charset parameter on the media type or within the format of the media type itself. Note: User agents are not required to be able to render the characters associated with the ISO-8859-1 character set. However, they must be able to interpret their meaning to whatever extent is required to properly handle messages in that character set.. 5.4.3 Accept-Encoding The Accept-Encoding header field is similar to Accept, but lists the encoding-mechanisms and transfer-encoding values which are acceptable in the response. Accept-Encoding = "Accept-Encoding" ":" 1#( encoding-mechanism | transfer-encoding ) An example of its use is Accept-Encoding: compress, base64, gzip, quoted-printable The field value should never include the identity transfer-encoding values ("7bit", "8bit", and "binary") since they actually represent "no encoding." If no Accept-Encoding field is present in a request, it must be assumed that the client does not accept any encoding- mechanism and only the identity transfer-encodings. 5.4.4 Accept-Language The Accept-Language header field is similar to Accept, but lists the set of natural languages that are preferred as a response to the request. Accept-Language = "Accept-Language" ":" 1#language-tag The language-tag is described in Section 8.2. Languages are listed in the order of their preference to the user. For example, Accept-Language: dk, en-gb would mean: "Send me a Danish version if you have it; else a British English version." If the server cannot fulfill the request with one or more of the languages given, or if the languages only represent a subset of a multi-linguistic Entity-Body, it is acceptable to serve the request in an unspecified language. 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. 5.4.5 Authorization A user agent that wishes to authenticate itself with a server (usually, but not necessarily, after receiving a "401 Unauthorized" response), may do so by including an Authorization header field with the request. The Authorization field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested. Authorization = "Authorization" ":" credentials HTTP access authentication is described in Section 10. If a request is authenticated and a realm specified, the same credentials should be valid for all other requests within this realm. 5.4.6 From The From 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 addr-spec in RFC 822: From = "From" ":" addr-spec An example is: From: webmaster@w3.org This header field may be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It should not be used as an insecure form of access protection. The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents should include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end. The Internet e-mail address in this field does not have to correspond to the Internet host which issued the request. (For example, when a request is passed through a proxy, then the original issuer's address should be used). The address should, if possible, be a valid Internet e-mail address, whether or not it is in fact an Internet e-mail address or the Internet e-mail representation of an address on some other mail system. 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. 5.4.7 If-Modified-Since The If-Modified-Since header field is used with the GET method to make it conditional: if the requested resource has not been modified since the time specified in this field, a copy of the resource will not be returned from the server; instead, a "304 Not Modified" response will be returned without any Entity-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 The purpose of this feature is to allow efficient updates of local cache information with a minimum amount of transaction overhead. The same functionality can be obtained, though with much greater overhead, by issuing a HEAD request and following it with a GET request if the server indicates that the entity has been modified. 5.4.8 Pragma The Pragma header field is used to specify directives that must be applied to all servers along the request chain (where relevant). The directives typically specify behavior that prevents intermediate proxies from changing the nature of the request. Although multiple pragma directives can be listed as part of the request, HTTP/1.0 only defines semantics for the "no-cache" directive. Pragma = "Pragma" ":" 1#pragma-directive pragma-directive = "no-cache" | extension-pragma extension-pragma = token When the "no-cache" directive is present, a caching proxy must forward the request toward the origin server even if it has a cached copy of what is being requested. This allows a client to insist upon receiving an authoritative response to its request. It also allows a client to refresh a cached copy which has become corrupted or is known to be stale. Pragmas must be passed through by a proxy even when they have significance to that proxy. This is necessary in cases when the request has to go through many proxies, and the pragma may affect all of them. It is not possible to specify a pragma for a specific proxy; however, any pragma-directive not relevant to a gateway or proxy should be ignored. 5.4.9 Referer The Referer field allows the client to specify, for the server's benefit, the address (URI) of the document (or element within the document) from which the Request-URI was obtained. This allows a server to generate lists of back-links to documents, for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The format of the field is: Referer = "Referer" ":" URI Example: Referer: http://info.cern.ch/hypertext/DataSources/Overview.html If a partial URI is given, it should be interpreted relative to the Request-URI. Note: Because the source of a link may be considered private information or may reveal an otherwise secure 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. 5.4.10 User-Agent The User-Agent 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. Although it is not required, user agents should always include this field with requests. The field can contain multiple tokens specifying the product name, with an optional slash and version designator, and other products which form a significant part of the user agent. By convention, the products are listed in order of their significance for identifying the application. User-Agent = "User-Agent" ":" 1*( product ) product = token ["/" product-version] product-version = token Example: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Product tokens should be short and to the point -- use of this field for advertizing or other non-essential information is explicitly deprecated and will be considered as non-conformance to the protocol. 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). The User-Agent field may include additional information within comments that are not part of the value of the field. Note: Some current proxy applications append their product information to the list in the User-Agent field. This is no longer recommended, since it makes machine interpretation of these fields ambiguous. Instead, proxies should use the Forwarded header described in Section 4.3.2. 6. Response After receiving and interpreting a request message, a server responds in the form of an HTTP response message. Response = Simple-Response | Full-Response Simple-Response= [ Entity-Body ] Full-Response = Status-Line ; Section 6.1 *( General-Header ; Section 4.3 | Response-Header ; Section 6.3 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2 A Simple-Response should only be sent in response to an HTTP/0.9 Simple-Request or if the server only supports the more limited HTTP/0.9 protocol. If a client sends an HTTP/1.0 Full-Request and receives a response that does not begin with a Status-Line, it should assume that the response is a Simple-Response and parse it accordingly. Note that the Simple-Response consists only of the entity body and is terminated by the server closing the connection. 6.1 Status-Line The first line of a Full-Response message is the Status-Line, consisting of the protocol version followed by a numeric status code and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence. Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF Since a status line always begins with the protocol version's "HTTP/" 1*DIGIT "." 1*DIGIT (e.g. "HTTP/1.0"), the presence of that expression is considered sufficient to differentiate a Full-Response from a Simple-Response. Although the Simple-Response format may allow such an expression to occur at the beginning of an entity body (and thus cause a misinterpretation of the message if it was given in response to a Full-Request), the likelihood of such an occurrence is negligible. 6.2 Status Codes and Reason Phrases The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. 