path: root/doc/rfc/rfc1738.txt

Network Working Group                                     T. Berners-Lee
Request for Comments: 1738                                          CERN
Category: Standards Track                                    L. Masinter
                                                       Xerox Corporation
                                                             M. McCahill
                                                 University of Minnesota
                                                                 Editors
                                                           December 1994


                    Uniform Resource Locators (URL)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document specifies a Uniform Resource Locator (URL), the syntax
   and semantics of formalized information for location and access of
   resources via the Internet.

1. Introduction

   This document describes the syntax and semantics for a compact string
   representation for a resource available via the Internet.  These
   strings are called "Uniform Resource Locators" (URLs).

   The specification is derived from concepts introduced by the World-
   Wide Web global information initiative, whose use of such objects
   dates from 1990 and is described in "Universal Resource Identifiers
   in WWW", RFC 1630. The specification of URLs is designed to meet the
   requirements laid out in "Functional Requirements for Internet
   Resource Locators" [12].

   This document was written by the URI working group of the Internet
   Engineering Task Force.  Comments may be addressed to the editors, or
   to the URI-WG <uri@bunyip.com>. Discussions of the group are archived
   at <URL:http://www.acl.lanl.gov/URI/archive/uri-archive.index.html>








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2. General URL Syntax

   Just as there are many different methods of access to resources,
   there are several schemes for describing the location of such
   resources.

   The generic syntax for URLs provides a framework for new schemes to
   be established using protocols other than those defined in this
   document.

   URLs are used to `locate' resources, by providing an abstract
   identification of the resource location.  Having located a resource,
   a system may perform a variety of operations on the resource, as
   might be characterized by such words as `access', `update',
   `replace', `find attributes'. In general, only the `access' method
   needs to be specified for any URL scheme.

2.1. The main parts of URLs

   A full BNF description of the URL syntax is given in Section 5.

   In general, URLs are written as follows:

       <scheme>:<scheme-specific-part>

   A URL contains the name of the scheme being used (<scheme>) followed
   by a colon and then a string (the <scheme-specific-part>) whose
   interpretation depends on the scheme.

   Scheme names consist of a sequence of characters. The lower case
   letters "a"--"z", digits, and the characters plus ("+"), period
   ("."), and hyphen ("-") are allowed. For resiliency, programs
   interpreting URLs should treat upper case letters as equivalent to
   lower case in scheme names (e.g., allow "HTTP" as well as "http").

2.2. URL Character Encoding Issues

   URLs are sequences of characters, i.e., letters, digits, and special
   characters. A URLs may be represented in a variety of ways: e.g., ink
   on paper, or a sequence of octets in a coded character set. The
   interpretation of a URL depends only on the identity of the
   characters used.

   In most URL schemes, the sequences of characters in different parts
   of a URL are used to represent sequences of octets used in Internet
   protocols. For example, in the ftp scheme, the host name, directory
   name and file names are such sequences of octets, represented by
   parts of the URL.  Within those parts, an octet may be represented by



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   the chararacter which has that octet as its code within the US-ASCII
   [20] coded character set.

   In addition, octets may be encoded by a character triplet consisting
   of the character "%" followed by the two hexadecimal digits (from
   "0123456789ABCDEF") which forming the hexadecimal value of the octet.
   (The characters "abcdef" may also be used in hexadecimal encodings.)

   Octets must be encoded if they have no corresponding graphic
   character within the US-ASCII coded character set, if the use of the
   corresponding character is unsafe, or if the corresponding character
   is reserved for some other interpretation within the particular URL
   scheme.

   No corresponding graphic US-ASCII:

   URLs are written only with the graphic printable characters of the
   US-ASCII coded character set. The octets 80-FF hexadecimal are not
   used in US-ASCII, and the octets 00-1F and 7F hexadecimal represent
   control characters; these must be encoded.

   Unsafe:

   Characters can be unsafe for a number of reasons.  The space
   character is unsafe because significant spaces may disappear and
   insignificant spaces may be introduced when URLs are transcribed or
   typeset or subjected to the treatment of word-processing programs.
   The characters "<" and ">" are unsafe because they are used as the
   delimiters around URLs in free text; the quote mark (""") is used to
   delimit URLs in some systems.  The character "#" is unsafe and should
   always be encoded because it is used in World Wide Web and in other
   systems to delimit a URL from a fragment/anchor identifier that might
   follow it.  The character "%" is unsafe because it is used for
   encodings of other characters.  Other characters are unsafe because
   gateways and other transport agents are known to sometimes modify
   such characters. These characters are "{", "}", "|", "\", "^", "~",
   "[", "]", and "`".

   All unsafe characters must always be encoded within a URL. For
   example, the character "#" must be encoded within URLs even in
   systems that do not normally deal with fragment or anchor
   identifiers, so that if the URL is copied into another system that
   does use them, it will not be necessary to change the URL encoding.








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   Reserved:

   Many URL schemes reserve certain characters for a special meaning:
   their appearance in the scheme-specific part of the URL has a
   designated semantics. If the character corresponding to an octet is
   reserved in a scheme, the octet must be encoded.  The characters ";",
   "/", "?", ":", "@", "=" and "&" are the characters which may be
   reserved for special meaning within a scheme. No other characters may
   be reserved within a scheme.

