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rfc:rfc1945

Network Working Group T. Berners-Lee Request for Comments: 1945 MIT/LCS Category: Informational R. Fielding

                                                             UC Irvine
                                                            H. Frystyk
                                                               MIT/LCS
                                                              May 1996
              Hypertext Transfer Protocol -- HTTP/1.0

Status of This Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

IESG Note:

 The IESG has concerns about this protocol, and expects this document
 to be replaced relatively soon by a standards track document.

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 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 common usage of
 the protocol referred to as "HTTP/1.0".

Table of Contents

 1.  Introduction ..............................................  4
     1.1  Purpose ..............................................  4
     1.2  Terminology ..........................................  4
     1.3  Overall Operation ....................................  6
     1.4  HTTP and MIME ........................................  8
 2.  Notational Conventions and Generic Grammar ................  8
     2.1  Augmented BNF ........................................  8
     2.2  Basic Rules .......................................... 10
 3.  Protocol Parameters ....................................... 12

Berners-Lee, et al Informational [Page 1] RFC 1945 HTTP/1.0 May 1996

     3.1  HTTP Version ......................................... 12
     3.2  Uniform Resource Identifiers ......................... 14
          3.2.1  General Syntax ................................ 14
          3.2.2  http URL ...................................... 15
     3.3  Date/Time Formats .................................... 15
     3.4  Character Sets ....................................... 17
     3.5  Content Codings ...................................... 18
     3.6  Media Types .......................................... 19
          3.6.1  Canonicalization and Text Defaults ............ 19
          3.6.2  Multipart Types ............................... 20
     3.7  Product Tokens ....................................... 20
 4.  HTTP Message .............................................. 21
     4.1  Message Types ........................................ 21
     4.2  Message Headers ...................................... 22
     4.3  General Header Fields ................................ 23
 5.  Request ................................................... 23
     5.1  Request-Line ......................................... 23
          5.1.1  Method ........................................ 24
          5.1.2  Request-URI ................................... 24
     5.2  Request Header Fields ................................ 25
 6.  Response .................................................. 25
     6.1  Status-Line .......................................... 26
          6.1.1  Status Code and Reason Phrase ................. 26
     6.2  Response Header Fields ............................... 28
 7.  Entity .................................................... 28
     7.1  Entity Header Fields ................................. 29
     7.2  Entity Body .......................................... 29
          7.2.1  Type .......................................... 29
          7.2.2  Length ........................................ 30
 8.  Method Definitions ........................................ 30
     8.1  GET .................................................. 31
     8.2  HEAD ................................................. 31
     8.3  POST ................................................. 31
 9.  Status Code Definitions ................................... 32
     9.1  Informational 1xx .................................... 32
     9.2  Successful 2xx ....................................... 32
     9.3  Redirection 3xx ...................................... 34
     9.4  Client Error 4xx ..................................... 35
     9.5  Server Error 5xx ..................................... 37
 10. Header Field Definitions .................................. 37
     10.1  Allow ............................................... 38
     10.2  Authorization ....................................... 38
     10.3  Content-Encoding .................................... 39
     10.4  Content-Length ...................................... 39
     10.5  Content-Type ........................................ 40
     10.6  Date ................................................ 40
     10.7  Expires ............................................. 41
     10.8  From ................................................ 42

Berners-Lee, et al Informational [Page 2] RFC 1945 HTTP/1.0 May 1996

     10.9  If-Modified-Since ................................... 42
     10.10 Last-Modified ....................................... 43
     10.11 Location ............................................ 44
     10.12 Pragma .............................................. 44
     10.13 Referer ............................................. 44
     10.14 Server .............................................. 45
     10.15 User-Agent .......................................... 46
     10.16 WWW-Authenticate .................................... 46
 11. Access Authentication ..................................... 47
     11.1  Basic Authentication Scheme ......................... 48
 12. Security Considerations ................................... 49
     12.1  Authentication of Clients ........................... 49
     12.2  Safe Methods ........................................ 49
     12.3  Abuse of Server Log Information ..................... 50
     12.4  Transfer of Sensitive Information ................... 50
     12.5  Attacks Based On File and Path Names ................ 51
 13. Acknowledgments ........................................... 51
 14. References ................................................ 52
 15. Authors' Addresses ........................................ 54
 Appendix A.   Internet Media Type message/http ................ 55
 Appendix B.   Tolerant Applications ........................... 55
 Appendix C.   Relationship to MIME ............................ 56
     C.1  Conversion to Canonical Form ......................... 56
     C.2  Conversion of Date Formats ........................... 57
     C.3  Introduction of Content-Encoding ..................... 57
     C.4  No Content-Transfer-Encoding ......................... 57
     C.5  HTTP Header Fields in Multipart Body-Parts ........... 57
 Appendix D.   Additional Features ............................. 57
     D.1  Additional Request Methods ........................... 58
          D.1.1  PUT ........................................... 58
          D.1.2  DELETE ........................................ 58
          D.1.3  LINK .......................................... 58
          D.1.4  UNLINK ........................................ 58
     D.2  Additional Header Field Definitions .................. 58
          D.2.1  Accept ........................................ 58
          D.2.2  Accept-Charset ................................ 59
          D.2.3  Accept-Encoding ............................... 59
          D.2.4  Accept-Language ............................... 59
          D.2.5  Content-Language .............................. 59
          D.2.6  Link .......................................... 59
          D.2.7  MIME-Version .................................. 59
          D.2.8  Retry-After ................................... 60
          D.2.9  Title ......................................... 60
          D.2.10 URI ........................................... 60

Berners-Lee, et al Informational [Page 3] RFC 1945 HTTP/1.0 May 1996

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 common usage of the protocol referred too as
 "HTTP/1.0". This specification describes the features that seem to be
 consistently implemented in most HTTP/1.0 clients and servers. The
 specification is split into two sections. Those features of HTTP for
 which implementations are usually consistent are described in the
 main body of this document. Those features which have few or
 inconsistent implementations are listed in Appendix D.
 Practical information systems require more functionality than simple
 retrieval, including search, front-end update, and annotation. HTTP
 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 Uniform Resource Identifier (URI) [2], as a location
 (URL) [4] or name (URN) [16], 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 [7] and the Multipurpose Internet Mail
 Extensions (MIME) [5].
 HTTP is also used as a generic protocol for communication between
 user agents and proxies/gateways to other Internet protocols, such as
 SMTP [12], NNTP [11], FTP [14], Gopher [1], and WAIS [8], allowing
 basic hypermedia access to resources available from diverse
 applications and simplifying the implementation of user agents.

1.2 Terminology

 This specification uses a number of terms to refer to the roles
 played by participants in, and objects of, the HTTP communication.
 connection
     A transport layer virtual circuit established between two
     application programs for the purpose of communication.
 message
     The basic unit of HTTP communication, consisting of a structured
     sequence of octets matching the syntax defined in Section 4 and
     transmitted via the connection.

Berners-Lee, et al Informational [Page 4] RFC 1945 HTTP/1.0 May 1996

 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 (Section 3.2).
 entity
     A particular representation or rendition of a data resource, or
     reply from a service 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
     An application program that establishes connections for the
     purpose of sending requests.
 user agent
     The client which initiates a request. These are often browsers,
     editors, spiders (web-traversing robots), or other end user
     tools.
 server
     An application program that accepts connections in order to
     service requests by sending back responses.
 origin server
     The server on which a given resource resides or is to be created.
 proxy
     An intermediary program which acts as both a server and a client
     for the purpose of making requests on behalf of other clients.
     Requests are serviced internally or by passing them, with
     possible translation, on to other servers. A proxy must
     interpret and, if necessary, rewrite a request message before

Berners-Lee, et al Informational [Page 5] RFC 1945 HTTP/1.0 May 1996

     forwarding it. Proxies are often used as client-side portals
     through network firewalls and as helper applications for
     handling requests via protocols not implemented by the user
     agent.
 gateway
     A server which acts as an intermediary for some other server.
     Unlike a proxy, a gateway receives requests as if it were the
     origin server for the requested resource; the requesting client
     may not be aware that it is communicating with a gateway.
     Gateways are often used as server-side portals through network
     firewalls and as protocol translators for access to resources
     stored on non-HTTP systems.
 tunnel
     A tunnel is an intermediary program which is acting as a blind
     relay between two connections. Once active, a tunnel is not
     considered a party to the HTTP communication, though the tunnel
     may have been initiated by an HTTP request. The tunnel ceases to
     exist when both ends of the relayed connections are closed.
     Tunnels are used when a portal is necessary and the intermediary
     cannot, or should not, interpret the relayed communication.
 cache
     A program's local store of response messages and the subsystem
     that controls its message storage, retrieval, and deletion. A
     cache stores cachable responses in order to reduce the response
     time and network bandwidth consumption on future, equivalent
     requests. Any client or server may include a cache, though a
     cache cannot be used by a server while it is acting as a tunnel.
 Any given program may be capable of being both a client and a server;
 our use of these terms refers only to the role being performed by the
 program for a particular connection, rather than to the program's
 capabilities in general. Likewise, any server may act as an origin
 server, proxy, gateway, or tunnel, switching behavior based on the
 nature of each request.