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 - Not used, but reserved for future use 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/1.0, 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 = "200" ; OK | "201" ; Created | "202" ; Accepted | "203" ; Provisional Information | "204" ; No Content | "300" ; Multiple Choices | "301" ; Moved Permanently | "302" ; Moved Temporarily | "303" ; Method | "304" ; Not Modified | "400" ; Bad Request | "401" ; Unauthorized | "402" ; Payment Required | "403" ; Forbidden | "404" ; Not Found | "405" ; Method Not Allowed | "406" ; None Acceptable | "407" ; Proxy Authentication Required | "408" ; Request Timeout | "409" ; Conflict | "410" ; Gone | "500" ; Internal Server Error | "501" ; Not Implemented | "502" ; Bad Gateway | "503" ; Service Unavailable | "504" ; Gateway Timeout | extension-code extension-code = 3DIGIT Reason-Phrase = token *( SP token ) HTTP status codes are extensible and should be registered with the IANA. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications are required to understand the class of any status code (as indicated by the first digit) and to treat the response as being equivalent to the x00 status code of that class. For example, if an unknown status code of 421 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 are encouraged to present the entity returned with the response to the user, since that entity is likely to include human-readable information which will explain the unusual status. Each Status-Code is described below, including a description of which method(s) it can follow and any metainformation required in the response. 6.2.1 Successful 2xx This class of status codes indicates that the client's request was successfully received, understood, and accepted. 200 OK o Following: any method o Required headers: none The request has been fulfilled and an entity corresponding to the requested resource is being sent in the response. If the HEAD method was used, the response should only contain the Entity-Header information and no Entity-Body. 201 Created o Following: any method that may request a new resource be created o Required headers: URI-header 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 URI-header field of the response. The origin server is encouraged, but not obliged, to actually create the resource before using this Status-Code. If the action cannot be carried out immediately, or within a clearly defined timeframe, the server should respond with "202 Accepted" instead. Of the methods defined by this specification, only PUT and POST can create a resource. 202 Accepted o Following: any method o Required headers: none 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 Accepted" 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 enacted. 203 Provisional Information o Following: GET, HEAD o Required headers: none 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. 204 No Content o Following: any method o Required headers: none 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. This response is primarily intended to allow input for scripts or other actions to take place without causing a change to the user agent's current document view. 6.2.2 Redirection 3xx This class of status code indicates that further action needs to be taken by the client in order to fulfill the request. The action required can sometimes be carried out by the client without interaction with the user, but it is strongly recommended that this only takes place if the method used in the request is idempotent (GET or HEAD). 300 Multiple Choices o Following: any method o Required headers: none The requested resource is available at one or more locations and a preferred location could not be determined via content negotiation. Unless it was a HEAD request, the response must include an entity containing a formatted list of resource characteristics and locations from which the 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. 301 Moved Permanently o Following: any method o Required headers: URI-header, Location The requested resource has been assigned a new permanent URI and any future references to this resource must be done using the returned URI. Clients with link editing capabilities are encouraged to automatically relink references to the Request-URI to the new reference returned by the server, where possible. It is possible for the server to send this status code in response to a request using the PUT, POST, or DELETE methods. However, as this might change the conditions under which the request was issued, the user agent must not automatically redirect the request unless it can be confirmed by the user. 302 Moved Temporarily o Following: any method o Required headers: URI-header, Location The requested resource resides temporarily under a different URI. Since the redirection may be altered on occasion, the client should on future requests from the user continue to use the original Request-URI and not the URI returned in the URI-header field and Location fields. It is possible for the server to send this status code in response to a request using the PUT, POST, or DELETE methods. However, as this might change the conditions under which the request was issued, the user agent must not automatically redirect the request unless it can be confirmed by the user. 303 Method This code is obsolete. 304 Not Modified o Following: conditional GET o Required headers: none If the client has performed a conditional GET request and access is allowed, but the document has not been modified since the date and time specified in the If-Modified-Since field, the server shall respond with this status code and not send an Entity-Body to the client. Header fields contained in the response should only include information which is relevant to cache managers and which may have changed independently of the entity's Last-Modified date. Examples of relevant header fields include: Date, Server, and Expires. 6.2.3 Client Error 4xx The 4xx class of status codes is intended for cases in which the client seems to have erred. If the client has not completed the request when a 4xx code is received, it should immediately cease sending data to the server. Except when responding to a HEAD request, the server is encouraged to 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. 400 Bad Request The request had bad syntax or was inherently impossible to be satisfied. The client is discouraged from repeating the request without modifications. 401 Unauthorized o Required headers: WWW-Authenticate The request requires user authentication. The response must include a WWW-Authenticate header field (Section 6.3.4) containing a challenge applicable to the requested resource. The client may repeat the request with a suitable Authorization header field. HTTP access authentication is explained in Section 10. 402 Payment Required This code is not currently supported, but is reserved for future use. 403 Forbidden The request is forbidden because of some reason that remains unknown to the client. Authorization will not help and the request should not be repeated. This status code can be used if the server does not want to make public why the request cannot be fulfilled. 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. 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 method) that an old resource is permanently unavailable and has no forwarding address. 405 Method Not Allowed o Required headers: Allow 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 method's for the requested resource. 406 None Acceptable o Required headers: Content-*, where applicable to the Request-URI The server has found a resource matching the Request-URI, but not one that satisfies the conditions identified by the Accept and Accept-Encoding request headers. The response must include (when applicable) the Content-Type, Content-Encoding, and Content- Language of the resource, and is encouraged to include the resource's complete metainformation. No Entity-Body can be included in the response. 407 Proxy Authentication Required This code is reserved for future use. It is similar to "401 Unauthorized", but indicates that the client must first authenticate itself with the proxy. HTTP/1.0 does not provide a means for proxy authentication -- this feature will be available in future versions. 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. 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 may 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 Conflict" response to indicate that it can't complete the PUT. In this case, the response entity may contain a list of the differences between the two versions. 