   Usually a URL has the same interpretation when an octet is
   represented by a character and when it encoded. However, this is not
   true for reserved characters: encoding a character reserved for a
   particular scheme may change the semantics of a URL.

   Thus, only alphanumerics, the special characters "$-_.+!*'(),", and
   reserved characters used for their reserved purposes may be used
   unencoded within a URL.

   On the other hand, characters that are not required to be encoded
   (including alphanumerics) may be encoded within the scheme-specific
   part of a URL, as long as they are not being used for a reserved
   purpose.

2.3 Hierarchical schemes and relative links

   In some cases, URLs are used to locate resources that contain
   pointers to other resources. In some cases, those pointers are
   represented as relative links where the expression of the location of
   the second resource is in terms of "in the same place as this one
   except with the following relative path". Relative links are not
   described in this document. However, the use of relative links
   depends on the original URL containing a hierarchical structure
   against which the relative link is based.

   Some URL schemes (such as the ftp, http, and file schemes) contain
   names that can be considered hierarchical; the components of the
   hierarchy are separated by "/".













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3. Specific Schemes

   The mapping for some existing standard and experimental protocols is
   outlined in the BNF syntax definition.  Notes on particular protocols
   follow. The schemes covered are:

   ftp                     File Transfer protocol
   http                    Hypertext Transfer Protocol
   gopher                  The Gopher protocol
   mailto                  Electronic mail address
   news                    USENET news
   nntp                    USENET news using NNTP access
   telnet                  Reference to interactive sessions
   wais                    Wide Area Information Servers
   file                    Host-specific file names
   prospero                Prospero Directory Service

   Other schemes may be specified by future specifications. Section 4 of
   this document describes how new schemes may be registered, and lists
   some scheme names that are under development.

3.1. Common Internet Scheme Syntax

   While the syntax for the rest of the URL may vary depending on the
   particular scheme selected, URL schemes that involve the direct use
   of an IP-based protocol to a specified host on the Internet use a
   common syntax for the scheme-specific data:

        //<user>:<password>@<host>:<port>/<url-path>

   Some or all of the parts "<user>:<password>@", ":<password>",
   ":<port>", and "/<url-path>" may be excluded.  The scheme specific
   data start with a double slash "//" to indicate that it complies with
   the common Internet scheme syntax. The different components obey the
   following rules:

    user
        An optional user name. Some schemes (e.g., ftp) allow the
        specification of a user name.

    password
        An optional password. If present, it follows the user
        name separated from it by a colon.

   The user name (and password), if present, are followed by a
   commercial at-sign "@". Within the user and password field, any ":",
   "@", or "/" must be encoded.




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   Note that an empty user name or password is different than no user
   name or password; there is no way to specify a password without
   specifying a user name. E.g., <URL:ftp://@host.com/> has an empty
   user name and no password, <URL:ftp://host.com/> has no user name,
   while <URL:ftp://foo:@host.com/> has a user name of "foo" and an
   empty password.

    host
        The fully qualified domain name of a network host, or its IP
        address as a set of four decimal digit groups separated by
        ".". Fully qualified domain names take the form as described
        in Section 3.5 of RFC 1034 [13] and Section 2.1 of RFC 1123
        [5]: a sequence of domain labels separated by ".", each domain
        label starting and ending with an alphanumerical character and
        possibly also containing "-" characters. The rightmost domain
        label will never start with a digit, though, which
        syntactically distinguishes all domain names from the IP
        addresses.

    port
        The port number to connect to. Most schemes designate
        protocols that have a default port number. Another port number
        may optionally be supplied, in decimal, separated from the
        host by a colon. If the port is omitted, the colon is as well.

    url-path
        The rest of the locator consists of data specific to the
        scheme, and is known as the "url-path". It supplies the
        details of how the specified resource can be accessed. Note
        that the "/" between the host (or port) and the url-path is
        NOT part of the url-path.

   The url-path syntax depends on the scheme being used, as does the
   manner in which it is interpreted.

3.2. FTP

   The FTP URL scheme is used to designate files and directories on
   Internet hosts accessible using the FTP protocol (RFC959).

   A FTP URL follow the syntax described in Section 3.1.  If :<port> is
   omitted, the port defaults to 21.









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3.2.1. FTP Name and Password

   A user name and password may be supplied; they are used in the ftp
   "USER" and "PASS" commands after first making the connection to the
   FTP server.  If no user name or password is supplied and one is
   requested by the FTP server, the conventions for "anonymous" FTP are
   to be used, as follows:

        The user name "anonymous" is supplied.

        The password is supplied as the Internet e-mail address
        of the end user accessing the resource.

   If the URL supplies a user name but no password, and the remote
   server requests a password, the program interpreting the FTP URL
   should request one from the user.

3.2.2. FTP url-path

   The url-path of a FTP URL has the following syntax:

        <cwd1>/<cwd2>/.../<cwdN>/<name>;type=<typecode>

   Where <cwd1> through <cwdN> and <name> are (possibly encoded) strings
   and <typecode> is one of the characters "a", "i", or "d".  The part
   ";type=<typecode>" may be omitted. The <cwdx> and <name> parts may be
   empty. The whole url-path may be omitted, including the "/"
   delimiting it from the prefix containing user, password, host, and
   port.