1.3 Overall Operation

 The HTTP protocol is based on a request/response paradigm. A client
 establishes a connection with 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

Berners-Lee, et al Informational [Page 6] RFC 1945 HTTP/1.0 May 1996

 status line, including the message's protocol version and a success
 or error code, followed by a MIME-like message containing server
 information, entity metainformation, and possible body content.
 Most HTTP communication is initiated by a user agent and consists of
 a request to be applied to a resource on some origin server. In the
 simplest case, this may be accomplished via a single connection (v)
 between the user agent (UA) and the origin server (O).
        request chain ------------------------>
     UA -------------------v------------------- O
        <----------------------- response chain
 A more complicated situation occurs when one or more intermediaries
 are present in the request/response chain. There are three common
 forms of intermediary: proxy, gateway, and tunnel. A proxy is a
 forwarding agent, receiving requests for a URI in its absolute form,
 rewriting all or parts of the message, and forwarding the reformatted
 request toward the server identified by the URI. A gateway is a
 receiving agent, acting as a layer above some other server(s) and, if
 necessary, translating the requests to the underlying server's
 protocol. A tunnel acts as a relay point between two connections
 without changing the messages; tunnels are used when the
 communication needs to pass through an intermediary (such as a
 firewall) even when the intermediary cannot understand the contents
 of the messages.
        request chain -------------------------------------->
     UA -----v----- A -----v----- B -----v----- C -----v----- O
        <------------------------------------- response chain
 The figure above shows three intermediaries (A, B, and C) between the
 user agent and origin server. A request or response message that
 travels the whole chain must pass through four separate connections.
 This distinction is important because some HTTP communication options
 may apply only to the connection with the nearest, non-tunnel
 neighbor, only to the end-points of the chain, or to all connections
 along the chain. Although the diagram is linear, each participant may
 be engaged in multiple, simultaneous communications. For example, B
 may be receiving requests from many clients other than A, and/or
 forwarding requests to servers other than C, at the same time that it
 is handling A's request.
 Any party to the communication which is not acting as a tunnel may
 employ an internal cache for handling requests. The effect of a cache
 is that the request/response chain is shortened if one of the
 participants along the chain has a cached response applicable to that
 request. The following illustrates the resulting chain if B has a

Berners-Lee, et al Informational [Page 7] RFC 1945 HTTP/1.0 May 1996

 cached copy of an earlier response from O (via C) for a request which
 has not been cached by UA or A.
        request chain ---------->
     UA -----v----- A -----v----- B - - - - - - C - - - - - - O
        <--------- response chain
 Not all responses are cachable, and some requests may contain
 modifiers which place special requirements on cache behavior. Some
 HTTP/1.0 applications use heuristics to describe what is or is not a
 "cachable" response, but these rules are not standardized.
 On the Internet, HTTP communication generally takes place over TCP/IP
 connections. The default port is TCP 80 [15], but other ports can be
 used. This does not preclude HTTP from being implemented on top of
 any other protocol on the Internet, or on other networks. HTTP only
 presumes a reliable transport; any protocol that provides such
 guarantees can be used, and 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.
 Except for experimental applications, current practice requires that
 the connection be established by the client prior to each request and
 closed by the server after sending the response. Both clients and
 servers should be aware that either party may close the connection
 prematurely, due to user action, automated time-out, or program
 failure, and should handle such closing in a predictable fashion. In
 any case, the closing of the connection by either or both parties
 always terminates the current request, regardless of its status.

1.4 HTTP and MIME

 HTTP/1.0 uses many of the constructs defined for MIME, as defined in
 RFC 1521 [5]. Appendix C describes the ways in which the context of
 HTTP allows for different use of Internet Media Types than is
 typically found in Internet mail, and gives the rationale for those
 differences.

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 [7]. Implementors will need to be familiar with the
 notation in order to understand this specification. The augmented BNF
 includes the following constructs:

Berners-Lee, et al Informational [Page 8] RFC 1945 HTTP/1.0 May 1996

 name = definition
     The name of a rule is simply the name itself (without any
     enclosing "<" and ">") and is separated from its definition by
     the equal character "=". Whitespace is only significant in that
     indentation of continuation lines is used to indicate a rule
     definition that spans more than one line. Certain basic rules
     are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc.
     Angle brackets are used within definitions whenever their
     presence will facilitate discerning the use of rule names.
 "literal"
     Quotation marks surround literal text. Unless stated otherwise,
     the text is case-insensitive.
 rule1 | rule2
     Elements separated by a bar ("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 "<n>*<m>element" indicating at least <n> and at
     most <m> occurrences of element. Default values are 0 and
     infinity so that "*(element)" allows any number, including zero;
     "1*element" requires at least one; and "1*2element" allows one
     or two.
 [rule]
     Square brackets enclose optional elements; "[foo bar]" is
     equivalent to "*1(foo bar)".
 N rule
     Specific repetition: "<n>(element)" is equivalent to
     "<n>*<n>(element)"; that is, exactly <n> occurrences of
     (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a
     string of three alphabetic characters.

Berners-Lee, et al Informational [Page 9] RFC 1945 HTTP/1.0 May 1996

 #rule
     A construct "#" is defined, similar to "*", for defining lists
     of elements. The full form is "<n>#<m>element" indicating at
     least <n> and at most <m> elements, each separated by one or
     more commas (",") and optional linear whitespace (LWS). This
     makes the usual form of lists very easy; a rule such as
     "( *LWS element *( *LWS "," *LWS element ))" can be shown as
     "1#element". Wherever this construct is used, null elements are
     allowed, but do not contribute to the count of elements present.
     That is, "(element), , (element)" is permitted, but counts as
     only two elements. Therefore, where at least one element is
     required, at least one non-null element must be present. Default
     values are 0 and infinity so that "#(element)" allows any
     number, including zero; "1#element" requires at least one; and
     "1#2element" allows one or two.
 ; comment
     A semi-colon, set off some distance to the right of rule text,
     starts a comment that continues to the end of line. This is a
     simple way of including useful notes in parallel with the
     specifications.
 implied *LWS
     The grammar described by this specification is word-based.
     Except where noted otherwise, linear whitespace (LWS) can be
     included between any two adjacent words (token or
     quoted-string), and between adjacent tokens and delimiters
     (tspecials), without changing the interpretation of a field. At
     least one delimiter (tspecials) must exist between any two
     tokens, since they would otherwise be interpreted as a single
     token. 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 coded character set
 is defined by [17].
     OCTET          = <any 8-bit sequence of data>
     CHAR           = <any US-ASCII character (octets 0 - 127)>
     UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
     LOALPHA        = <any US-ASCII lowercase letter "a".."z">

Berners-Lee, et al Informational [Page 10] RFC 1945 HTTP/1.0 May 1996

     ALPHA          = UPALPHA | LOALPHA
     DIGIT          = <any US-ASCII digit "0".."9">
     CTL            = <any US-ASCII control character
                      (octets 0 - 31) and DEL (127)>
     CR             = <US-ASCII CR, carriage return (13)>
     LF             = <US-ASCII LF, linefeed (10)>
     SP             = <US-ASCII SP, space (32)>
     HT             = <US-ASCII HT, horizontal-tab (9)>
     <">            = <US-ASCII double-quote mark (34)>
 HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker
 for all protocol elements except the Entity-Body (see Appendix B for
 tolerant applications). The end-of-line marker within an Entity-Body
 is defined by its associated media type, as described in Section 3.6.
     CRLF           = CR LF
 HTTP/1.0 headers may be folded onto multiple lines if each
 continuation line begins with a space or horizontal tab. All linear
 whitespace, including folding, has the same semantics as SP.
     LWS            = [CRLF] 1*( SP | HT )
 However, folding of header lines is not expected by some
 applications, and should not be generated by HTTP/1.0 applications.
 The TEXT rule is only used for descriptive field contents and values
 that are not intended to be interpreted by the message parser. Words
 of *TEXT may contain octets from character sets other than US-ASCII.
     TEXT           = <any OCTET except CTLs,
                      but including LWS>
 Recipients of header field TEXT containing octets outside the US-
 ASCII character set may assume that they represent ISO-8859-1
 characters.
 Hexadecimal numeric characters are used in several protocol elements.
     HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                    | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
 Many HTTP/1.0 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

Berners-Lee, et al Informational [Page 11] RFC 1945 HTTP/1.0 May 1996

     token          = 1*<any CHAR except CTLs or tspecials>
     tspecials      = "(" | ")" | "<" | ">" | "@"
                    | "," | ";" | ":" | "\" | <">
                    | "/" | "[" | "]" | "?" | "="
                    | "{" | "}" | SP | HT
 Comments may be included in some HTTP header fields by surrounding
 the comment text with parentheses. Comments are only allowed in
 fields containing "comment" as part of their field value definition.
 In all other fields, parentheses are considered part of the field
 value.
     comment        = "(" *( ctext | comment ) ")"
     ctext          = <any TEXT excluding "(" and ")">
 A string of text is parsed as a single word if it is quoted using
 double-quote marks.
     quoted-string  = ( <"> *(qdtext) <"> )
     qdtext         = <any CHAR except <"> and CTLs,
                      but including LWS>
 Single-character quoting using the backslash ("\") character is not
 permitted in HTTP/1.0.

3. Protocol Parameters

3.1 HTTP Version

 HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
 of the protocol. The protocol versioning policy is intended to allow
 the sender to indicate the format of a message and its capacity for
 understanding further HTTP communication, rather than the features
 obtained via that communication. No change is made to the version
 number for the addition of message components which do not affect
 communication behavior or which only add to extensible field values.
 The <minor> number is incremented when the changes made to the
 protocol add features which do not change the general message parsing
 algorithm, but which may add to the message semantics and imply
 additional capabilities of the sender. The <major> number is
 incremented when the format of a message within the protocol is
 changed.
 The version of an HTTP message is indicated by an HTTP-Version field
 in the first line of the message. If the protocol version is not
 specified, the recipient must assume that the message is in the

Berners-Lee, et al Informational [Page 12] RFC 1945 HTTP/1.0 May 1996

 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 zeros 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/1.0 servers must:
    o recognize the format of the Request-Line for HTTP/0.9 and
      HTTP/1.0 requests;
    o understand any valid request in the format of HTTP/0.9 or
      HTTP/1.0;
    o respond appropriately with a message in the same protocol
      version used by the client.
 HTTP/1.0 clients must:
    o recognize the format of the Status-Line for HTTP/1.0 responses;
    o understand any valid response in the format of HTTP/0.9 or
      HTTP/1.0.
 Proxy and gateway applications must be careful in forwarding requests
 that are received in a format different than that of the
 application's native HTTP version. Since the protocol version
 indicates the protocol capability of the sender, a proxy/gateway must
 never send a message with a version indicator which is greater than
 its native version; if a higher version request is received, the
 proxy/gateway must either downgrade the request version or respond
 with an error. Requests with a version lower than that of the
 application's native format may be upgraded before being forwarded;
 the proxy/gateway's response to that request must follow the server
 requirements listed above.

Berners-Lee, et al Informational [Page 13] RFC 1945 HTTP/1.0 May 1996

3.2 Uniform Resource Identifiers

 URIs have been known by many names: WWW addresses, Universal Document
 Identifiers, Universal Resource Identifiers [2], and finally the
 combination of Uniform Resource Locators (URL) [4] and Names (URN)
 [16]. As far as HTTP is concerned, Uniform Resource Identifiers are
 simply formatted strings which identify--via name, location, or any
 other characteristic--a network resource.