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 are encouraged to 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" can be used instead. The "410 Gone" 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. 6.2.4 Server Errors 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. If the client has not completed the request when a 5xx code is received, it should immediately cease sending data to the server. Except when responding to a HEAD request, the server is encouraged to include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These response codes are applicable to any request method and there are no required header fields. 500 Internal Server Error The server encountered an unexpected condition which prevented it from fulfilling the request. 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. 502 Bad Gateway The server received an invalid response from the gateway or upstream server it accessed in attempting to complete the request. 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 a "500 Internal Server Error". Note: The presence 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. 504 Gateway Timeout The server did not receive a timely response from the gateway or upstream server it accessed in attempting to complete the request. 6.3 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 are not intended to give information about an Entity-Body returned in the response, but about the server itself. Response-Header= Public | Retry-After | Server | WWW-Authenticate | extension-header Although additional response header fields may be implemented via the extension mechanism, applications which do not recognize those fields should treat them as Entity-Header fields. 6.3.1 Public The Public header field lists the set of non-standard 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 Request-URI; the Allow header field (Section 7.1.1) should be used to indicate methods allowed for a particular URI. This does not prevent a client from trying other methods. The field value should not include the methods predefined for HTTP/1.0 in Section 5.2. Public = "Public" ":" 1#method Example of use: Public: OPTIONS, MGET, MHEAD This header field applies only to the current connection. 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. 6.3.2 Retry-After The Retry-After header field can be used with "503 Service Unavailable" to indicate how long the service is expected to be unavailable to the requesting client. The value of this field can be either an full 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: Wed, 14 Dec 1994 18:22:54 GMT Retry-After: 120 In the latter example, the delay is 2 minutes. 6.3.3 Server The Server header field contains information about the software being used by the origin server program handling the request. The field is analogous to the User-Agent field and has the following format: Server = "Server" ":" 1*( product ) Example: Server: CERN/3.0 libwww/2.17 If the response is being forwarded through a proxy, the proxy application must not add its data to the product list. Instead, it should include a Forwarded field, as described in Section 4.3.2. 6.3.4 WWW-Authenticate The WWW-Authenticate header field must be included as part of a "401 Unauthorized" response. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the Request-URI. WWW-Authenticate = "WWW-Authenticate" ":" challenge The HTTP access authentication process is described in Section 10. 7. Entity Full-Request and Full-Response messages may transfer an entity within some requests and responses. An entity consists of Entity- Header fields and (usually) an Entity-Body. 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 This section specifies the format and semantics of the 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. Entity-Header = Allow | Content-Encoding | Content-Language | Content-Length | Content-Transfer-Encoding | Content-Type | Derived-From | Expires | Last-Modified | Link | Location | Title | URI-header | Version | extension-header extension-header = HTTP-header Each entity header field is explained in the subsections below. Other header fields are allowed but cannot be assumed to be recognizable by the recipient. Unknown header fields should be ignored by the recipient and forwarded by proxies. Note: It has been proposed that any HTML metainformation element (allowed within the element of a document) be a valid candidate for an HTTP entity header. This document defines two header fields, Link and Title, which are both examples of this. Base will be used in future versions of HTTP. 7.1.1 Allow The Allow 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; it must be present in a response with status code "405 Method Not Allowed". Allow = "Allow" ":" 1#method Example of use: Allow: GET, HEAD, PUT This does not prevent a client from trying other methods. However, it is recommended that the indications given by this field be followed. This field has no default value; if left undefined, the set of allowed methods is defined by the origin server at the time of each request. If a response passes through a proxy which does not understand one or more of the methods indicated in the Allow header, the proxy must not modify the Allow header; the user agent may have other means of communicating with the origin server. 7.1.2 Content-Encoding The Content-Encoding header field is used as a modifier to the media-type. When present, its value indicates what additional encoding mechanism has been applied to the resource, and thus what decoding mechanism must be applied in order to obtain the media- type referenced by the Content-Type header field. The 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" ":" encoding-mechanism Encoding mechanisms are defined in Section 8.4. An example of its use is Content-Encoding: gzip The Content-Encoding is a characteristic of the resource identified by the Request-URI. Typically, the resource is stored with this encoding and is only decoded before rendering or analogous usage. 7.1.3 Content-Language The Content-Language 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. Content-Language = "Content-Language" ":" 1#language-tag Language tags are defined in Section 8.2. The primary purpose of Content-Language is to allow a selective consumer to identify and differentiate resources according to the consumer's own preferred language. Thus, if the body content is intended only for a Danish audience, the appropriate field is Content-Language: dk If no Content-Language is specified, the default is that the content is intended for all language audiences. This may mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended. Multiple languages may be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for Content-Language: mi, en However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin," which is clearly intended to be used by an English audience. In this case, the Content-Language should only include "en". Content-Language may be applied to any media type -- it should not be considered limited to textual documents. 7.1.4 Content-Length The Content-Length header field indicates the size of the Entity- Body (in decimal number of octets) sent to the recipient or, in the case of the HEAD method, the size of the Entity-Body that would have been sent had the request been a GET. Content-Length = "Content-Length" ":" 1*DIGIT An example is Content-Length: 3495 Although it is not required, applications are strongly encouraged to use this field to indicate the size of the Entity-Body to be transferred, regardless of the media type of the entity. Any Content-Length greater than or equal to zero is a valid value. Section 7.2.2 describes how to determine the length of an Entity- Body if a Content-Length is not given. 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 used whenever the entity's length can be determined prior to being transferred. 7.1.5 Content-Transfer-Encoding The Content-Transfer-Encoding (CTE) header indicates what (if any) type of transformation has been applied to the entity in order to safely transfer it between the sender and the recipient. This differs from the Content-Encoding in that the CTE is a property of the message, not of the original resource. Content-Transfer-Encoding = "Content-Transfer-Encoding" ":" transfer-encoding Transfer encodings are defined in Section 8.5. Because all HTTP transactions take place on an 8-bit clean connection, the default Content-Transfer-Encoding for all messages is binary. However, HTTP may be used to transfer MIME messages which already have a defined CTE. An example is: Content-Transfer-Encoding: quoted-printable Many older HTTP/1.0 applications do not understand the Content- Transfer-Encoding header. However, since it may appear in any MIME message (i.e. entities retrieved via a gateway to a MIME-conformant protocol), future HTTP/1.0 applications are required to understand it upon receipt. Gateways are the only HTTP applications that would generate a CTE. 7.1.6 Content-Type The Content-Type 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 8.1. An example of the field is Content-Type: text/html; charset=ISO-8859-4 The Content-Type header field has no default value. Further discussion of methods for identifying the media type of an entity is provided in Section 7.2.1. 