   The url-path is interpreted as a series of FTP commands as follows:

      Each of the <cwd> elements is to be supplied, sequentially, as the
      argument to a CWD (change working directory) command.

      If the typecode is "d", perform a NLST (name list) command with
      <name> as the argument, and interpret the results as a file
      directory listing.

      Otherwise, perform a TYPE command with <typecode> as the argument,
      and then access the file whose name is <name> (for example, using
      the RETR command.)

   Within a name or CWD component, the characters "/" and ";" are
   reserved and must be encoded. The components are decoded prior to
   their use in the FTP protocol.  In particular, if the appropriate FTP
   sequence to access a particular file requires supplying a string
   containing a "/" as an argument to a CWD or RETR command, it is



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   necessary to encode each "/".

   For example, the URL <URL:ftp://myname@host.dom/%2Fetc/motd> is
   interpreted by FTP-ing to "host.dom", logging in as "myname"
   (prompting for a password if it is asked for), and then executing
   "CWD /etc" and then "RETR motd". This has a different meaning from
   <URL:ftp://myname@host.dom/etc/motd> which would "CWD etc" and then
   "RETR motd"; the initial "CWD" might be executed relative to the
   default directory for "myname". On the other hand,
   <URL:ftp://myname@host.dom//etc/motd>, would "CWD " with a null
   argument, then "CWD etc", and then "RETR motd".

   FTP URLs may also be used for other operations; for example, it is
   possible to update a file on a remote file server, or infer
   information about it from the directory listings. The mechanism for
   doing so is not spelled out here.

3.2.3. FTP Typecode is Optional

   The entire ;type=<typecode> part of a FTP URL is optional. If it is
   omitted, the client program interpreting the URL must guess the
   appropriate mode to use. In general, the data content type of a file
   can only be guessed from the name, e.g., from the suffix of the name;
   the appropriate type code to be used for transfer of the file can
   then be deduced from the data content of the file.

3.2.4 Hierarchy

   For some file systems, the "/" used to denote the hierarchical
   structure of the URL corresponds to the delimiter used to construct a
   file name hierarchy, and thus, the filename will look similar to the
   URL path. This does NOT mean that the URL is a Unix filename.

3.2.5. Optimization

   Clients accessing resources via FTP may employ additional heuristics
   to optimize the interaction. For some FTP servers, for example, it
   may be reasonable to keep the control connection open while accessing
   multiple URLs from the same server. However, there is no common
   hierarchical model to the FTP protocol, so if a directory change
   command has been given, it is impossible in general to deduce what
   sequence should be given to navigate to another directory for a
   second retrieval, if the paths are different.  The only reliable
   algorithm is to disconnect and reestablish the control connection.







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3.3. HTTP

   The HTTP URL scheme is used to designate Internet resources
   accessible using HTTP (HyperText Transfer Protocol).

   The HTTP protocol is specified elsewhere. This specification only
   describes the syntax of HTTP URLs.

   An HTTP URL takes the form:

      http://<host>:<port>/<path>?<searchpart>

   where <host> and <port> are as described in Section 3.1. If :<port>
   is omitted, the port defaults to 80.  No user name or password is
   allowed.  <path> is an HTTP selector, and <searchpart> is a query
   string. The <path> is optional, as is the <searchpart> and its
   preceding "?". If neither <path> nor <searchpart> is present, the "/"
   may also be omitted.

   Within the <path> and <searchpart> components, "/", ";", "?" are
   reserved.  The "/" character may be used within HTTP to designate a
   hierarchical structure.

3.4. GOPHER

   The Gopher URL scheme is used to designate Internet resources
   accessible using the Gopher protocol.

   The base Gopher protocol is described in RFC 1436 and supports items
   and collections of items (directories). The Gopher+ protocol is a set
   of upward compatible extensions to the base Gopher protocol and is
   described in [2]. Gopher+ supports associating arbitrary sets of
   attributes and alternate data representations with Gopher items.
   Gopher URLs accommodate both Gopher and Gopher+ items and item
   attributes.

3.4.1. Gopher URL syntax

   A Gopher URL takes the form:

      gopher://<host>:<port>/<gopher-path>

   where <gopher-path> is one of

       <gophertype><selector>
       <gophertype><selector>%09<search>
       <gophertype><selector>%09<search>%09<gopher+_string>




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   If :<port> is omitted, the port defaults to 70.  <gophertype> is a
   single-character field to denote the Gopher type of the resource to
   which the URL refers. The entire <gopher-path> may also be empty, in
   which case the delimiting "/" is also optional and the <gophertype>
   defaults to "1".

   <selector> is the Gopher selector string.  In the Gopher protocol,
   Gopher selector strings are a sequence of octets which may contain
   any octets except 09 hexadecimal (US-ASCII HT or tab) 0A hexadecimal
   (US-ASCII character LF), and 0D (US-ASCII character CR).

   Gopher clients specify which item to retrieve by sending the Gopher
   selector string to a Gopher server.

   Within the <gopher-path>, no characters are reserved.

   Note that some Gopher <selector> strings begin with a copy of the
   <gophertype> character, in which case that character will occur twice
   consecutively. The Gopher selector string may be an empty string;
   this is how Gopher clients refer to the top-level directory on a
   Gopher server.