3.2.1 General Syntax

 URIs in HTTP can be represented in absolute form or relative to some
 known base URI [9], depending upon the context of their use. The two
 forms are differentiated by the fact that absolute URIs always begin
 with a scheme name followed by a colon.
     URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]
     absoluteURI    = scheme ":" *( uchar | reserved )
     relativeURI    = net_path | abs_path | rel_path
     net_path       = "//" net_loc [ abs_path ]
     abs_path       = "/" rel_path
     rel_path       = [ path ] [ ";" params ] [ "?" query ]
     path           = fsegment *( "/" segment )
     fsegment       = 1*pchar
     segment        = *pchar
     params         = param *( ";" param )
     param          = *( pchar | "/" )
     scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
     net_loc        = *( pchar | ";" | "?" )
     query          = *( uchar | reserved )
     fragment       = *( uchar | reserved )
     pchar          = uchar | ":" | "@" | "&" | "=" | "+"
     uchar          = unreserved | escape
     unreserved     = ALPHA | DIGIT | safe | extra | national
     escape         = "%" HEX HEX
     reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
     extra          = "!" | "*" | "'" | "(" | ")" | ","
     safe           = "$" | "-" | "_" | "."
     unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
     national       = <any OCTET excluding ALPHA, DIGIT,

Berners-Lee, et al Informational [Page 14] RFC 1945 HTTP/1.0 May 1996

                      reserved, extra, safe, and unsafe>
 For definitive information on URL syntax and semantics, see RFC 1738
 [4] and RFC 1808 [9]. The BNF above includes national characters not
 allowed in valid URLs as specified by RFC 1738, since HTTP servers
 are not restricted in the set of unreserved characters allowed to
 represent the rel_path part of addresses, and HTTP proxies may
 receive requests for URIs not defined by RFC 1738.

3.2.2 http URL

 The "http" scheme is used to locate network resources via the HTTP
 protocol. This section defines the scheme-specific syntax and
 semantics for http URLs.
     http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]
     host           = <A legal Internet host domain name
                       or IP address (in dotted-decimal form),
                       as defined by Section 2.1 of RFC 1123>
     port           = *DIGIT
 If the port is empty or not given, port 80 is assumed. The semantics
 are that the identified resource is located at the server listening
 for TCP connections on that port of that host, and the Request-URI
 for the resource is abs_path. If the abs_path is not present in the
 URL, it must be given as "/" when used as a Request-URI (Section
 5.1.2).
    Note: Although the HTTP protocol is independent of the transport
    layer protocol, the http URL only identifies resources by their
    TCP location, and thus non-TCP resources must be identified by
    some other URI scheme.
 The canonical form for "http" URLs is obtained by converting any
 UPALPHA characters in host to their LOALPHA equivalent (hostnames are
 case-insensitive), eliding the [ ":" port ] if the port is 80, and
 replacing an empty abs_path with "/".

3.3 Date/Time Formats

 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

Berners-Lee, et al Informational [Page 15] RFC 1945 HTTP/1.0 May 1996

 The first format is preferred as an Internet standard and represents
 a fixed-length subset of that defined by RFC 1123 [6] (an update to
 RFC 822 [7]). The second format is in common use, but is based on the
 obsolete RFC 850 [10] date format and lacks a four-digit year.
 HTTP/1.0 clients and servers that parse the date value should accept
 all three formats, though they must never generate the third
 (asctime) format.
    Note: Recipients of date values are encouraged to 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 proxies/gateways to SMTP or NNTP.
 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"
     Note: HTTP requirements for the date/time stamp format apply
     only to their usage within the protocol stream. Clients and
     servers are not required to use these formats for user

Berners-Lee, et al Informational [Page 16] RFC 1945 HTTP/1.0 May 1996

     presentation, request logging, etc.

3.4 Character Sets

 HTTP uses the same definition of the term "character set" as that
 described for MIME:
    The term "character set" is used in this document to refer to a
    method used with one or more tables to convert a sequence of
    octets into a sequence of characters. Note that unconditional
    conversion in the other direction is not required, in that not all
    characters may be available in a given character set and a
    character set may provide more than one sequence of octets to
    represent a particular character. This definition is intended to
    allow various kinds of character encodings, from simple single-
    table mappings such as US-ASCII to complex table switching methods
    such as those that use ISO 2022's techniques. However, the
    definition associated with a MIME character set name must fully
    specify the mapping to be performed from octets to characters. In
    particular, use of external profiling information to determine the
    exact mapping is not permitted.
    Note: This use of the term "character set" is more commonly
    referred to as a "character encoding." However, since HTTP and
    MIME share the same registry, it is important that the terminology
    also be shared.
 HTTP character sets are identified by case-insensitive tokens. The
 complete set of tokens are defined by the IANA Character Set registry
 [15]. 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. These character sets include those registered by RFC 1521
 [5] -- the US-ASCII [17] and ISO-8859 [18] character sets -- and
 other 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 charset
 value, any token that has a predefined value within the IANA
 Character Set registry [15] must represent the character set defined

Berners-Lee, et al Informational [Page 17] RFC 1945 HTTP/1.0 May 1996

 by that registry. Applications should limit their use of character
 sets to those defined by the IANA registry.
 The character set of an entity body should be labelled as the lowest
 common denominator of the character codes used within that body, with
 the exception that no label is preferred over the labels US-ASCII or
 ISO-8859-1.

3.5 Content Codings

 Content coding values are used to indicate an encoding transformation
 that has been applied to a resource. Content codings 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 in this encoding and only decoded before rendering or
 analogous usage.
     content-coding = "x-gzip" | "x-compress" | token
     Note: For future compatibility, HTTP/1.0 applications should
     consider "gzip" and "compress" to be equivalent to "x-gzip"
     and "x-compress", respectively.
 All content-coding values are case-insensitive. HTTP/1.0 uses
 content-coding values in the Content-Encoding (Section 10.3) header
 field. Although the value describes the content-coding, what is more
 important is that it indicates what decoding mechanism will be
 required to remove the encoding. Note that a single program may be
 capable of decoding multiple content-coding formats. Two values are
 defined by this specification:
 x-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.
 x-compress
     The encoding format produced by the file compression program
     "compress". This format is an adaptive Lempel-Ziv-Welch coding
     (LZW).
     Note: Use of program names for the identification of
     encoding formats is not desirable and should be discouraged
     for future encodings. Their use here is representative of
     historical practice, not good design.

Berners-Lee, et al Informational [Page 18] RFC 1945 HTTP/1.0 May 1996

3.6 Media Types

 HTTP uses Internet Media Types [13] in the Content-Type header field
 (Section 10.5) in order to provide open and extensible data typing.
     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 case-
 insensitive. Parameter values may or may not be case-sensitive,
 depending on the semantics of the parameter name. LWS must not be
 generated between the type and subtype, nor between an attribute and
 its value. Upon receipt of a media type with an unrecognized
 parameter, a user agent should treat the media type as if the
 unrecognized parameter and its value were not present.
 Some older HTTP applications do not recognize media type parameters.
 HTTP/1.0 applications should only use media type parameters when they
 are necessary to define the content of a message.
 Media-type values are registered with the Internet Assigned Number
 Authority (IANA [15]). The media type registration process is
 outlined in RFC 1590 [13]. Use of non-registered media types is
 discouraged.

3.6.1 Canonicalization and Text Defaults

 Internet media types are registered with a canonical form. In
 general, an Entity-Body transferred via HTTP must be represented in
 the appropriate canonical form prior to its transmission. If the body
 has been encoded with a Content-Encoding, the underlying data should
 be in canonical form prior to being encoded.
 Media subtypes of the "text" type use CRLF as the text line break
 when in canonical form. However, HTTP allows the transport of text
 media with plain CR or LF alone representing a line break when used
 consistently within the Entity-Body. HTTP applications must accept
 CRLF, bare CR, and bare LF as being representative of a line break in
 text media received via HTTP.

Berners-Lee, et al Informational [Page 19] RFC 1945 HTTP/1.0 May 1996

 In addition, if the text media is represented in a character set that
 does not use octets 13 and 10 for CR and LF respectively, as is the
 case for some multi-byte character sets, HTTP allows the use of
 whatever octet sequences are defined by that character set to
 represent the equivalent of CR and LF for line breaks. This
 flexibility regarding line breaks applies only to text media in the
 Entity-Body; a bare CR or LF should not be substituted for CRLF
 within any of the HTTP control structures (such as header fields and
 multipart boundaries).
 The "charset" parameter is used with some media types to define the
 character set (Section 3.4) of the data. When no explicit charset
 parameter is provided by the sender, media subtypes of the "text"
 type are defined to have a default charset value of "ISO-8859-1" when
 received via HTTP. Data in character sets other than "ISO-8859-1" or
 its subsets must be labelled with an appropriate charset value in
 order to be consistently interpreted by the recipient.
    Note: Many current HTTP servers provide data using charsets other
    than "ISO-8859-1" without proper labelling. This situation reduces
    interoperability and is not recommended. To compensate for this,
    some HTTP user agents provide a configuration option to allow the
    user to change the default interpretation of the media type
    character set when no charset parameter is given.

3.6.2 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 [15] 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. An HTTP
 user agent should follow the same or similar behavior as a MIME user
 agent does upon receipt of a multipart type. HTTP servers should not
 assume that all HTTP clients are prepared to handle multipart types.
 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. Multipart body-parts may contain HTTP
 header fields which are significant to the meaning of that part.

3.7 Product Tokens

 Product tokens are used to allow communicating applications to
 identify themselves via a simple product token, with an optional
 slash and version designator. Most fields using product tokens also
 allow subproducts which form a significant part of the application to

Berners-Lee, et al Informational [Page 20] RFC 1945 HTTP/1.0 May 1996

 be listed, separated by whitespace. By convention, the products are
 listed in order of their significance for identifying the
 application.
     product         = token ["/" product-version]
     product-version = token
 Examples:
     User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     Server: Apache/0.8.4
 Product tokens should be short and to the point -- use of them for
 advertizing or other non-essential information is explicitly
 forbidden. Although any token character may appear in a product-
 version, this token should only be used for a version identifier
 (i.e., successive versions of the same product should only differ in
 the product-version portion of the product value).