7.1.7 Derived-From The Derived-From 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" ":" version-tag An example use of the field is: Derived-From: 2.1.1 A longer example of version control is included in Appendix C. The Derived-From field is required for PUT requests if the entity being put was previously retrieved from the same URI and a Version header was included with the entity when it was last retrieved. 7.1.8 Expires The Expires field gives the date/time after which the entity should be considered stale. This allows information providers to suggest the volatility of the resource. Caching clients (including proxies) must not cache this copy of the resource beyond the date given, unless its status has been updated by a later check of the origin server. 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. However, information providers that know (or even suspect) that a resource will change by a certain date are strongly encouraged to include an Expires header with that date. The format is an absolute date and time as defined by HTTP-date in Section 3.3. Expires = "Expires" ":" HTTP-date An example of its use is Expires: Thu, 01 Dec 1994 16:00:00 GMT The Expires field has no default value. If the date given is equal to or earlier than the value of the Date header, the recipient must not cache the enclosed entity. If a resource is dynamic by nature, as is the case with many data-producing processes, copies of that resource should be given an appropriate Expires value which reflects that dynamism. The Expires field 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. Note: Applications are encouraged to be tolerant of bad or misinformed implementations of the Expires header. In particular, recipients may wish to recognize a delta-seconds value (any decimal integer) as representing the number of seconds after receipt of the message that its contents should be considered expired. Likewise, a value of zero (0) or an invalid date format may be considered equivalent to an "expires immediately." Although these values are not legitimate for HTTP/1.0, a robust implementation is always desirable. 7.1.9 Last-Modified The Last-Modified header field indicates the date and time at which the sender believes the resource was last modified. The exact semantics of this field are defined in terms of how the receiver should interpret it: if the receiver has a copy of this resource which is older than the date given by the Last-Modified field, that copy should be considered stale. 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 sender and the nature of the original resource. For files, it may be just the file system last-mod date. For entities with dynamically included parts, it may be the most recent of the set of last-mod dates of its component parts. For database gateways, it may be the last-update timestamp of the record. For virtual objects, it may be the last time the internal state changed. In any case, the recipient should only know (and care) about the result -- whatever gets stuck in the Last-Modified field -- and not worry about how that result was obtained. 7.1.10 Link The Link header provides a means for describing a relationship between the entity 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 element in HTML [4]. Link = "Link" ":" 1#("<" URI ">" [ ";" "rel" "=" relation ] [ ";" "rev" "=" relation ] [ ";" "title" "=" quoted-string ] ) relation = sgml-name sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" ) Relation values are not case-sensitive and may be extended within the constraints of the sgmlname syntax. There are no predefined link relationship values for HTTP/1.0. 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. Examples of usage include: Link: ; rel="Previous" Link: ; rev="Made"; title="Tim Berners-Lee" The first example indicates that the entity is previous to chapter2 in a logical navigation path. The second indicates that the publisher of the resource is identified by the given e-mail address. 7.1.11 Location The Location header field is an earlier form of the URI-header and is considered obsolete. However, HTTP/1.0 applications should continue to support the Location header in order to properly interface with older applications. The purpose of Location is identical to that of the URI-header, except that no variants can be specified and only one absolute location URL is allowed. Location = "Location" ":" URI An example is Location: http://info.cern.ch/hypertext/WWW/NewLocation.html 7.1.12 Title The Title header field indicates the title of the entity Title = "Title" ":" *text An example of the field is Title: Hypertext Transfer Protocol -- HTTP/1.0 This field is to be considered isomorphic with the element in HTML [4]. 7.1.13 URI The URI-header field may contain one or more Universal Resource Identifiers (URIs) by which the resource origin of the entity can be identified. There is no guarantee that the resource can be accessed using the URI(s) specified. This field is required for the 201, 301, and 302 response messages, and may be included in any message that contains resource metainformation. URI-header = "URI" ":" 1#( "<" URI ">" [ ";" vary ] ) vary = "vary" "=" <"> 1#vary-dimension <"> vary-dimension = "type" | "language" | "version" | "encoding" | "charset" | "user-agent" | extension-vary extension-vary = token Any URI specified in this field can be either absolute or relative to the URI given in the Request-Line. The URI-header improves upon the Location header field described in Section 7.1.11. For backwards compatibility with older clients, servers are encouraged to include both header fields in 301 and 302 responses. The URI-header may also be used by a client performing a POST request to suggest a URI for the new entity. Whether or not the suggested URI is used is entirely up to the server to decide. In any case, the server's response must include the actual URI(s) of the new resource if one is successfully created (status 201). If a URI refers to a set of variants, then the dimensions of that variance must be given with a vary parameter. One example is: URI: <http://info.cern.ch/hypertext/WWW/TheProject.multi>; vary="type,language" which indicates that the URI covers a group of entities that vary in media type and natural language. A request for that URI will result in a response that depends upon the client's request headers for Accept and Accept-Language. Similar dimensions exist for the Accept-Encoding, Accept-Charset, Version, and User-Agent header fields, as demonstrated in the following example. URI: <TheProject.ps>;vary="encoding,version", <TheProject.html>;vary="user-agent,charset,version" The vary parameter has an important effect on cache management, particularly for caching proxies which service a diverse set of user agents. Since the response to one user agent may differ from the response to a second user agent if the two agents have differing request profiles, a caching proxy must keep track of the content metainformation for resources with varying dimensions. Thus, the vary parameter tells the caching proxy what entity headers must be part of the key for caching that URI. When the caching proxy gets a request for that URI, it must forward the request toward the origin server if the request profile includes a variant dimension that has not already been cached. 7.1.14 Version The Version field defines the version tag associated with a rendition of an evolving entity. Together with the Derived-From field described in Section 7.1.7, 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. It should not be used to indicate derived works or renditions in different representations. Version = "Version" ":" version-tag version-tag = token | quoted-string Examples of the Version field include: Version: 2.1.2 Version: "Fred 19950116-12:26:48" Version: 2.5a4-omega7 The version tag should be considered opaque to all parties but the origin server. A user agent can request a particular version of an entity by including its tag in a Version header as part of the request. Similarly, a user agent may suggest a value for the version of an entity transferred via a PUT or POST request. However, only the origin server can reliably assign or increment the version tag of an entity. 