3.4.2 Specifying URLs for Gopher Search Engines

   If the URL refers to a search to be submitted to a Gopher search
   engine, the selector is followed by an encoded tab (%09) and the
   search string. To submit a search to a Gopher search engine, the
   Gopher client sends the <selector> string (after decoding), a tab,
   and the search string to the Gopher server.

3.4.3 URL syntax for Gopher+ items

   URLs for Gopher+ items have a second encoded tab (%09) and a Gopher+
   string. Note that in this case, the %09<search> string must be
   supplied, although the <search> element may be the empty string.

   The <gopher+_string> is used to represent information required for
   retrieval of the Gopher+ item. Gopher+ items may have alternate
   views, arbitrary sets of attributes, and may have electronic forms
   associated with them.

   To retrieve the data associated with a Gopher+ URL, a client will
   connect to the server and send the Gopher selector, followed by a tab
   and the search string (which may be empty), followed by a tab and the
   Gopher+ commands.






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3.4.4 Default Gopher+ data representation

   When a Gopher server returns a directory listing to a client, the
   Gopher+ items are tagged with either a "+" (denoting Gopher+ items)
   or a "?" (denoting Gopher+ items which have a +ASK form associated
   with them). A Gopher URL with a Gopher+ string consisting of only a
   "+" refers to the default view (data representation) of the item
   while a Gopher+ string containing only a "?" refer to an item with a
   Gopher electronic form associated with it.

3.4.5 Gopher+ items with electronic forms

   Gopher+ items which have a +ASK associated with them (i.e. Gopher+
   items tagged with a "?") require the client to fetch the item's +ASK
   attribute to get the form definition, and then ask the user to fill
   out the form and return the user's responses along with the selector
   string to retrieve the item.  Gopher+ clients know how to do this but
   depend on the "?" tag in the Gopher+ item description to know when to
   handle this case. The "?" is used in the Gopher+ string to be
   consistent with Gopher+ protocol's use of this symbol.

3.4.6 Gopher+ item attribute collections

   To refer to the Gopher+ attributes of an item, the Gopher URL's
   Gopher+ string consists of "!" or "$". "!" refers to the all of a
   Gopher+ item's attributes. "$" refers to all the item attributes for
   all items in a Gopher directory.

3.4.7 Referring to specific Gopher+ attributes

   To refer to specific attributes, the URL's gopher+_string is
   "!<attribute_name>" or "$<attribute_name>". For example, to refer to
   the attribute containing the abstract of an item, the gopher+_string
   would be "!+ABSTRACT".

   To refer to several attributes, the gopher+_string consists of the
   attribute names separated by coded spaces. For example,
   "!+ABSTRACT%20+SMELL" refers to the +ABSTRACT and +SMELL attributes
   of an item.

3.4.8 URL syntax for Gopher+ alternate views

   Gopher+ allows for optional alternate data representations (alternate
   views) of items. To retrieve a Gopher+ alternate view, a Gopher+
   client sends the appropriate view and language identifier (found in
   the item's +VIEW attribute). To refer to a specific Gopher+ alternate
   view, the URL's Gopher+ string would be in the form:




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        +<view_name>%20<language_name>

   For example, a Gopher+ string of "+application/postscript%20Es_ES"
   refers to the Spanish language postscript alternate view of a Gopher+
   item.

3.4.9 URL syntax for Gopher+ electronic forms

   The gopher+_string for a URL that refers to an item referenced by a
   Gopher+ electronic form (an ASK block) filled out with specific
   values is a coded version of what the client sends to the server.
   The gopher+_string is of the form:

+%091%0D%0A+-1%0D%0A<ask_item1_value>%0D%0A<ask_item2_value>%0D%0A.%0D%0A

   To retrieve this item, the Gopher client sends:

       <a_gopher_selector><tab>+<tab>1<cr><lf>
       +-1<cr><lf>
       <ask_item1_value><cr><lf>
       <ask_item2_value><cr><lf>
       .<cr><lf>

   to the Gopher server.

3.5. MAILTO

   The mailto URL scheme is used to designate the Internet mailing
   address of an individual or service. No additional information other
   than an Internet mailing address is present or implied.

   A mailto URL takes the form:

        mailto:<rfc822-addr-spec>

   where <rfc822-addr-spec> is (the encoding of an) addr-spec, as
   specified in RFC 822 [6]. Within mailto URLs, there are no reserved
   characters.

   Note that the percent sign ("%") is commonly used within RFC 822
   addresses and must be encoded.

   Unlike many URLs, the mailto scheme does not represent a data object
   to be accessed directly; there is no sense in which it designates an
   object. It has a different use than the message/external-body type in
   MIME.





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3.6. NEWS

   The news URL scheme is used to refer to either news groups or
   individual articles of USENET news, as specified in RFC 1036.

   A news URL takes one of two forms:

     news:<newsgroup-name>
     news:<message-id>

   A <newsgroup-name> is a period-delimited hierarchical name, such as
   "comp.infosystems.www.misc". A <message-id> corresponds to the
   Message-ID of section 2.1.5 of RFC 1036, without the enclosing "<"
   and ">"; it takes the form <unique>@<full_domain_name>.  A message
   identifier may be distinguished from a news group name by the
   presence of the commercial at "@" character. No additional characters
   are reserved within the components of a news URL.

