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Versions: 07 RFC 1738

Uniform Resource Locators                                 T. Berners-Lee
draft-ietf-uri-url-07.txt                                    L. Masinter
Expires March 13, 1995                                       M. McCahill
                                                                 Editors
                                                       September 7, 1994

                  Uniform Resource Locators (URL)

Status of this memo

     This document is an Internet-Draft.  Internet-Drafts are
     working documents of the Internet Engineering Task Force
     (IETF), its areas, and its working groups.  Note that other
     groups may also distribute working documents as
     Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months.  Internet-Drafts may be updated, replaced, or obsoleted
     by other documents at any time.  It is not appropriate to use
     Internet-Drafts as reference material or to cite them other
     than as a ``working draft'' or ``work in progress.''

     To learn the current status of any Internet-Draft, please check
     the 1id-abstracts.txt listing contained in the Internet-Drafts
     Shadow Directories on ds.internic.net, nic.nordu.net,
     ftp.isi.edu, or munnari.oz.au.

     This Internet Draft expires April 7, 1995.

0. 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

   The specification of a Uniform Resource Locator, defined in this
   document, 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 is based on the "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 editor,
   Tim Berners-Lee <timbl@info.cern.ch>, 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>

2. Definitions

   This document describes the syntax for "Uniform Resource Locators"
   (URLs): a compact representation of the location and access method
   for a resource available via the Internet. 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 provides a framework for new URL schemes to be
   established using as yet undefined protocols.

   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. URL SYNTAX

   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 lower case letters "a"--"z", digits, and
   the characters plus ("+"), period ("."), and hyphen ("-"). 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").

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

2.2. Encoding of reserved and unsafe characters

   URLs are represented as a sequence of characters taken from the NVT
   ASCII character set.

   Characters and other 8-bit bytes are _encoded_ by representing them
   as a percent sign "%" followed by two hexadecimal digits (0-9,
   A-F).

   In any circumstance, only printable ASCII characters are allowed in
   URLs: URLs may not contain space or other non-printable characters.
   If it is necessary to designate a byte in a component of a URL that
   would otherwise be represented by a space or a non-printable
   character, it is necessary to represent that byte encoded.

   There are a number of printable ASCII characters whose use in URLs
   is _unsafe_; characters can be unsafe for a number of reasons.  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 sometimes
   known to modify such characters.

   All unsafe characters should always be encoded within a URL. For
   example, the character "#" should 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.

   In general, only alphanumerics, reserved characters used for their
   reserved purposes, "$", "-", "_", ".", and "+" may be used
   unencoded.

   On the other hand, even safe characters such as alphanumerics _may_
   be encoded, as long as they are not being used for a reserved
   purpose.

   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 it is necessary to designate a byte in a
   component of a URL that would otherwise be represented by a
   reserved character, it is necessary to represent that byte 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 a byte is
   represented by a character and when it is represented by its hex
   encoding. However, this is not true for reserved characters:
   encoding a reserved character for a particular scheme may change
   the semantics of a URL.

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 "/".

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 initial part of the scheme-specific data:

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

   This initial part starts with a double slash "//" to indicate its
   presence, and continues until the following slash "/", if any.
   Within this section are:

    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.

   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 digits separated by periods.
        Fully qualified domain names take the form as described in
        Section 3.5 of RFC 1034: a sequence of parts separated by
        period.

    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 use, 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.

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 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 necessary to encode each "/" as %2F.

   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.

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

   If :<port> is omitted, the port defaults to 70.  <gophertype> is
   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 8-bit bytes which may
   contain any characters other than tab, return, or linefeed.  Gopher
   clients specify which item to retrieve by sending the Gopher
   selector string to a Gopher server.

   Within the <gopher-path>, no additional characters have a reserved
   interpretation.

   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
   optionally by a tab and the search string (if the <search> element
   is not empty), followed by a tab and the Gopher+ commands.

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:

        +view_name%20language_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%0Aask_item1_value%0D%0Aask_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. Within mailto URLs, no additional characters
   are reserved within the <rfc822-addr-spec> component.

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

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.

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 port defaults to 23; the <user>
   and <password> segments are completely optional (a <password>
   requires a <user> element.)

   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 [6]; the WAIS protocol is described in RFC 1625 [17].

   A WAIS URLs takes one 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.

   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
   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. Experimental prefixes
   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 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 | "+" | "-" | "." ]
   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       = alpha *uchar
   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 [ "/" 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

   ; 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"
   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 | national
   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.

   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, and many others have helped refine the current draft.

APPENDIX: Recommendations for URLs in Context

   URIs, including URLs, are intended to be transmitted though
   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. 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>.

REFERENCES

   [1] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
       Torrey, D., and Alberti, B., "The Internet Gopher Protocol:
       A distributed document search and retrieval protocol",
       RFC 1436, <URL:ftp://ds.internic.net/rfc/rfc1436.txt>,
       March 1993.

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

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

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

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

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

   [7] Deutsch, P., Emtage, A. & Marine, A. "How to Use Anonymous
       FTP", RFC1635, <URL: ftp://ds.internic.net/rfc/rfc1635.txt>,
       May 1994.

   [8] Horton, M. and Adams, R., "Standard For Interchange of USENET
       messages", RFC 1036, <URL:ftp://ds.internic.net/rfc/rfc1036.txt>,
       December 1987.

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

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

  [11] Kantor, B. and Lapsley, P., "Network News Transfer Protocol:
       A Proposed Standard for the Stream-Based Transmission of News",
       RFC977, <URL:ftp://ds.internic.net/rfc/rfc977.txt>,
       February 1986.

  [12] Kunze, J., "Functional Requirements for Internet Resource
       Locators", Internet-Draft (work in progress), <URL:ftp://ds.in
       ternic.net/internet-drafts/draft-ietf-uri-irl-fun-req-01.txt>,
       July 1994.

  [13] Mockapetris, P., "Domain Names - Concepts and Facilities",
       RFC1034, USC-ISI, <URL:ftp://ds.internic.net/rfc/rfc1034.txt>,
       November, 1987.

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

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

  [16] Sollins, K. and Masinter, L. "Requirements for Uniform Resource
       Names", Internet-Draft (work in progress), <URL:ftp://ds.inter
       nic.net/internet-drafts/draft-sollins-urn-00.txt>

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

  [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",
       Internet Draft, November 1991, now expired.

EDITORS' ADDRESSES

Tim Berners-Lee
World-Wide Web project
CERN,
1211 Geneva 23,
Switzerland
Tel: +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
Tel: (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
Tel: (612) 625 1300
EMail: mpm@boombox.micro.umn.edu


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