7.2 Entity Body The entity-body (if any) sent with an HTTP/1.0 request or response is in a format and encoding defined by the Entity-Header fields. Entity-Body = *OCTET An entity-body is included with a request message only when the request method calls for one. This specification defines two request methods, "POST" and "PUT", that allow an entity-body. In general, the presence of an entity-body in a request is signaled by the inclusion of a Content-Length and/or Content-Transfer-Encoding header field in the request message headers. Note: Most current implementations of the POST and PUT methods require a valid Content-Length header field. This can cause problems for some systems that do not know the size of the entity-body before transmission. Experimental implementations (and future versions of HTTP) may use a packetized Content-Transfer-Encoding to obviate the need for a Content-Length. For response messages, whether or not an entity-body is included with a message is dependent on both the request method and the response code. All responses to the HEAD request method must not include a body, even though the presence of Content header fields may lead one to believe they should. Similarly, the responses "204 No Content", "304 Not Modified", and "406 None Acceptable" must not include a body. 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, Content-Encoding, and Content-Transfer-Encoding. These define a three-layer, ordered encoding model: entity-body <- Content-Transfer-Encoding( Content-Encoding( Content-Type ) ) The default for both encodings is none (i.e., the identity function). A Content-Type specifies the media type of the underlying data. A Content-Encoding may be used to indicate an additional encoding mechanism applied to the type (usually for the purpose of data compression) that is a property of the resource requested. A Content-Transfer-Encoding may be applied by a transport agent to ensure safe and/or proper transfer of the message. Note that the Content-Transfer-Encoding is a property of the message, not of the resource. The Content-Type header field has no default value. If and only if the media type is not given by a Content-Type header (as is always the case for Simple-Response messages), the receiver may attempt to guess the media type via inspection of its content and/or the name extension(s) of the URL used to access the resource. If the media type remains unknown, the receiver should treat it as type "application/octet-stream". 7.2.2 Length When an Entity-Body is included with a message, the length of that body may be determined in one of several ways. If a Content-Length header field is present, its value in bytes (number of octets) represents the length of the Entity-Body. Otherwise, the body length is determined by the Content-Type (for types with an explicit end-of-body delimiter), the Content-Transfer-Encoding (for packetized encodings), or the closing of the connection by the server. Note that the latter cannot be used to indicate the end of a request body, since it leaves no possibility for the server to send back a response. Note: Some older servers supply an invalid Content-Length when sending a document that contains server-side includes dynamically inserted into the data stream. It must be emphasized that this will not be tolerated by future versions of HTTP. Unless the client knows that it is receiving a response from a compliant server, it should not depend on the Content-Length value being correct. 8. Content Parameters 8.1 Media Types HTTP uses Internet Media Types [15], formerly referred to as MIME Content-Types [6], in order to provide open and extensible data typing and type negotiation. For mail applications, where there is no type negotiation between sender and receiver, it is reasonable to put strict limits on the set of allowed media types. With HTTP, however, user agents can identify acceptable media types as part of the connection, and thus are allowed more freedom in the use of non- registered types. The following grammar for media types is a superset of that for MIME because it does not restrict itself to the official IANA and x-token types. media-type = type "/" subtype *( ";" parameter ) type = token subtype = token Parameters may follow the type/subtype in the form of attribute/value pairs. parameter = attribute "=" value attribute = token value = token | quoted-string The type, subtype, and parameter attribute names are not case- sensitive. Parameter values may or may not be case-sensitive, depending on the semantics of the parameter name. No LWS is allowed between the type and subtype, nor between an attribute and its value. If a given media-type value has been registered by the IANA, any use of that value must be indicative of the registered data format. Although HTTP allows the use of non-registered media types, such usage must not conflict with the IANA registry. Data providers are strongly encouraged to register their media types with IANA via the procedures outlined in RFC 1590 [15]. All media-type's registered by IANA must be preferred over extension tokens. However, HTTP does not limit conforming applications to the use of officially registered media types, nor does it encourage the use of an "x-" prefix for unofficial types outside of explicitly short experimental use between consenting applications. 8.1.1 Canonicalization and Text Defaults Media types are registered in a canonical form. In general, entity bodies transferred via HTTP must be represented in the appropriate canonical form prior to transmission. If the body has been encoded via a Content-Encoding and/or Content-Transfer-Encoding, the data must be in canonical form prior to that encoding. However, HTTP modifies the canonical form requirements for media of primary type "text" and for "application" types consisting of text-like records. HTTP redefines the canonical form of text media to allow multiple octet sequences to indicate a text line break. In addition to the preferred form of CRLF, HTTP applications must accept a bare CR or LF alone as representing a single line break in text media. Furthermore, if the text media is represented in a character set which 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 sequence(s) is defined by that character set to represent the equivalent of CRLF, bare CR, and bare LF. It is assumed that any recipient capable of using such a character set will know the appropriate octet sequence for representing line breaks within that character set. Note: This interpretation of line breaks applies only to the contents of an Entity-Body and only after any Content- Transfer-Encoding and/or Content-Encoding has been removed. All other HTTP constructs use CRLF exclusively to indicate a line break. Encoding mechanisms define their own line break requirements. A recipient of an HTTP text entity should translate the received entity line breaks to the local line break conventions before saving the entity external to the application and its cache; whether this translation takes place immediately upon receipt of the entity, or only when prompted by the user, is entirely up to the individual application. HTTP also redefines the default character set for text media in an entity body. If a textual media type defines a charset parameter with a registered default value of "US-ASCII", HTTP changes the default to be "ISO-8859-1". Since the character set ISO-8859-1 [19] is a superset of USASCII [18], this has no effect upon the interpretation of entity bodies which only contain octets within the US-ASCII set (0 - 127). The presence of a charset parameter value in a Content-Type header field overrides the default. HTTP does not require that the character set of an entity body be labelled as the lowest common denominator of the character codes used within a document. 8.1.2 Multipart Types MIME provides for a number of "multipart" types -- encapsulations of several entities within a single message's Entity-Body. The multipart types registered by IANA [17] do not have any special meaning for HTTP/1.0, though user agents may need to understand each type in order to correctly interpret the purpose of each body- part. Ideally, an HTTP user agent should follow the same or similar behavior as a MIME user agent does upon receipt of a multipart type. As in MIME [6], all multipart types share a common syntax 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, multipart body-parts may contain HTTP header fields which are significant to the meaning of that part. A URI-header field (Section 7.1.13) should be included in the body- part for each enclosed entity that can be identified by a URI. 8.2 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. The HTTP/1.0 protocol uses language tags within the Accept-Language and Content-Language header fields. The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or more parts: A primary language tag and a (possibly empty) series of subtags: language-tag = primary-tag *( "-" subtag ) primary-tag = 1*8ALPHA subtag = 1*8ALPHA Whitespace is not allowed within the tag and all tags are to be treated as case insensitive. The namespace 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. Note: Earlier versions of this document specified an incomplete language tag, where values were limited to ISO 639 language abbreviations with an optional ISO 3166 country code appended after an underscore ("_") or slash ("/") character. This format was abandoned in favor of the recently proposed standard for Internet protocols. 