   If <newsgroup-name> is "*" (as in <URL:news:*>), it is used to refer
   to "all available news groups".

   The news URLs are unusual in that by themselves, they do not contain
   sufficient information to locate a single resource, but, rather, are
   location-independent.

3.7. NNTP

   The nntp URL scheme is an alternative method of referencing news
   articles, useful for specifying news articles from NNTP servers (RFC
   977).

   A nntp URL take the form:

      nntp://<host>:<port>/<newsgroup-name>/<article-number>

   where <host> and <port> are as described in Section 3.1. If :<port>
   is omitted, the port defaults to 119.

   The <newsgroup-name> is the name of the group, while the <article-
   number> is the numeric id of the article within that newsgroup.

   Note that while nntp: URLs specify a unique location for the article
   resource, most NNTP servers currently on the Internet today are
   configured only to allow access from local clients, and thus nntp
   URLs do not designate globally accessible resources. Thus, the news:
   form of URL is preferred as a way of identifying news articles.





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3.8. TELNET

   The Telnet URL scheme is used to designate interactive services that
   may be accessed by the Telnet protocol.

   A telnet URL takes the form:

       telnet://<user>:<password>@<host>:<port>/

   as specified in Section 3.1. The final "/" character may be omitted.
   If :<port> is omitted, the port defaults to 23.  The :<password> can
   be omitted, as well as the whole <user>:<password> part.

   This URL does not designate a data object, but rather an interactive
   service. Remote interactive services vary widely in the means by
   which they allow remote logins; in practice, the <user> and
   <password> supplied are advisory only: clients accessing a telnet URL
   merely advise the user of the suggested username and password.

3.9.  WAIS

   The WAIS URL scheme is used to designate WAIS databases, searches, or
   individual documents available from a WAIS database. WAIS is
   described in [7]. The WAIS protocol is described in RFC 1625 [17];
   Although the WAIS protocol is based on Z39.50-1988, the WAIS URL
   scheme is not intended for use with arbitrary Z39.50 services.

   A WAIS URL takes one of the following forms:

     wais://<host>:<port>/<database>
     wais://<host>:<port>/<database>?<search>
     wais://<host>:<port>/<database>/<wtype>/<wpath>

   where <host> and <port> are as described in Section 3.1. If :<port>
   is omitted, the port defaults to 210.  The first form designates a
   WAIS database that is available for searching. The second form
   designates a particular search.  <database> is the name of the WAIS
   database being queried.

   The third form designates a particular document within a WAIS
   database to be retrieved. In this form <wtype> is the WAIS
   designation of the type of the object. Many WAIS implementations
   require that a client know the "type" of an object prior to
   retrieval, the type being returned along with the internal object
   identifier in the search response.  The <wtype> is included in the
   URL in order to allow the client interpreting the URL adequate
   information to actually retrieve the document.




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   The <wpath> of a WAIS URL consists of the WAIS document-id, encoded
   as necessary using the method described in Section 2.2. The WAIS
   document-id should be treated opaquely; it may only be decomposed by
   the server that issued it.

3.10 FILES

   The file URL scheme is used to designate files accessible on a
   particular host computer. This scheme, unlike most other URL schemes,
   does not designate a resource that is universally accessible over the
   Internet.

   A file URL takes the form:

       file://<host>/<path>

   where <host> is the fully qualified domain name of the system on
   which the <path> is accessible, and <path> is a hierarchical
   directory path of the form <directory>/<directory>/.../<name>.

   For example, a VMS file

     DISK$USER:[MY.NOTES]NOTE123456.TXT

   might become

     <URL:file://vms.host.edu/disk$user/my/notes/note12345.txt>

   As a special case, <host> can be the string "localhost" or the empty
   string; this is interpreted as `the machine from which the URL is
   being interpreted'.

   The file URL scheme is unusual in that it does not specify an
   Internet protocol or access method for such files; as such, its
   utility in network protocols between hosts is limited.

3.11 PROSPERO

   The Prospero URL scheme is used to designate resources that are
   accessed via the Prospero Directory Service. The Prospero protocol is
   described elsewhere [14].

   A prospero URLs takes the form:

      prospero://<host>:<port>/<hsoname>;<field>=<value>

   where <host> and <port> are as described in Section 3.1. If :<port>
   is omitted, the port defaults to 1525. No username or password is



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   allowed.

   The <hsoname> is the host-specific object name in the Prospero
   protocol, suitably encoded.  This name is opaque and interpreted by
   the Prospero server.  The semicolon ";" is reserved and may not
   appear without quoting in the <hsoname>.

   Prospero URLs are interpreted by contacting a Prospero directory
   server on the specified host and port to determine appropriate access
   methods for a resource, which might themselves be represented as
   different URLs. External Prospero links are represented as URLs of
   the underlying access method and are not represented as Prospero
   URLs.