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 [7] for transferring entities. Both messages may include optional
 header fields (also known as "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 5.2
                       | 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 6.2

Berners-Lee, et al Informational [Page 21] RFC 1945 HTTP/1.0 May 1996

                       | 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 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 5.2), Response-Header (Section 6.2), and
 Entity-Header (Section 7.1) fields, follow the same generic format as
 that given in Section 3.1 of RFC 822 [7]. Each header field consists
 of a name followed immediately by a colon (":"), a single space (SP)
 character, and the field value. Field names are case-insensitive.
 Header fields can be extended over multiple lines by preceding each
 extra line with at least one SP or HT, though this is not
 recommended.
     HTTP-header    = field-name ":" [ field-value ] CRLF
     field-name     = token
     field-value    = *( field-content | LWS )
     field-content  = <the OCTETs making up the field-value
                      and consisting of either *TEXT or combinations
                      of token, tspecials, and quoted-string>
 The order in which header fields are received is not significant.
 However, it is "good practice" to send General-Header fields first,
 followed by Request-Header or Response-Header fields prior to the
 Entity-Header fields.
 Multiple HTTP-header fields with the same field-name may be present
 in a message if and only if the entire field-value for that header
 field is defined as a comma-separated list [i.e., #(values)]. It must
 be possible to combine the multiple header fields into one "field-
 name: field-value" pair, without changing the semantics of the
 message, by appending each subsequent field-value to the first, each
 separated by a comma.

Berners-Lee, et al Informational [Page 22] RFC 1945 HTTP/1.0 May 1996

4.3 General Header Fields

 There are a few header fields which have general applicability for
 both request and response messages, but which do not apply to the
 entity being transferred. These headers apply only to the message
 being transmitted.
     General-Header = Date                     ; Section 10.6
                    | Pragma                   ; Section 10.12
 General header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields may be given the semantics of general
 header fields if all parties in the communication recognize them to
 be general header fields. Unrecognized header fields are treated as
 Entity-Header fields.

5. Request

 A request message from a client to a server includes, within the
 first line of that message, the method to be applied to the resource,
 the identifier of the resource, and the protocol version in use. 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 5.2
                       | 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

Berners-Lee, et al Informational [Page 23] RFC 1945 HTTP/1.0 May 1996

 Note that the difference between a Simple-Request and the Request-
 Line of a Full-Request is the presence of the HTTP-Version field and
 the availability of methods other than GET.

5.1.1 Method

 The Method token indicates the method to be performed on the resource
 identified by the Request-URI. The method is case-sensitive.
     Method         = "GET"                    ; Section 8.1
                    | "HEAD"                   ; Section 8.2
                    | "POST"                   ; Section 8.3
                    | extension-method
     extension-method = token
 The list of methods acceptable by a specific resource can change
 dynamically; the client is notified through the return code of the
 response if a method is not allowed on a resource. Servers should
 return the status code 501 (not implemented) if the method is
 unrecognized or not implemented.
 The methods commonly used by HTTP/1.0 applications are fully defined
 in Section 8.

5.1.2 Request-URI

 The Request-URI is a Uniform Resource Identifier (Section 3.2) and
 identifies the resource upon which to apply the request.
     Request-URI    = absoluteURI | abs_path
 The two options for Request-URI are dependent on the nature of the
 request.
 The absoluteURI form is only allowed when the request is being made
 to a proxy. The proxy is requested to forward the request and return
 the response. If the request is GET or HEAD and a prior response is
 cached, the proxy may use the cached message if it passes any
 restrictions in the Expires header field. Note that the proxy may
 forward the request on to another proxy or directly to the server
 specified by the absoluteURI. In order to avoid request loops, a
 proxy must be able to recognize all of its server names, including
 any aliases, local variations, and the numeric IP address. An example
 Request-Line would be:
     GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.0

Berners-Lee, et al Informational [Page 24] RFC 1945 HTTP/1.0 May 1996

 The most common form of Request-URI is that used to identify a
 resource on an origin server or gateway. In this case, only the
 absolute path of the URI is transmitted (see Section 3.2.1,
 abs_path). For example, a client wishing to retrieve the resource
 above directly from the origin server would create a TCP connection
 to port 80 of the host "www.w3.org" and send the line:
     GET /pub/WWW/TheProject.html HTTP/1.0
 followed by the remainder of the Full-Request. Note that the absolute
 path cannot be empty; if none is present in the original URI, it must
 be given as "/" (the server root).
 The Request-URI is transmitted as an encoded string, where some
 characters may be escaped using the "% HEX HEX" encoding defined by
 RFC 1738 [4]. The origin server must decode the Request-URI in order
 to properly interpret the request.

5.2 Request Header Fields

 The request header fields allow the client to pass additional
 information about the request, and about the client itself, to the
 server. These fields act as request modifiers, with semantics
 equivalent to the parameters on a programming language method
 (procedure) invocation.
     Request-Header = Authorization            ; Section 10.2
                    | From                     ; Section 10.8
                    | If-Modified-Since        ; Section 10.9
                    | Referer                  ; Section 10.13
                    | User-Agent               ; Section 10.15
 Request-Header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields may be given the semantics of request
 header fields if all parties in the communication recognize them to
 be request header fields. Unrecognized header fields are treated as
 Entity-Header fields.

6. Response

 After receiving and interpreting a request message, a server responds
 in the form of an HTTP response message.
     Response        = Simple-Response | Full-Response
     Simple-Response = [ Entity-Body ]

Berners-Lee, et al Informational [Page 25] RFC 1945 HTTP/1.0 May 1996

     Full-Response   = Status-Line             ; Section 6.1
                       *( General-Header       ; Section 4.3
                        | Response-Header      ; Section 6.2
                        | 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 and
 status code
     "HTTP/" 1*DIGIT "." 1*DIGIT SP 3DIGIT SP
 (e.g., "HTTP/1.0 200 "), the presence of that expression is
 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, most HTTP/0.9 servers are limited to responses of type
 "text/html" and therefore would never generate such a response.

6.1.1 Status Code and Reason Phrase

 The Status-Code element is a 3-digit integer result code of the
 attempt to understand and satisfy the request. 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.

Berners-Lee, et al Informational [Page 26] RFC 1945 HTTP/1.0 May 1996

 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. These codes are fully defined in Section 9.
     Status-Code    = "200"   ; OK
                    | "201"   ; Created
                    | "202"   ; Accepted
                    | "204"   ; No Content
                    | "301"   ; Moved Permanently
                    | "302"   ; Moved Temporarily
                    | "304"   ; Not Modified
                    | "400"   ; Bad Request
                    | "401"   ; Unauthorized
                    | "403"   ; Forbidden
                    | "404"   ; Not Found
                    | "500"   ; Internal Server Error
                    | "501"   ; Not Implemented
                    | "502"   ; Bad Gateway
                    | "503"   ; Service Unavailable
                    | extension-code
     extension-code = 3DIGIT
     Reason-Phrase  = *<TEXT, excluding CR, LF>
 HTTP status codes are extensible, but the above codes are the only
 ones generally recognized in current practice. HTTP applications are
 not required to understand the meaning of all registered status

Berners-Lee, et al Informational [Page 27] RFC 1945 HTTP/1.0 May 1996

 codes, though such understanding is obviously desirable. However,
 applications must understand the class of any status code, as
 indicated by the first digit, and treat any unrecognized response as
 being equivalent to the x00 status code of that class, with the
 exception that an unrecognized response must not be cached. For
 example, if an unrecognized status code of 431 is received by the
 client, it can safely assume that there was something wrong with its
 request and treat the response as if it had received a 400 status
 code. In such cases, user agents should present to the user the
 entity returned with the response, since that entity is likely to
 include human-readable information which will explain the unusual
 status.

6.2 Response Header Fields

 The response header fields allow the server to pass additional
 information about the response which cannot be placed in the Status-
 Line. These header fields give information about the server and about
 further access to the resource identified by the Request-URI.
     Response-Header = Location                ; Section 10.11
                     | Server                  ; Section 10.14
                     | WWW-Authenticate        ; Section 10.16
 Response-Header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields may be given the semantics of response
 header fields if all parties in the communication recognize them to
  be response header fields. Unrecognized header fields are treated as
 Entity-Header fields.

7. Entity

 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.

Berners-Lee, et al Informational [Page 28] RFC 1945 HTTP/1.0 May 1996

7.1 Entity Header Fields

 Entity-Header fields define optional metainformation about the
 Entity-Body or, if no body is present, about the resource identified
 by the request.
     Entity-Header  = Allow                    ; Section 10.1
                    | Content-Encoding         ; Section 10.3
                    | Content-Length           ; Section 10.4
                    | Content-Type             ; Section 10.5
                    | Expires                  ; Section 10.7
                    | Last-Modified            ; Section 10.10
                    | extension-header
     extension-header = HTTP-header
 The extension-header mechanism allows additional Entity-Header fields
 to be defined without changing the protocol, but these fields cannot
 be assumed to be recognizable by the recipient. Unrecognized header
 fields should be ignored by the recipient and forwarded by proxies.

7.2 Entity Body

 The entity body (if any) sent with an HTTP request or response is in
 a format and encoding defined by the Entity-Header fields.
     Entity-Body    = *OCTET
 An entity body is included with a request message only when the
 request method calls for one. The presence of an entity body in a
 request is signaled by the inclusion of a Content-Length header field
 in the request message headers. HTTP/1.0 requests containing an
 entity body must include a valid Content-Length header field.
 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 entity header fields may lead one
 to believe they do. All 1xx (informational), 204 (no content), and
 304 (not modified) responses must not include a body. All other
 responses must include an entity body or a Content-Length header
 field defined with a value of zero (0).