8.3 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. Character sets are identified by case-insensitive tokens. The complete set of allowed charset values are defined by the IANA Character Set registry [17]. However, because that registry does not define a single, consistent token for each character set, we define here the preferred names for those character sets most likely to be used with HTTP entities. This set of charset values includes those registered by RFC 1521 [6] -- the US-ASCII [18] and ISO8859 [19] character sets -- and other character set names specifically recommended for use within MIME charset parameters. charset = "US-ASCII" | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3" | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6" | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9" | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR" | "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8" | token Although HTTP allows an arbitrary token to be used as a character set value, any token that has a predefined value within the IANA Character Set registry [17] must represent the character set defined by that registry. Applications are encouraged, but not required, to limit their use of character sets to those defined by the IANA registry. 8.4 Encoding Mechanisms Encoding mechanism values are used to indicate an encoding transformation that has been or can be applied to a resource. Encoding mechanisms are primarily used to allow a document to be compressed or encrypted without losing the identity of its underlying media type. Typically, the resource is stored with this encoding and is only decoded before rendering or analogous usage. encoding-mechanism = "gzip" | "compress" | token Note: For historical reasons, HTTP/1.0 applications should consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress", respectively. All encoding-mechanism values are case-insensitive. HTTP/1.0 uses encoding-mechanism values in the Accept-Encoding (Section 5.4.3) and Content-Encoding (Section 7.1.2) header fields. Although the value describes the encoding-mechanism, what is more important is that it indicates what decoding mechanism will be required to remove the encoding. Note that a single program may be capable of decoding multiple encoding-mechanism formats. Two values are defined by this specification: gzip An encoding format produced by the file compression program "gzip" (GNU zip) developed by Jean-loup Gailly. This format is typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC. Gzip is available from the GNU project at <URL:ftp://prep.ai.mit.edu/pub/gnu/>. compress The encoding format produced by the file compression program "compress". This format is an adaptive Lempel-Ziv- Welch coding (LZW). 8.5 Transfer Encodings Transfer encoding 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. Transfer encodings are only used with entities destined for or retrieved from MIME-conformant systems, and thus will rarely occur in an HTTP/1.0 message. This differs from an encoding- mechanism in that the transfer encoding is a property of the message, not of the original resource. transfer-encoding = "binary" | "8bit" | "7bit" | "quoted-printable" | "base64" | token All transfer-encoding values are case-insensitive. HTTP/1.0 uses transfer-encoding values in the Accept-Encoding (Section 5.4.3) and Content-Transfer-Encoding (Section 7.1.5) header fields. The values "7bit", "8bit", and "binary" are used to indicate that no transfer encoding has been performed. Instead, they describe the sort of encoding that might be needed for transmission through an unsafe transport system. Binary indicates that the body may contain any set of octets. 8bit adds the restrictions that CR and LF characters only occur as part of CRLF line separators, all lines are short (less than 1000 octets), and no NULs (octet 0) are present. 7bit adds a further restriction that all octets are 7-bit US-ASCII characters. The "quoted-printable" and "base64" values indicate that the associated encoding (as defined in MIME [6]) has been applied to the body. These encodings consist entirely of 7-bit US-ASCII characters. 9. Content Negotiation Content negotiation is an optional feature of the HTTP protocol. It is designed to allow for preemptive selection of a preferred content representation, within a single request-response round-trip, and without intervention from the user. However, this may not always be desirable for the user and is sometimes unnecessary for the content provider. Implementors are encouraged to provide mechanisms whereby the amount of preemptive content negotiation, and the parameters of that negotiation, are configurable by the user and server maintainer. The first step in the negotiation algorithm is for the server to determine whether or not there are any content variants for the requested resource. Content variants may be in the form of multiple preexisting entities or a set of dynamic conversion filters. If there are no variant forms of the resource, the "negotiation" is limited to whether or not that single media type is acceptable under the constraints given by the Accept request header field (if any). If variants are available, those variants that are completely unacceptable should be removed from consideration first. Unacceptable variants include those with a Content-Encoding not listed in an Accept-Encoding field, those with a character subset (other than the default ISO-8859-1) not listed in an Accept-Charset field, and those with a media type not within any of the media ranges of an explicitly constrained Accept field (or listed with a zero quality parameter). If no acceptable variants remain at this point, the server should respond with a "406 None Acceptable" response message. If more than one variant remains, and at least one has a Content-Language within those listed by an Accept-Language field, any variants which do not match the language constraint are removed from further consideration. If multiple choices still remain, the selection is further winnowed by calculating and comparing the relative quality of the available media types. The calculated weights are normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. The following parameters are included in the calculation: q The quality factor chosen by the user agent (and configurable by the user) that represents the desirability of that media type. In this case, desirability is usually a measure of the clients ability to faithfully represent the contents of that media type to the user. The value is in the range [0,1], where the default value is 1. qs The quality factor, as chosen by the server (via some unspecified mechanism), to represent the relative quality of a particular variant representation of the source. For example, a picture originally in JPEG form would have a lower qs when translated to the XBM format, and much lower qs when translated to an ASCII-art representation. Note, however, that this is a function of the source -- an original piece of ASCII-art may degrade in quality if it is captured in JPEG form. The qs value has the same range and default as q. mxb The maximum number of bytes in the Entity-Body accepted by the client. The default value is mxb=undefined (i.e. infinity). bs The actual number of bytes in the Entity-Body for a particular source representation. This should equal the value sent as Content-Length. The default value is 0. The discrete mapping function is defined as: { if mxb=undefined, then (qs * q) } Q(q,qs,mxb,bs) = { if mxb >= bs, then (qs * q) } { if mxb < bs, then 0 } The variants with a maximal value for the Q function represent the preferred representation(s) of the entity. If multiple representations exist for a single media type (as would be the case if they only varied by Content-Encoding), then the smallest representation (lowest bs) is preferred. Finally, there may still be multiple choices available to the user. If so, the server may either choose one (possibly at random) from those available and respond with "200 OK", or it may respond with "300 Multiple Choices" and include an entity describing the choices. In the latter case, the entity should either be of type "text/html' (such that the user can choose from among the choices by following an exact link) or of some type that would allow the user agent to perform the selection automatically (no such type is available at the time of this writing). The "300 Multiple Choices" response can be given even if the server does not perform any winnowing of the representation choices via the content negotiation algorithm described above. Furthermore, it may include choices that were not considered as part of the negotiation algorithm and resources that may be located at other servers. 