   Note that a slash "/" may appear in the <hsoname> without quoting and
   no significance may be assumed by the application.  Though slashes
   may indicate hierarchical structure on the server, such structure is
   not guaranteed. Note that many <hsoname>s begin with a slash, in
   which case the host or port will be followed by a double slash: the
   slash from the URL syntax, followed by the initial slash from the
   <hsoname>. (E.g., <URL:prospero://host.dom//pros/name> designates a
   <hsoname> of "/pros/name".)

   In addition, after the <hsoname>, optional fields and values
   associated with a Prospero link may be specified as part of the URL.
   When present, each field/value pair is separated from each other and
   from the rest of the URL by a ";" (semicolon).  The name of the field
   and its value are separated by a "=" (equal sign). If present, these
   fields serve to identify the target of the URL.  For example, the
   OBJECT-VERSION field can be specified to identify a specific version
   of an object.

4. REGISTRATION OF NEW SCHEMES

   A new scheme may be introduced by defining a mapping onto a
   conforming URL syntax, using a new prefix. URLs for experimental
   schemes may be used by mutual agreement between parties. Scheme names
   starting with the characters "x-" are reserved for experimental
   purposes.

   The Internet Assigned Numbers Authority (IANA) will maintain a
   registry of URL schemes. Any submission of a new URL scheme must
   include a definition of an algorithm for accessing of resources
   within that scheme and the syntax for representing such a scheme.

   URL schemes must have demonstrable utility and operability.  One way
   to provide such a demonstration is via a gateway which provides
   objects in the new scheme for clients using an existing protocol.  If



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   the new scheme does not locate resources that are data objects, the
   properties of names in the new space must be clearly defined.

   New schemes should try to follow the same syntactic conventions of
   existing schemes, where appropriate.  It is likewise recommended
   that, where a protocol allows for retrieval by URL, that the client
   software have provision for being configured to use specific gateway
   locators for indirect access through new naming schemes.

   The following scheme have been proposed at various times, but this
   document does not define their syntax or use at this time. It is
   suggested that IANA reserve their scheme names for future definition:

   afs              Andrew File System global file names.
   mid              Message identifiers for electronic mail.
   cid              Content identifiers for MIME body parts.
   nfs              Network File System (NFS) file names.
   tn3270           Interactive 3270 emulation sessions.
   mailserver       Access to data available from mail servers.
   z39.50           Access to ANSI Z39.50 services.

5. BNF for specific URL schemes

   This is a BNF-like description of the Uniform Resource Locator
   syntax, using the conventions of RFC822, except that "|" is used to
   designate alternatives, and brackets [] are used around optional or
   repeated elements. Briefly, literals are quoted with "", optional
   elements are enclosed in [brackets], and elements may be preceded
   with <n>* to designate n or more repetitions of the following
   element; n defaults to 0.

; The generic form of a URL is:

genericurl     = scheme ":" schemepart

; Specific predefined schemes are defined here; new schemes
; may be registered with IANA

url            = httpurl | ftpurl | newsurl |
                 nntpurl | telneturl | gopherurl |
                 waisurl | mailtourl | fileurl |
                 prosperourl | otherurl

; new schemes follow the general syntax
otherurl       = genericurl

; the scheme is in lower case; interpreters should use case-ignore
scheme         = 1*[ lowalpha | digit | "+" | "-" | "." ]



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schemepart     = *xchar | ip-schemepart


; URL schemeparts for ip based protocols:

ip-schemepart  = "//" login [ "/" urlpath ]

login          = [ user [ ":" password ] "@" ] hostport
hostport       = host [ ":" port ]
host           = hostname | hostnumber
hostname       = *[ domainlabel "." ] toplabel
domainlabel    = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit
toplabel       = alpha | alpha *[ alphadigit | "-" ] alphadigit
alphadigit     = alpha | digit
hostnumber     = digits "." digits "." digits "." digits
port           = digits
user           = *[ uchar | ";" | "?" | "&" | "=" ]
password       = *[ uchar | ";" | "?" | "&" | "=" ]
urlpath        = *xchar    ; depends on protocol see section 3.1

; The predefined schemes:

; FTP (see also RFC959)

ftpurl         = "ftp://" login [ "/" fpath [ ";type=" ftptype ]]
fpath          = fsegment *[ "/" fsegment ]
fsegment       = *[ uchar | "?" | ":" | "@" | "&" | "=" ]
ftptype        = "A" | "I" | "D" | "a" | "i" | "d"

; FILE

fileurl        = "file://" [ host | "localhost" ] "/" fpath

; HTTP

httpurl        = "http://" hostport [ "/" hpath [ "?" search ]]
hpath          = hsegment *[ "/" hsegment ]
hsegment       = *[ uchar | ";" | ":" | "@" | "&" | "=" ]
search         = *[ uchar | ";" | ":" | "@" | "&" | "=" ]

; GOPHER (see also RFC1436)

gopherurl      = "gopher://" hostport [ / [ gtype [ selector
                 [ "%09" search [ "%09" gopher+_string ] ] ] ] ]
gtype          = xchar
selector       = *xchar
gopher+_string = *xchar




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; MAILTO (see also RFC822)

mailtourl      = "mailto:" encoded822addr
encoded822addr = 1*xchar               ; further defined in RFC822