7.2.1 Type

 When an Entity-Body is included with a message, the data type of that
 body is determined via the header fields Content-Type and Content-
 Encoding. These define a two-layer, ordered encoding model:

Berners-Lee, et al Informational [Page 29] RFC 1945 HTTP/1.0 May 1996

     entity-body := Content-Encoding( Content-Type( data ) )
 A Content-Type specifies the media type of the underlying data. A
 Content-Encoding may be used to indicate any additional content
 coding applied to the type, usually for the purpose of data
 compression, that is a property of the resource requested. The
 default for the content encoding is none (i.e., the identity
 function).
 Any HTTP/1.0 message containing an entity body should include a
 Content-Type header field defining the media type of that body. If
 and only if the media type is not given by a Content-Type header, as
 is the case for Simple-Response messages, the recipient may attempt
 to guess the media type via inspection of its content and/or the name
 extension(s) of the URL used to identify the resource. If the media
 type remains unknown, the recipient should treat it as type
 "application/octet-stream".

7.2.2 Length

 When an Entity-Body is included with a message, the length of that
 body may be determined in one of two ways. If a Content-Length header
 field is present, its value in bytes represents the length of the
 Entity-Body. Otherwise, the body length is determined by the closing
 of the connection by the server.
 Closing the connection cannot be used to indicate the end of a
 request body, since it leaves no possibility for the server to send
 back a response. Therefore, HTTP/1.0 requests containing an entity
 body must include a valid Content-Length header field. If a request
 contains an entity body and Content-Length is not specified, and the
 server does not recognize or cannot calculate the length from other
 fields, then the server should send a 400 (bad request) 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. Method Definitions

 The set of common methods for HTTP/1.0 is defined below. Although
 this set can be expanded, additional methods cannot be assumed to
 share the same semantics for separately extended clients and servers.

Berners-Lee, et al Informational [Page 30] RFC 1945 HTTP/1.0 May 1996

8.1 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, as described in Section 10.9. 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.

8.2 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" request analogous to the conditional
 GET. If an If-Modified-Since header field is included with a HEAD
 request, it should be ignored.

8.3 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, such as the result of submitting a
      form [3], to a data-handling process;
    o Extending a database through an append operation.

Berners-Lee, et al Informational [Page 31] RFC 1945 HTTP/1.0 May 1996

 The actual function performed by the POST method is determined by the
 server and is usually dependent on the Request-URI. The posted entity
 is subordinate to that URI in the same way that a file is subordinate
 to a directory containing it, a news article is subordinate to a
 newsgroup to which it is posted, or a record is subordinate to a
 database.
 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 an entity (preferably of type
 "text/html") which describes the status of the request and refers to
 the new resource.
 A valid Content-Length is required on all HTTP/1.0 POST requests. An
 HTTP/1.0 server should respond with a 400 (bad request) message if it
 cannot determine the length of the request message's content.
 Applications must not cache responses to a POST request because the
 application has no way of knowing that the server would return an
 equivalent response on some future request.

9. Status Code Definitions

 Each Status-Code is described below, including a description of which
 method(s) it can follow and any metainformation required in the
 response.

9.1 Informational 1xx

 This class of status code indicates a provisional response,
 consisting only of the Status-Line and optional headers, and is
 terminated by an empty line. HTTP/1.0 does not define any 1xx status
 codes and they are not a valid response to a HTTP/1.0 request.
 However, they may be useful for experimental applications which are
 outside the scope of this specification.

9.2 Successful 2xx

 This class of status code indicates that the client's request was
 successfully received, understood, and accepted.

Berners-Lee, et al Informational [Page 32] RFC 1945 HTTP/1.0 May 1996

 200 OK
 The request has succeeded. The information returned with the
 response is dependent on the method used in the request, as follows:
 GET    an entity corresponding to the requested resource is sent
        in the response;
 HEAD   the response must only contain the header information and
        no Entity-Body;
 POST   an entity describing or containing the result of the action.
 201 Created
 The request has been fulfilled and resulted in a new resource being
 created. The newly created resource can be referenced by the URI(s)
 returned in the entity of the response. The origin server should
 create the resource before using this Status-Code. If the action
 cannot be carried out immediately, the server must include in the
 response body a description of when the resource will be available;
 otherwise, the server should respond with 202 (accepted).
 Of the methods defined by this specification, only POST can create a
 resource.
 202 Accepted
 The request has been accepted for processing, but the processing
 has not been completed. The request may or may not eventually be
 acted upon, as it may be disallowed when processing actually takes
 place. There is no facility for re-sending a status code from an
 asynchronous operation such as this.
 The 202 response is intentionally non-committal. Its purpose is to
 allow a server to accept a request for some other process (perhaps
 a batch-oriented process that is only run once per day) without
 requiring that the user agent's connection to the server persist
 until the process is completed. The entity returned with this
 response should include an indication of the request's current
 status and either a pointer to a status monitor or some estimate of
 when the user can expect the request to be fulfilled.
 204 No Content
 The server has fulfilled the request but there is no new
 information to send back. If the client is a user agent, it should
 not change its document view from that which caused the request to

Berners-Lee, et al Informational [Page 33] RFC 1945 HTTP/1.0 May 1996

 be generated. 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 active document view. The response may include
 new metainformation in the form of entity headers, which should
 apply to the document currently in the user agent's active view.

9.3 Redirection 3xx

 This class of status code indicates that further action needs to be
 taken by the user agent in order to fulfill the request. The action
 required may be carried out by the user agent without interaction
 with the user if and only if the method used in the subsequent
 request is GET or HEAD. A user agent should never automatically
 redirect a request more than 5 times, since such redirections usually
 indicate an infinite loop.
 300 Multiple Choices
 This response code is not directly used by HTTP/1.0 applications,
 but serves as the default for interpreting the 3xx class of
 responses.
 The requested resource is available at one or more locations.
 Unless it was a HEAD request, the response should include an entity
 containing a list of resource characteristics and locations from
 which the user or user agent can choose the one most appropriate.
 If the server has a preferred choice, it should include the URL in
 a Location field; user agents may use this field value for
 automatic redirection.
 301 Moved Permanently
 The requested resource has been assigned a new permanent URL and
 any future references to this resource should be done using that
 URL. Clients with link editing capabilities should automatically
 relink references to the Request-URI to the new reference returned
 by the server, where possible.
 The new URL must be given by the Location field in the response.
 Unless it was a HEAD request, the Entity-Body of the response
 should contain a short note with a hyperlink to the new URL.
 If the 301 status code is received in response to a request using
 the POST method, the user agent must not automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.

Berners-Lee, et al Informational [Page 34] RFC 1945 HTTP/1.0 May 1996

     Note: When automatically redirecting a POST request after
     receiving a 301 status code, some existing user agents will
     erroneously change it into a GET request.
 302 Moved Temporarily
 The requested resource resides temporarily under a different URL.
 Since the redirection may be altered on occasion, the client should
 continue to use the Request-URI for future requests.
 The URL must be given by the Location field in the response. Unless
 it was a HEAD request, the Entity-Body of the response should
 contain a short note with a hyperlink to the new URI(s).
 If the 302 status code is received in response to a request using
 the POST method, the user agent must not automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.
     Note: When automatically redirecting a POST request after
     receiving a 302 status code, some existing user agents will
     erroneously change it into a GET request.
 304 Not Modified
 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 must
 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 or which may have
 changed independently of the entity's Last-Modified date. Examples
 of relevant header fields include: Date, Server, and Expires. A
 cache should update its cached entity to reflect any new field
 values given in the 304 response.

9.4 Client Error 4xx

 The 4xx class of status code 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 should include an entity containing an explanation of the
 error situation, and whether it is a temporary or permanent
 condition. These status codes are applicable to any request method.

Berners-Lee, et al Informational [Page 35] RFC 1945 HTTP/1.0 May 1996

    Note: If the client is sending data, server implementations on TCP
    should be careful to ensure that the client acknowledges receipt
    of the packet(s) containing the response prior to closing the
    input connection. If the client continues sending data to the
    server after the close, the server's controller will send a reset
    packet to the client, which may erase the client's unacknowledged
    input buffers before they can be read and interpreted by the HTTP
    application.
 400 Bad Request
 The request could not be understood by the server due to malformed
 syntax. The client should not repeat the request without
 modifications.
 401 Unauthorized
 The request requires user authentication. The response must include
 a WWW-Authenticate header field (Section 10.16) containing a
 challenge applicable to the requested resource. The client may
 repeat the request with a suitable Authorization header field
 (Section 10.2). If the request already included Authorization
 credentials, then the 401 response indicates that authorization has
 been refused for those credentials. If the 401 response contains
 the same challenge as the prior response, and the user agent has
 already attempted authentication at least once, then the user
 should be presented the entity that was given in the response,
 since that entity may include relevant diagnostic information. HTTP
 access authentication is explained in Section 11.
 403 Forbidden
 The server understood the request, but is refusing to fulfill it.
 Authorization will not help and the request should not be repeated.
 If the request method was not HEAD and the server wishes to make
 public why the request has not been fulfilled, it should describe
 the reason for the refusal in the entity body. This status code is
 commonly used when the server does not wish to reveal exactly why
 the request has been refused, or when no other response is
 applicable.
 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.

Berners-Lee, et al Informational [Page 36] RFC 1945 HTTP/1.0 May 1996

9.5 Server Error 5xx

 Response status codes beginning with the digit "5" indicate cases in
 which the server is aware that it has erred or is incapable of
 performing the request. 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
 should 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, while acting as a gateway or proxy, received an invalid
 response from the upstream server it accessed in attempting to
 fulfill 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.
     Note: The existence of the 503 status code does not imply
     that a server must use it when becoming overloaded. Some
     servers may wish to simply refuse the connection.

10. Header Field Definitions

 This section defines the syntax and semantics of all commonly used
 HTTP/1.0 header fields. For general and entity header fields, both
 sender and recipient refer to either the client or the server,
 depending on who sends and who receives the message.

Berners-Lee, et al Informational [Page 37] RFC 1945 HTTP/1.0 May 1996

10.1 Allow

 The Allow entity-header field lists the set of methods supported by
 the resource identified by the Request-URI. The purpose of this field
 is strictly to inform the recipient of valid methods associated with
 the resource. The Allow header field is not permitted in a request
 using the POST method, and thus should be ignored if it is received
 as part of a POST entity.
     Allow          = "Allow" ":" 1#method
  Example of use:
     Allow: GET, HEAD
 This field cannot prevent a client from trying other methods.
 However, the indications given by the Allow header field value should
 be followed. The actual set of allowed methods is defined by the
 origin server at the time of each request.
 A proxy must not modify the Allow header field even if it does not
 understand all the methods specified, since the user agent may have
 other means of communicating with the origin server.
 The Allow header field does not indicate what methods are implemented
 by the server.