10. Access Authentication HTTP provides a simple challenge-response authorization mechanism which may be used by a server to challenge a client request and by a client to provide authentication information. The mechanism uses an extensible 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 = "Basic" | 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 the challenge applicable to the requested resource. challenge = auth-scheme 1*LWS realm [ "," 1#auth-param ] realm = "Realm" "=" quoted-string The realm attribute is required for all access authentication schemes which issue a challenge. The realm value, in combination with the root URL of the server being accessed, defines the authorization space. These realms allow the protected resources on a server to be partitioned into a set of authorization spaces, each with its own authentication scheme and/or 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 response), may do so by including an Authorization 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. credentials = auth-scheme [ 1*LWS encoded-cookie ] #auth-param encoded-cookie = 1*<any CHAR except CTLs or tspecials, but including "=" and "/"> The domain over which credentials can be automatically applied by a user agent is determined by the authorization space. If a request is authenticated, the credentials can be reused for all other requests within that authorization space for a period of time determined by the authentication scheme, parameters, and/or user preference. The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms may be used at the transport level, via message encapsulation, and/or 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 access authentication. That is, they must forward the WWW-Authenticate and Authorization headers untouched. HTTP/1.0 does not provide a means for a client to be authenticated with a proxy -- this feature will be available in future versions of HTTP. Note: The names Proxy-Authenticate and Proxy-Authorization have been suggested as headers, analogous to WWW-Authenticate and Authorization, but applying only to the immediate connection with a proxy. 10.1 Basic Authentication Scheme The basic authentication scheme is based on the model that the client 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. The server will service the request only if it can validate the user-ID and password for the domain of the Request-URI. basic-challenge = "Basic" SP realm The client sends the user-ID and password (separated by a single colon ":" character) within a base64 [6] encoded-cookie in the credentials. basic-credentials = "Basic" SP basic-cookie basic-cookie = <base64 encoding of userid-password> userid-password = [ token ] ":" *text There are no optional authentication parameters for the basic scheme. For example, if the user agent wishes to send the user-ID "Aladdin" and password "open sesame", it would use the following header field: Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== The basic authentication scheme is a non-secure method of filtering unauthorized access to resources on an HTTP server. It is based on the assumption that the connection between the client and the server can be regarded as a trusted carrier. As this is not generally true on an open network, the basic authentication scheme should be used accordingly. In spite of this, clients are strongly encouraged to implement the scheme in order to communicate with servers that use it. 11. Security Considerations This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.0 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. 11.1 Authentication of Clients As mentioned in Section 10.1, the Basic authentication scheme is not considered to be a secure method of user authentication, nor does it prevent the Entity-Body from being transmitted in clear text across the physical network used as the carrier. HTTP/1.0 does not prevent additional authentication schemes and encryption mechanisms to be employed to increase security. 11.2 Idempotent Methods The writers of client software should be aware that the software represents the user in their interactions over the net, 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. The link "click here to subscribe"--causing the reading of a special "magic" document--is open to abuse by others making a link "click here to see a pretty picture." These methods should be considered "safe" and should not have side effects. This allows the client software to represent other methods (such as POST, PUT and DELETE) in a special way, so that the user is aware of the fact that an action is being requested. 11.3 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. Two header fields are worth special mention in this context: Referer and From. 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 have indicated a secure document's URI, whose revelation itself would be a breach of security. 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 active 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. 12. Acknowledgments This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822 [8]. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME [6]. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP/1.0 and Internet mail. The HTTP protocol has evolved considerably over the past three 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, Phillip M. Hallam-Baker, Haringkon W. Lie, Ari Luotonen, Rob McCool, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining aspects of the protocol for early versions of this specification. This document has benefited greatly from the comments of all those participating in the HTTPWG. In addition to those already mentioned, the following individuals have contributed to this specification: Gary Adams Harald Tveit Alvestrand Keith Ball Brian Behlendorf Paul Burchard Maurizio Codogno Mike Cowlishaw Michael A. Dolan John Franks Alex Hopmann Bob Jernigan Martijn Koster Dave Kristol Daniel LaLiberte Albert Lunde John C. Mallery Larry Masinter Mitra Gavin Nicol Marc Salomon Chuck Shotton Eric W. Sink Simon E. Spero 13. References [1] H. T. Alvestrand. "Tags for the identification of languages." RFC 1766, UNINETT, March 1995. [2] F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, and B. Alberti. "The Internet Gopher Protocol: A distributed document search and retrieval protocol." RFC 1436, University of Minnesota, March 1993. [3] T. Berners-Lee. "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] T. Berners-Lee and D. Connolly. "HyperText Markup Language Specification - 2.0." Work in Progress (draft-ietf-html-spec- 01.txt), CERN, HaL Computer Systems, February 1995. [5] T. Berners-Lee, L. Masinter, and M. McCahill. "Uniform Resource Locators (URL)." RFC 1738, CERN, Xerox PARC, University of Minnesota, October 1994. [6] N. Borenstein and N. Freed. "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies." RFC 1521, Bellcore, Innosoft, September 1993. [7] R. Braden. "Requirements for Internet hosts - application and support." STD 3, RFC 1123, IETF, October 1989. [8] D. H. Crocker. "Standard for the Format of ARPA Internet Text Messages." STD 11, RFC 822, UDEL, August 1982. [9] F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J. Sui, and M. Grinbaum. "WAIS Interface Protocol Prototype Functional Specification." (v1.5), Thinking Machines Corporation, April 1990. [10] R. T. Fielding. "Relative Uniform Resource Locators." Work in Progress (draft-ietf-uri-relative-url-05.txt), UC Irvine, January 1995. [11] M. Horton and R. Adams. "Standard for interchange of USENET messages." RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Center for Seismic Studies, December 1987. [12] B. Kantor 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. [13] K. Moore. "MIME (Multipurpose Internet Mail Extensions) Part Two: Message Header Extensions for Non-ASCII Text." RFC 1522, University of Tennessee, September 1993. [14] J. Postel. "Simple Mail Transfer Protocol." STD 10, RFC 821, USC/ISI, August 1982. [15] J. Postel. "Media Type Registration Procedure." RFC 1590, USC/ISI, March 1994. [16] J. Postel and J. K. Reynolds. "File Transfer Protocol (FTP)." STD 9, RFC 959, USC/ISI, October 1985. [17] J. Reynolds and J. Postel. "Assigned Numbers." STD 2, RFC 1700, USC/ISI, October 1994. [18] US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986. [19] ISO-8859. International Standard -- Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990. 