; NEWS (see also RFC1036)

newsurl        = "news:" grouppart
grouppart      = "*" | group | article
group          = alpha *[ alpha | digit | "-" | "." | "+" | "_" ]
article        = 1*[ uchar | ";" | "/" | "?" | ":" | "&" | "=" ] "@" host

; NNTP (see also RFC977)

nntpurl        = "nntp://" hostport "/" group [ "/" digits ]

; TELNET

telneturl      = "telnet://" login [ "/" ]

; WAIS (see also RFC1625)

waisurl        = waisdatabase | waisindex | waisdoc
waisdatabase   = "wais://" hostport "/" database
waisindex      = "wais://" hostport "/" database "?" search
waisdoc        = "wais://" hostport "/" database "/" wtype "/" wpath
database       = *uchar
wtype          = *uchar
wpath          = *uchar

; PROSPERO

prosperourl    = "prospero://" hostport "/" ppath *[ fieldspec ]
ppath          = psegment *[ "/" psegment ]
psegment       = *[ uchar | "?" | ":" | "@" | "&" | "=" ]
fieldspec      = ";" fieldname "=" fieldvalue
fieldname      = *[ uchar | "?" | ":" | "@" | "&" ]
fieldvalue     = *[ uchar | "?" | ":" | "@" | "&" ]

; Miscellaneous definitions

lowalpha       = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                 "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                 "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                 "y" | "z"
hialpha        = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                 "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                 "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"



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alpha          = lowalpha | hialpha
digit          = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                 "8" | "9"
safe           = "$" | "-" | "_" | "." | "+"
extra          = "!" | "*" | "'" | "(" | ")" | ","
national       = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
punctuation    = "<" | ">" | "#" | "%" | <">


reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="
hex            = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                 "a" | "b" | "c" | "d" | "e" | "f"
escape         = "%" hex hex

unreserved     = alpha | digit | safe | extra
uchar          = unreserved | escape
xchar          = unreserved | reserved | escape
digits         = 1*digit

6. Security Considerations

   The URL scheme does not in itself pose a security threat. Users
   should beware that there is no general guarantee that a URL which at
   one time points to a given object continues to do so, and does not
   even at some later time point to a different object due to the
   movement of objects on servers.

   A URL-related security threat is that it is sometimes possible to
   construct a URL such that an attempt to perform a harmless idempotent
   operation such as the retrieval of the object will in fact cause a
   possibly damaging remote operation to occur.  The unsafe URL is
   typically constructed by specifying a port number other than that
   reserved for the network protocol in question.  The client
   unwittingly contacts a server which is in fact running a different
   protocol.  The content of the URL contains instructions which when
   interpreted according to this other protocol cause an unexpected
   operation. An example has been the use of gopher URLs to cause a rude
   message to be sent via a SMTP server.  Caution should be used when
   using any URL which specifies a port number other than the default
   for the protocol, especially when it is a number within the reserved
   space.

   Care should be taken when URLs contain embedded encoded delimiters
   for a given protocol (for example, CR and LF characters for telnet
   protocols) that these are not unencoded before transmission.  This
   would violate the protocol but could be used to simulate an extra
   operation or parameter, again causing an unexpected and possible
   harmful remote operation to be performed.



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   The use of URLs containing passwords that should be secret is clearly
   unwise.

7. Acknowledgements

   This paper builds on the basic WWW design (RFC 1630) and much
   discussion of these issues by many people on the network. The
   discussion was particularly stimulated by articles by Clifford Lynch,
   Brewster Kahle [10] and Wengyik Yeong [18]. Contributions from John
   Curran, Clifford Neuman, Ed Vielmetti and later the IETF URL BOF and
   URI working group were incorporated.

   Most recently, careful readings and comments by Dan Connolly, Ned
   Freed, Roy Fielding, Guido van Rossum, Michael Dolan, Bert Bos, John
   Kunze, Olle Jarnefors, Peter Svanberg and many others have helped
   refine this RFC.



































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APPENDIX: Recommendations for URLs in Context

   URIs, including URLs, are intended to be transmitted through
   protocols which provide a context for their interpretation.

   In some cases, it will be necessary to distinguish URLs from other
   possible data structures in a syntactic structure. In this case, is
   recommended that URLs be preceeded with a prefix consisting of the
   characters "URL:". For example, this prefix may be used to
   distinguish URLs from other kinds of URIs.

   In addition, there are many occasions when URLs are included in other
   kinds of text; examples include electronic mail, USENET news
   messages, or printed on paper. In such cases, it is convenient to
   have a separate syntactic wrapper that delimits the URL and separates
   it from the rest of the text, and in particular from punctuation
   marks that might be mistaken for part of the URL. For this purpose,
   is recommended that angle brackets ("<" and ">"), along with the
   prefix "URL:", be used to delimit the boundaries of the URL.  This
   wrapper does not form part of the URL and should not be used in
   contexts in which delimiters are already specified.

   In the case where a fragment/anchor identifier is associated with a
   URL (following a "#"), the identifier would be placed within the
   brackets as well.

   In some cases, extra whitespace (spaces, linebreaks, tabs, etc.) may
   need to be added to break long URLs across lines.  The whitespace
   should be ignored when extracting the URL.