10.2 Authorization

 A user agent that wishes to authenticate itself with a server--
 usually, but not necessarily, after receiving a 401 response--may do
 so by including an Authorization request-header field with the
 request. The Authorization field value consists of credentials
 containing the authentication information of the user agent for the
 realm of the resource being requested.
     Authorization  = "Authorization" ":" credentials
 HTTP access authentication is described in Section 11. If a request
 is authenticated and a realm specified, the same credentials should
 be valid for all other requests within this realm.
 Responses to requests containing an Authorization field are not
 cachable.

Berners-Lee, et al Informational [Page 38] RFC 1945 HTTP/1.0 May 1996

10.3 Content-Encoding

 The Content-Encoding entity-header field is used as a modifier to the
 media-type. When present, its value indicates what additional content
 coding 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" ":" content-coding
 Content codings are defined in Section 3.5. An example of its use is
     Content-Encoding: x-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.

10.4 Content-Length

 The Content-Length entity-header field indicates the size of the
 Entity-Body, in decimal number of octets, sent to the recipient or,
 in the case of the HEAD method, the size of the Entity-Body that
 would have been sent had the request been a GET.
     Content-Length = "Content-Length" ":" 1*DIGIT
 An example is
     Content-Length: 3495
 Applications should use this field to indicate the size of the
 Entity-Body to be transferred, regardless of the media type of the
 entity. A valid Content-Length field value is required on all
 HTTP/1.0 request messages containing an entity body.
 Any Content-Length greater than or equal to zero is a valid value.
 Section 7.2.2 describes how to determine the length of a response
 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.

Berners-Lee, et al Informational [Page 39] RFC 1945 HTTP/1.0 May 1996

10.5 Content-Type

 The Content-Type entity-header field indicates the media type of the
 Entity-Body sent to the recipient or, in the case of the HEAD method,
 the media type that would have been sent had the request been a GET.
     Content-Type   = "Content-Type" ":" media-type
 Media types are defined in Section 3.6. An example of the field is
     Content-Type: text/html
 Further discussion of methods for identifying the media type of an
 entity is provided in Section 7.2.1.

10.6 Date

 The Date general-header field represents the date and time at which
 the message was originated, having the same semantics as orig-date in
 RFC 822. The field value is an HTTP-date, as described in Section
 3.3.
     Date           = "Date" ":" HTTP-date
 An example is
     Date: Tue, 15 Nov 1994 08:12:31 GMT
 If a message is received via direct connection with the user agent
 (in the case of requests) or the origin server (in the case of
 responses), then the date can be assumed to be the current date at
 the receiving end. However, since the date--as it is believed by the
 origin--is important for evaluating cached responses, origin servers
 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 POST request, and even then it is optional. A received message
 which does not have a Date header field should be assigned one by the
 recipient if the message will be cached by that recipient or
 gatewayed via a protocol which requires a Date.
 In theory, the date should represent the moment just before the
 entity is generated. In practice, the date can be generated at any
 time during the message origination without affecting its semantic
 value.
    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)

Berners-Lee, et al Informational [Page 40] RFC 1945 HTTP/1.0 May 1996

    usage.

10.7 Expires

 The Expires entity-header field gives the date/time after which the
 entity should be considered stale. This allows information providers
 to suggest the volatility of the resource, or a date after which the
 information may no longer be valid. Applications must not cache this
 entity beyond the date given. 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
 should 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
 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, entities from 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.
 User agents often have history mechanisms, such as "Back" buttons and
 history lists, which can be used to redisplay an entity retrieved
 earlier in a session. By default, the Expires field does not apply to
 history mechanisms. If the entity is still in storage, a history
 mechanism should display it even if the entity has expired, unless
 the user has specifically configured the agent to refresh expired
 history documents.
    Note: Applications are encouraged to be tolerant of bad or
    misinformed implementations of the Expires header. A value of zero
    (0) or an invalid date format should be considered equivalent to
    an "expires immediately." Although these values are not legitimate
    for HTTP/1.0, a robust implementation is always desirable.

Berners-Lee, et al Informational [Page 41] RFC 1945 HTTP/1.0 May 1996

10.8 From

 The From request-header field, if given, should contain an Internet
 e-mail address for the human user who controls the requesting user
 agent. The address should be machine-usable, as defined by mailbox in
 RFC 822 [7] (as updated by RFC 1123 [6]):
     From           = "From" ":" mailbox
 An example is:
     From: webmaster@w3.org
 This header field may be used for logging purposes and as a means for
 identifying the source of invalid or unwanted requests. It should not
 be used as an insecure form of access protection. The interpretation
 of this field is that the request is being performed on behalf of the
 person given, who accepts responsibility for the method performed. In
 particular, robot agents should include this header so that the
 person responsible for running the robot can be contacted if problems
 occur on the receiving end.
 The Internet e-mail address in this field may be separate from the
 Internet host which issued the request. For example, when a request
 is passed through a proxy, the original issuer's address should be
 used.
    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.

10.9 If-Modified-Since

 The If-Modified-Since request-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

Berners-Lee, et al Informational [Page 42] RFC 1945 HTTP/1.0 May 1996

 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. A date which is later than the server's current
       time is invalid.
    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 a valid
       If-Modified-Since date, the server shall return a 304 (not
       modified) response.
 The purpose of this feature is to allow efficient updates of cached
 information with a minimum amount of transaction overhead.

10.10 Last-Modified

 The Last-Modified entity-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 recipient
 should interpret it:  if the recipient 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-modified time. For entities with
 dynamically included parts, it may be the most recent of the set of
 last-modify times for its component parts. For database gateways, it
 may be the last-update timestamp of the record. For virtual objects,
 it may be the last time the internal state changed.
 An origin server must not send a Last-Modified date which is later
 than the server's time of message origination. In such cases, where
 the resource's last modification would indicate some time in the

Berners-Lee, et al Informational [Page 43] RFC 1945 HTTP/1.0 May 1996

 future, the server must replace that date with the message
 origination date.

10.11 Location

 The Location response-header field defines the exact location of the
 resource that was identified by the Request-URI. For 3xx responses,
 the location must indicate the server's preferred URL for automatic
 redirection to the resource. Only one absolute URL is allowed.
     Location       = "Location" ":" absoluteURI
 An example is
     Location: http://www.w3.org/hypertext/WWW/NewLocation.html

10.12 Pragma

 The Pragma general-header field is used to include implementation-
 specific directives that may apply to any recipient along the
 request/response chain. All pragma directives specify optional
 behavior from the viewpoint of the protocol; however, some systems
 may require that behavior be consistent with the directives.
     Pragma           = "Pragma" ":" 1#pragma-directive
     pragma-directive = "no-cache" | extension-pragma
     extension-pragma = token [ "=" word ]
 When the "no-cache" directive is present in a request message, an
 application should forward the request toward the origin server even
 if it has a cached copy of what is being requested. This allows a
 client to insist upon receiving an authoritative response to its
 request. It also allows a client to refresh a cached copy which is
 known to be corrupted or stale.
 Pragma directives must be passed through by a proxy or gateway
 application, regardless of their significance to that application,
 since the directives may be applicable to all recipients along the
 request/response chain. It is not possible to specify a pragma for a
 specific recipient; however, any pragma directive not relevant to a
 recipient should be ignored by that recipient.

10.13 Referer

 The Referer request-header field allows the client to specify, for
 the server's benefit, the address (URI) of the resource from which
 the Request-URI was obtained. This allows a server to generate lists

Berners-Lee, et al Informational [Page 44] RFC 1945 HTTP/1.0 May 1996

 of back-links to resources for interest, logging, optimized caching,
 etc. It also allows obsolete or mistyped links to be traced for
 maintenance. The Referer field must not be sent if the Request-URI
 was obtained from a source that does not have its own URI, such as
 input from the user keyboard.
     Referer        = "Referer" ":" ( absoluteURI | relativeURI )
 Example:
     Referer: http://www.w3.org/hypertext/DataSources/Overview.html
 If a partial URI is given, it should be interpreted relative to the
 Request-URI. The URI must not include a fragment.
    Note: Because the source of a link may be private information or
    may reveal an otherwise private information source, it is strongly
    recommended that the user be able to select whether or not the
    Referer field is sent. For example, a browser client could have a
    toggle switch for browsing openly/anonymously, which would
    respectively enable/disable the sending of Referer and From
    information.

10.14 Server

 The Server response-header field contains information about the
 software used by the origin server to handle the request. The field
 can contain multiple product tokens (Section 3.7) and comments
 identifying the server and any significant subproducts. By
 convention, the product tokens are listed in order of their
 significance for identifying the application.
     Server         = "Server" ":" 1*( product | comment )
 Example:
     Server: CERN/3.0 libwww/2.17
 If the response is being forwarded through a proxy, the proxy
 application must not add its data to the product list.
    Note: Revealing the specific software version of the server may
    allow the server machine to become more vulnerable to attacks
    against software that is known to contain security holes. Server
    implementors are encouraged to make this field a configurable
    option.

Berners-Lee, et al Informational [Page 45] RFC 1945 HTTP/1.0 May 1996

    Note: Some existing servers fail to restrict themselves to the
    product token syntax within the Server field.

10.15 User-Agent

 The User-Agent request-header field contains information about the
 user agent originating the request. This is for statistical purposes,
 the tracing of protocol violations, and automated recognition of user
 agents for the sake of tailoring responses to avoid particular user
 agent limitations. Although it is not required, user agents should
 include this field with requests. The field can contain multiple
 product tokens (Section 3.7) and comments identifying the agent and
 any subproducts which form a significant part of the user agent. By
 convention, the product tokens are listed in order of their
 significance for identifying the application.
     User-Agent     = "User-Agent" ":" 1*( product | comment )
 Example:
     User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     Note: Some current proxy applications append their product
     information to the list in the User-Agent field. This is not
     recommended, since it makes machine interpretation of these
     fields ambiguous.
     Note: Some existing clients fail to restrict themselves to
     the product token syntax within the User-Agent field.