14. Authors' Addresses Tim Berners-Lee Director, W3 Consortium MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, U.S.A. Tel: +1 (617) 253 9670 Fax: +1 (617) 258 8682 Email: timbl@w3.org Roy T. Fielding Department of Information and Computer Science University of California Irvine, CA 92717-3425, U.S.A. Tel: +1 (714) 824-4049 Fax: +1 (714) 824-4056 Email: fielding@ics.uci.edu Henrik Frystyk Nielsen World-Wide Web Project CERN, 1211 Geneva 23, Switzerland Tel: +41 (22) 767 8265 Fax: +41 (22) 767 8730 Email: frystyk@w3.org Appendices These appendices are provided for informational reasons only -- they do not form a part of the HTTP/1.0 specification. A. Internet Media Type message/http In addition to defining the HTTP/1.0 protocol, this document serves as the specification for the Internet media type "message/http". The following is to be registered with IANA [15]. Media Type name: message Media subtype name: http Required parameters: none Optional parameters: version, type version: The HTTP-Version number of the enclosed message (e.g. "1.0"). If not present, the version can be determined from the first line of the body. type: 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 B. Minimum Compliance Early reviews of this specification have indicated the need for a statement of the minimum requirements for an implementation to be considered in compliance with HTTP/1.0. This section will be written soon. Note: The primary difficulty in determining a standard for minimum compliance rests in the fact that HTTP is a flexible protocol which can be used for many purposes. The requirements for special purpose applications often differ from those of general purpose applications. B.1 Servers Servers have a special responsibility for being honest when generating their responses to requesting clients. The Status-Code sent in the Status-Line must correspond to the actual action taken by the server. This is especially the case when the method used in the request is one of PUT, POST, DELETE, LINK, and UNLINK. If a Status-Code of 200 is returned, the client must be able to assume that the action has been carried out. If the server is not able to fulfill the requested action immediately, the correct status code to use is "202 Accepted". The methods GET and HEAD must be supported by all general-purpose servers. Servers which provide Last-Modified dates for resources must also support the conditional GET method. C. Tolerant Applications While it may be appropriate for testing applications to verify full conformance to this specification, it is recommended that operational applications be tolerant of deviations. This appendix mentions the most important topics where tolerance is recommended. C.1 Request-Line, Status-Line, and Header Fields 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 and HTAB characters between fields, even though only a single SP is specified. The line terminator for HTTP-header fields should be 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. We recommend that servers allocate URIs free of "variant" characters (characters whose representation differs in some of the national variant character sets), punctuation characters, and spaces. This makes URIs easier to handle by humans when the need arises (such as for debugging or transmission through non hypertext systems). D. Relationship to MIME HTTP/1.0 reuses many of the constructs defined for Internet Mail (RFC 822 [8]) and the Multipurpose Internet Mail Extensions (MIME [6]) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, HTTP is not a MIME-conforming application. HTTP's performance requirements differ substantially from those of Internet mail. Since it is not limited by the restrictions of existing mail protocols and gateways, HTTP does not obey some of the constraints imposed by RFC 822 and MIME for mail transport. This appendix describes specific areas where HTTP differs from MIME. Gateways to MIME-compliant protocols must be aware of these differences and provide the appropriate conversions where necessary. No conversion should be necessary for a MIME-conforming entity to be transferred using HTTP. D.1 Conversion to Canonical Form MIME requires that an entity be converted to canonical form prior to being transferred, as described in Appendix G of RFC 1521 [6]. Although HTTP does require media types to be transferred in canonical form, it changes the definition of "canonical form" for text-based media types as described in Section 8.1.1. D.1.1 Representation of Line Breaks MIME requires that the canonical form of any text type represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. Since HTTP allows CRLF, bare CR, and bare LF (or the octet sequence(s) to which they would be translated for the given character set) to indicate a line break within text content, recipients of an HTTP message cannot rely upon receiving MIME- canonical line breaks in text. Where it is possible, a gateway from HTTP to a MIME-conformant protocol should translate all line breaks within text/* media types to the MIME canonical form of CRLF. However, this may be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF (as is the case for some multi-byte character sets). D.1.2 Default Character Set MIME requires that all subtypes of the top-level Content-Type "text" have a default character set of US-ASCII [18]. In contrast, HTTP defines the default character set for "text" to be ISO88591 [19] (a superset of US-ASCII). Therefore, if a text/* media type given in the Content-Type header field does not already include an explicit charset parameter, the parameter ;charset="iso-8859-1" should be added by the gateway if the entity contains any octets greater than 127. D.2 Default Content-Transfer-Encoding The default Content-Transfer-Encoding (CTE) for all MIME messages is "7bit". In contrast, HTTP defines the default CTE to be "binary". Therefore, if an entity does not include an explicit CTE header field, the gateway should apply either the "quoted-printable" or "base64" transfer encodings and add the appropriate Content-Transfer-Encoding field. At a minimum, the explicit CTE field of Content-Transfer-Encoding: binary should be added by the gateway if it is unwilling to apply a mail- safe encoding. D.3 Introduction of Content-Encoding MIME does not include any concept equivalent to HTTP's Content- Encoding header field. Since this acts as a modifier on the media type, gateways to MIME-conformant protocols should either change the value of the Content-Type header field or decode the Entity- Body before forwarding the message. Note: Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<encoding-mechanisms>" to perform an equivalent function as Content-Encoding. However, this parameter is not part of the MIME specification at the time of this writing. E. Example of Version Control This appendix gives an example on how the Entity-Header fields Version and Derived-From can be used to apply version control to the creation and parallel development of a work. In order to simplify the example, only two user agents (A and B) are considered together with an origin server S. o A sends a POST request to S, including the header "Version: 1.0" and an entity o S replies "201 Created" to A, including the header "Version: 1.0" and a URI-header which should be used for future references o A starts editing the entity o B sends a GET request to S o S replies "200 OK" to B, including the entity with a header "Version: 1.0" o B starts editing the entity o B sends a PUT request to S, including the entity and a header "Derived-From: 1.0" o S replies "204 No Content" to B, including a header "Version: 1.1" but no entity o A sends a PUT request to S, including the entity and a header "Derived-From: 1.0" o S replies "409 Conflict" to A, including "Version: 1.1" and the list of problems with merging A's changes to 1.0 with those already applied for B and version 1.1 o A merges B's changes with its own, possibly with help from the user of A o A sends a PUT request to S including the entity and the header "Derived-From: 1.1" o S replies "204 No Content" to A, including the header "Version: 1.2" but no entity The example can be expanded to any number of involved user agents, though the likelihood of conflicts and the difficulty of resolving them may increase.