   No whitespace should be introduced after a hyphen ("-") character.
   Because some typesetters and printers may (erroneously) introduce a
   hyphen at the end of line when breaking a line, the interpreter of a
   URL containing a line break immediately after a hyphen should ignore
   all unencoded whitespace around the line break, and should be aware
   that the hyphen may or may not actually be part of the URL.

   Examples:

      Yes, Jim, I found it under <URL:ftp://info.cern.ch/pub/www/doc;
      type=d> but you can probably pick it up from <URL:ftp://ds.in
      ternic.net/rfc>.  Note the warning in <URL:http://ds.internic.
      net/instructions/overview.html#WARNING>.








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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.
       <URL:ftp://ds.internic.net/rfc/rfc1436.txt;type=a>

   [2] Anklesaria, F., Lindner, P., McCahill, M., Torrey, D.,
       Johnson, D., and B. Alberti, "Gopher+: Upward compatible
       enhancements to the Internet Gopher protocol",
       University of Minnesota, July 1993.
       <URL:ftp://boombox.micro.umn.edu/pub/gopher/gopher_protocol
       /Gopher+/Gopher+.txt>

   [3] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
       Unifying Syntax for the Expression of Names and Addresses of
       Objects on the Network as used in the World-Wide Web", RFC
       1630, CERN, June 1994.
       <URL:ftp://ds.internic.net/rfc/rfc1630.txt>

   [4] Berners-Lee, T., "Hypertext Transfer Protocol (HTTP)",
       CERN, November 1993.
       <URL:ftp://info.cern.ch/pub/www/doc/http-spec.txt.Z>

   [5] Braden, R., Editor, "Requirements for Internet Hosts --
       Application and Support", STD 3, RFC 1123, IETF, October 1989.
       <URL:ftp://ds.internic.net/rfc/rfc1123.txt>

   [6] Crocker, D. "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, UDEL, April 1982.
       <URL:ftp://ds.internic.net/rfc/rfc822.txt>

   [7] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
       Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
       Functional Specification", (v1.5), Thinking Machines
       Corporation, April 1990.
       <URL:ftp://quake.think.com/pub/wais/doc/protspec.txt>

   [8] Horton, M. and R. Adams, "Standard For Interchange of USENET
       Messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic
       Studies, December 1987.
       <URL:ftp://ds.internic.net/rfc/rfc1036.txt>

   [9] Huitema, C., "Naming: Strategies and Techniques", Computer
       Networks and ISDN Systems 23 (1991) 107-110.





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  [10] Kahle, B., "Document Identifiers, or International Standard
       Book Numbers for the Electronic Age", 1991.
       <URL:ftp://quake.think.com/pub/wais/doc/doc-ids.txt>

  [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.
       <URL:ftp://ds.internic.net/rfc/rfc977.txt>

  [12] Kunze, J., "Functional Requirements for Internet Resource
       Locators", Work in Progress, December 1994.
       <URL:ftp://ds.internic.net/internet-drafts
       /draft-ietf-uri-irl-fun-req-02.txt>

  [13] Mockapetris, P., "Domain Names - Concepts and Facilities",
       STD 13, RFC 1034, USC/Information Sciences Institute,
       November 1987.
       <URL:ftp://ds.internic.net/rfc/rfc1034.txt>

  [14] Neuman, B., and S. Augart, "The Prospero Protocol",
       USC/Information Sciences Institute, June 1993.
       <URL:ftp://prospero.isi.edu/pub/prospero/doc
       /prospero-protocol.PS.Z>

  [15] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)",
       STD 9, RFC 959, USC/Information Sciences Institute,
       October 1985.
       <URL:ftp://ds.internic.net/rfc/rfc959.txt>

  [16] Sollins, K. and L. Masinter, "Functional Requirements for
       Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
       December 1994.
       <URL:ftp://ds.internic.net/rfc/rfc1737.txt>

  [17] St. Pierre, M, Fullton, J., Gamiel, K., Goldman, J., Kahle, B.,
       Kunze, J., Morris, H., and F. Schiettecatte, "WAIS over
       Z39.50-1988", RFC 1625, WAIS, Inc., CNIDR, Thinking Machines
       Corp., UC Berkeley, FS Consulting, June 1994.
       <URL:ftp://ds.internic.net/rfc/rfc1625.txt>

  [18] Yeong, W. "Towards Networked Information Retrieval", Technical
       report 91-06-25-01, Performance Systems International, Inc.
       <URL:ftp://uu.psi.com/wp/nir.txt>, June 1991.

  [19] Yeong, W., "Representing Public Archives in the Directory",
       Work in Progress, November 1991.





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  [20] "Coded Character Set -- 7-bit American Standard Code for
       Information Interchange", ANSI X3.4-1986.

Editors' Addresses

Tim Berners-Lee
World-Wide Web project
CERN,
1211 Geneva 23,
Switzerland

Phone: +41 (22)767 3755
Fax: +41 (22)767 7155
EMail: timbl@info.cern.ch


Larry Masinter
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94034

Phone: (415) 812-4365
Fax: (415) 812-4333
EMail: masinter@parc.xerox.com


Mark McCahill
Computer and Information Services,
University of Minnesota
Room 152 Shepherd Labs
100 Union Street SE
Minneapolis, MN 55455

Phone: (612) 625 1300
EMail: mpm@boombox.micro.umn.edu
















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