10.16 WWW-Authenticate

 The WWW-Authenticate response-header field must be included in 401
 (unauthorized) response messages. The field value consists of at
 least one challenge that indicates the authentication scheme(s) and
 parameters applicable to the Request-URI.
     WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
 The HTTP access authentication process is described in Section 11.
 User agents must take special care in parsing the WWW-Authenticate
 field value if it contains more than one challenge, or if more than
 one WWW-Authenticate header field is provided, since the contents of
 a challenge may itself contain a comma-separated list of
 authentication parameters.

Berners-Lee, et al Informational [Page 46] RFC 1945 HTTP/1.0 May 1996

11. Access Authentication

 HTTP provides a simple challenge-response authentication mechanism
 which may be used by a server to challenge a client request and by a
 client to provide authentication information. It uses an extensible,
 case-insensitive token to identify the authentication scheme,
 followed by a comma-separated list of attribute-value pairs which
 carry the parameters necessary for achieving authentication via that
 scheme.
     auth-scheme    = token
     auth-param     = token "=" quoted-string
 The 401 (unauthorized) response message is used by an origin server
 to challenge the authorization of a user agent. This response must
 include a WWW-Authenticate header field containing at least one
 challenge applicable to the requested resource.
     challenge      = auth-scheme 1*SP realm *( "," auth-param )
     realm          = "realm" "=" realm-value
     realm-value    = quoted-string
 The realm attribute (case-insensitive) is required for all
 authentication schemes which issue a challenge. The realm value
 (case-sensitive), in combination with the canonical root URL of the
 server being accessed, defines the protection space. These realms
 allow the protected resources on a server to be partitioned into a
 set of protection spaces, each with its own authentication scheme
 and/or authorization database. The realm value is a string, generally
 assigned by the origin server, which may have additional semantics
 specific to the authentication scheme.
 A user agent that wishes to authenticate itself with a server--
 usually, but not necessarily, after receiving a 401 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    = basic-credentials
                    | ( auth-scheme #auth-param )
 The domain over which credentials can be automatically applied by a
 user agent is determined by the protection space. If a prior request
 has been authorized, the same credentials may be reused for all other
 requests within that protection space for a period of time determined

Berners-Lee, et al Informational [Page 47] RFC 1945 HTTP/1.0 May 1996

 by the authentication scheme, parameters, and/or user preference.
 Unless otherwise defined by the authentication scheme, a single
 protection space cannot extend outside the scope of its server.
 If the server does not wish to accept the credentials sent with a
 request, it should return a 403 (forbidden) response.
 The HTTP protocol does not restrict applications to this simple
 challenge-response mechanism for access authentication. Additional
 mechanisms may be used, such as encryption at the transport level or
 via message encapsulation, and with additional header fields
 specifying authentication information. However, these additional
 mechanisms are not defined by this specification.
 Proxies must be completely transparent regarding user agent
 authentication. That is, they must forward the WWW-Authenticate and
 Authorization headers untouched, and must not cache the response to a
 request containing Authorization. HTTP/1.0 does not provide a means
 for a client to be authenticated with a proxy.

11.1 Basic Authentication Scheme

 The "basic" authentication scheme is based on the model that the user
 agent must authenticate itself with a user-ID and a password for each
 realm. The realm value should be considered an opaque string which
 can only be compared for equality with other realms on that server.
 The server will authorize the request only if it can validate the
 user-ID and password for the protection space of the Request-URI.
 There are no optional authentication parameters.
 Upon receipt of an unauthorized request for a URI within the
 protection space, the server should respond with a challenge like the
 following:
     WWW-Authenticate: Basic realm="WallyWorld"
 where "WallyWorld" is the string assigned by the server to identify
 the protection space of the Request-URI.
 To receive authorization, the client sends the user-ID and password,
 separated by a single colon (":") character, within a base64 [5]
 encoded string in the credentials.
     basic-credentials = "Basic" SP basic-cookie
     basic-cookie      = <base64 [5] encoding of userid-password,
                          except not limited to 76 char/line>

Berners-Lee, et al Informational [Page 48] RFC 1945 HTTP/1.0 May 1996

     userid-password   = [ token ] ":" *TEXT
 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 should implement the scheme in
 order to communicate with servers that use it.

12. 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.

12.1 Authentication of Clients

 As mentioned in Section 11.1, the Basic authentication scheme is not
 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 from being employed
 to increase security.

12.2 Safe Methods

 The writers of client software should be aware that the software
 represents the user in their interactions over the Internet, and
 should be careful to allow the user to be aware of any actions they
 may take which may have an unexpected significance to themselves or
 others.
 In particular, the convention has been established that the GET and
 HEAD methods should never have the significance of taking an action
 other than retrieval. These methods should be considered "safe." This
 allows user agents to represent other methods, such as POST, in a
 special way, so that the user is made aware of the fact that a
 possibly unsafe action is being requested.

Berners-Lee, et al Informational [Page 49] RFC 1945 HTTP/1.0 May 1996

 Naturally, it is not possible to ensure that the server does not
 generate side-effects as a result of performing a GET request; in
 fact, some dynamic resources consider that a feature. The important
 distinction here is that the user did not request the side-effects,
 so therefore cannot be held accountable for them.

12.3 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.

12.4 Transfer of Sensitive Information

 Like any generic data transfer protocol, HTTP cannot regulate the
 content of the data that is transferred, nor is there any a priori
 method of determining the sensitivity of any particular piece of
 information within the context of any given request. Therefore,
 applications should supply as much control over this information as
 possible to the provider of that information. Three header fields are
 worth special mention in this context: Server, Referer and From.
 Revealing the specific software version of the server may allow the
 server machine to become more vulnerable to attacks against software
 that is known to contain security holes. Implementors should make the
 Server header field a configurable option.
 The Referer field allows reading patterns to be studied and reverse
 links drawn. Although it can be very useful, its power can be abused
 if user details are not separated from the information contained in
 the Referer. Even when the personal information has been removed, the
 Referer field may indicate a private document's URI whose publication
 would be inappropriate.
 The information sent in the From field might conflict with the user's
 privacy interests or their site's security policy, and hence it
 should not be transmitted without the user being able to disable,
 enable, and modify the contents of the field. The user must be able
 to set the contents of this field within a user preference or
 application defaults configuration.
 We suggest, though do not require, that a convenient toggle interface
 be provided for the user to enable or disable the sending of From and
 Referer information.

Berners-Lee, et al Informational [Page 50] RFC 1945 HTTP/1.0 May 1996

12.5 Attacks Based On File and Path Names

 Implementations of HTTP origin servers should be careful to restrict
 the documents returned by HTTP requests to be only those that were
 intended by the server administrators. If an HTTP server translates
 HTTP URIs directly into file system calls, the server must take
 special care not to serve files that were not intended to be
 delivered to HTTP clients. For example, Unix, Microsoft Windows, and
 other operating systems use ".." as a path component to indicate a
 directory level above the current one. On such a system, an HTTP
 server must disallow any such construct in the Request-URI if it
 would otherwise allow access to a resource outside those intended to
 be accessible via the HTTP server. Similarly, files intended for
 reference only internally to the server (such as access control
 files, configuration files, and script code) must be protected from
 inappropriate retrieval, since they might contain sensitive
 information. Experience has shown that minor bugs in such HTTP server
 implementations have turned into security risks.

13. Acknowledgments

 This specification makes heavy use of the augmented BNF and generic
 constructs defined by David H. Crocker for RFC 822 [7]. Similarly, it
 reuses many of the definitions provided by Nathaniel Borenstein and
 Ned Freed for MIME [5]. We hope that their inclusion in this
 specification will help reduce past confusion over the relationship
 between HTTP/1.0 and Internet mail message formats.
 The HTTP protocol has evolved considerably over the past four years.
 It has benefited from a large and active developer community--the
 many people who have participated on the www-talk mailing list--and
 it is that community which has been most responsible for the success
 of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
 Cailliau, Daniel W. Connolly, Bob Denny, Jean-Francois Groff, Phillip
 M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou
 Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve
 special recognition for their efforts in defining aspects of the
 protocol for early versions of this specification.
 Paul Hoffman contributed sections regarding the informational status
 of this document and Appendices C and D.

Berners-Lee, et al Informational [Page 51] RFC 1945 HTTP/1.0 May 1996

 This document has benefited greatly from the comments of all those
 participating in the HTTP-WG. In addition to those already mentioned,
 the following individuals have contributed to this specification:
     Gary Adams                         Harald Tveit Alvestrand
     Keith Ball                         Brian Behlendorf
     Paul Burchard                      Maurizio Codogno
     Mike Cowlishaw                     Roman Czyborra
     Michael A. Dolan                   John Franks
     Jim Gettys                         Marc Hedlund
     Koen Holtman                       Alex Hopmann
     Bob Jernigan                       Shel Kaphan
     Martijn Koster                     Dave Kristol
     Daniel LaLiberte                   Paul Leach
     Albert Lunde                       John C. Mallery
     Larry Masinter                     Mitra
     Jeffrey Mogul                      Gavin Nicol
     Bill Perry                         Jeffrey Perry
     Owen Rees                          Luigi Rizzo
     David Robinson                     Marc Salomon
     Rich Salz                          Jim Seidman
     Chuck Shotton                      Eric W. Sink
     Simon E. Spero                     Robert S. Thau
     Francois Yergeau                   Mary Ellen Zurko
     Jean-Philippe Martin-Flatin

14. References

 [1]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
      Torrey, D., and B. Alberti, "The Internet Gopher Protocol: A
      Distributed Document Search and Retrieval Protocol", RFC 1436,
      University of Minnesota, March 1993.
 [2]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
      Unifying Syntax for the Expression of Names and Addresses of
      Objects on the Network as used in the World-Wide Web",
      RFC 1630, CERN, June 1994.
 [3]  Berners-Lee, T., and D. Connolly, "Hypertext Markup Language -
      2.0", RFC 1866, MIT/W3C, November 1995.
 [4]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
      Resource Locators (URL)", RFC 1738, CERN, Xerox PARC,
      University of Minnesota, December 1994.

Berners-Lee, et al Informational [Page 52] RFC 1945 HTTP/1.0 May 1996

 [5]  Borenstein, N., 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.
 [6]  Braden, R., "Requirements for Internet hosts - Application and
      Support", STD 3, RFC 1123, IETF, October 1989.
 [7]  Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages", STD 11, RFC 822, UDEL, August 1982.
 [8]  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.
 [9]  Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
      UC Irvine, June 1995.
 [10] Horton, M., and R. Adams, "Standard for interchange of USENET
      Messages", RFC 1036 (Obsoletes RFC 850), AT&T Bell
      Laboratories, Center for Seismic Studies, December 1987.
 [11] Kantor, B., and P. Lapsley, "Network News Transfer Protocol:
      A Proposed Standard for the Stream-Based Transmission of News",
      RFC 977, UC San Diego, UC Berkeley, February 1986.
 [12] Postel, J., "Simple Mail Transfer Protocol." STD 10, RFC 821,
      USC/ISI, August 1982.
 [13] Postel, J., "Media Type Registration Procedure." RFC 1590,
      USC/ISI, March 1994.
 [14] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)",
      STD 9, RFC 959, USC/ISI, October 1985.
 [15] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
      1700, USC/ISI, October 1994.
 [16] Sollins, K., and L. Masinter, "Functional Requirements for
      Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
      December 1994.
 [17] US-ASCII. Coded Character Set - 7-Bit American Standard Code
      for Information Interchange. Standard ANSI X3.4-1986, ANSI,
      1986.

Berners-Lee, et al Informational [Page 53] RFC 1945 HTTP/1.0 May 1996

 [18] 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.

15. Authors' Addresses

 Tim Berners-Lee
 Director, W3 Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, U.S.A.
 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.
 Fax: +1 (714) 824-4056
 EMail: fielding@ics.uci.edu
 Henrik Frystyk Nielsen
 W3 Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, U.S.A.
 Fax: +1 (617) 258 8682
 EMail: frystyk@w3.org

Berners-Lee, et al Informational [Page 54] RFC 1945 HTTP/1.0 May 1996

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 [13].
     Media Type name:         message
     Media subtype name:      http
     Required parameters:     none
     Optional parameters:     version, msgtype
            version: The HTTP-Version number of the enclosed message
                     (e.g., "1.0"). If not present, the version can be
                     determined from the first line of the body.
            msgtype: The message type -- "request" or "response". If
                     not present, the type can be determined from the
                     first line of the body.
     Encoding considerations: only "7bit", "8bit", or "binary" are
                              permitted
     Security considerations: none

B. Tolerant Applications

 Although this document specifies the requirements for the generation
 of HTTP/1.0 messages, not all applications will be correct in their
 implementation. We therefore recommend that operational applications
 be tolerant of deviations whenever those deviations can be
 interpreted unambiguously.
 Clients should be tolerant in parsing the Status-Line and servers
 tolerant when parsing the Request-Line. In particular, they should
 accept any amount of SP or HT characters between fields, even though
 only a single SP is required.
 The line terminator for HTTP-header fields is the sequence CRLF.
 However, we recommend that applications, when parsing such headers,
 recognize a single LF as a line terminator and ignore the leading CR.

Berners-Lee, et al Informational [Page 55] RFC 1945 HTTP/1.0 May 1996

C. Relationship to MIME

 HTTP/1.0 uses many of the constructs defined for Internet Mail (RFC
 822 [7]) and the Multipurpose Internet Mail Extensions (MIME [5]) to
 allow entities to be transmitted in an open variety of
 representations and with extensible mechanisms. However, RFC 1521
 discusses mail, and HTTP has a few features that are different than
 those described in RFC 1521. These differences were carefully chosen
 to optimize performance over binary connections, to allow greater
 freedom in the use of new media types, to make date comparisons
 easier, and to acknowledge the practice of some early HTTP servers
 and clients.
 At the time of this writing, it is expected that RFC 1521 will be
 revised. The revisions may include some of the practices found in
 HTTP/1.0 but not in RFC 1521.
 This appendix describes specific areas where HTTP differs from RFC
 1521. Proxies and gateways to strict MIME environments should be
 aware of these differences and provide the appropriate conversions
 where necessary. Proxies and gateways from MIME environments to HTTP
 also need to be aware of the differences because some conversions may
 be required.

C.1 Conversion to Canonical Form

 RFC 1521 requires that an Internet mail entity be converted to
 canonical form prior to being transferred, as described in Appendix G
 of RFC 1521 [5]. Section 3.6.1 of this document describes the forms
 allowed for subtypes of the "text" media type when transmitted over
 HTTP.
 RFC 1521 requires that content with a Content-Type of "text"
 represent line breaks as CRLF and forbids the use of CR or LF outside
 of line break sequences. HTTP allows CRLF, bare CR, and bare LF to
 indicate a line break within text content when a message is
 transmitted over HTTP.
 Where it is possible, a proxy or gateway from HTTP to a strict RFC
 1521 environment should translate all line breaks within the text
 media types described in Section 3.6.1 of this document to the RFC
 1521 canonical form of CRLF. Note, however, that 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.

Berners-Lee, et al Informational [Page 56] RFC 1945 HTTP/1.0 May 1996

C.2 Conversion of Date Formats

 HTTP/1.0 uses a restricted set of date formats (Section 3.3) to
 simplify the process of date comparison. Proxies and gateways from
 other protocols should ensure that any Date header field present in a
 message conforms to one of the HTTP/1.0 formats and rewrite the date
 if necessary.

C.3 Introduction of Content-Encoding

 RFC 1521 does not include any concept equivalent to HTTP/1.0's
 Content-Encoding header field. Since this acts as a modifier on the
 media type, proxies and gateways from HTTP to MIME-compliant
 protocols must either change the value of the Content-Type header
 field or decode the Entity-Body before forwarding the message. (Some
 experimental applications of Content-Type for Internet mail have used
 a media-type parameter of ";conversions=<content-coding>" to perform
 an equivalent function as Content-Encoding. However, this parameter
 is not part of RFC 1521.)

C.4 No Content-Transfer-Encoding

 HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
 1521. Proxies and gateways from MIME-compliant protocols to HTTP must
 remove any non-identity CTE ("quoted-printable" or "base64") encoding
 prior to delivering the response message to an HTTP client.
 Proxies and gateways from HTTP to MIME-compliant protocols are
 responsible for ensuring that the message is in the correct format
 and encoding for safe transport on that protocol, where "safe
 transport" is defined by the limitations of the protocol being used.
 Such a proxy or gateway should label the data with an appropriate
 Content-Transfer-Encoding if doing so will improve the likelihood of
 safe transport over the destination protocol.

C.5 HTTP Header Fields in Multipart Body-Parts

 In RFC 1521, most header fields in multipart body-parts are generally
 ignored unless the field name begins with "Content-". In HTTP/1.0,
 multipart body-parts may contain any HTTP header fields which are
 significant to the meaning of that part.

D. Additional Features

 This appendix documents protocol elements used by some existing HTTP
 implementations, but not consistently and correctly across most
 HTTP/1.0 applications. Implementors should be aware of these
 features, but cannot rely upon their presence in, or interoperability

Berners-Lee, et al Informational [Page 57] RFC 1945 HTTP/1.0 May 1996

 with, other HTTP/1.0 applications.

D.1 Additional Request Methods

D.1.1 PUT

 The PUT method requests that the enclosed entity be stored under the
 supplied Request-URI. If the Request-URI refers to an already
 existing resource, the enclosed entity should be considered as a
 modified version of the one residing on the origin server. If the
 Request-URI does not point to an existing resource, and that URI is
 capable of being defined as a new resource by the requesting user
 agent, the origin server can create the resource with that URI.
 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 data to be processed. That resource may be a data-accepting
 process, a gateway to some other protocol, or a separate entity that
 accepts annotations. In contrast, the URI in a PUT request identifies
 the entity enclosed with the request -- the user agent knows what URI
 is intended and the server should not apply the request to some other
 resource.

D.1.2 DELETE

 The DELETE method requests that the origin server delete the resource
 identified by the Request-URI.

D.1.3 LINK

 The LINK method establishes one or more Link relationships between
 the existing resource identified by the Request-URI and other
 existing resources.

D.1.4 UNLINK

 The UNLINK method removes one or more Link relationships from the
 existing resource identified by the Request-URI.

D.2 Additional Header Field Definitions

D.2.1 Accept

 The Accept request-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

Berners-Lee, et al Informational [Page 58] RFC 1945 HTTP/1.0 May 1996

 of that type. The set of ranges given by the client should represent
 what types are acceptable given the context of the request.

D.2.2 Accept-Charset

 The Accept-Charset request-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.

D.2.3 Accept-Encoding

 The Accept-Encoding request-header field is similar to Accept, but
 restricts the content-coding values which are acceptable in the
 response.

D.2.4 Accept-Language

 The Accept-Language request-header field is similar to Accept, but
 restricts the set of natural languages that are preferred as a
 response to the request.

D.2.5 Content-Language

 The Content-Language entity-header field describes the natural
 language(s) of the intended audience for the enclosed entity. Note
 that this may not be equivalent to all the languages used within the
 entity.

D.2.6 Link

 The Link entity-header field 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.

D.2.7 MIME-Version

 HTTP messages may include a single MIME-Version general-header field
 to indicate what version of the MIME protocol was used to construct
 the message. Use of the MIME-Version header field, as defined by RFC
 1521 [5], should indicate that the message is MIME-conformant.
 Unfortunately, some older HTTP/1.0 servers send it indiscriminately,
 and thus this field should be ignored.

Berners-Lee, et al Informational [Page 59] RFC 1945 HTTP/1.0 May 1996

D.2.8 Retry-After

 The Retry-After response-header field can be used with a 503 (service
 unavailable) response to indicate how long the service is expected to
 be unavailable to the requesting client. The value of this field can
 be either an HTTP-date or an integer number of seconds (in decimal)
 after the time of the response.

D.2.9 Title

 The Title entity-header field indicates the title of the entity.

D.2.10 URI

 The URI entity-header field may contain some or all of the Uniform
 Resource Identifiers (Section 3.2) by which the Request-URI resource
 can be identified. There is no guarantee that the resource can be
 accessed using the URI(s) specified.

Berners-Lee, et al Informational [Page 60]

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