Use it as reference for URL syntax. Take a print out.-Javed

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 <>. Discussions of the group are archived    at <URL:> Berners-Lee, Masinter & McCahill                                [Page 1] RFC 1738            Uniform Resource Locators (URL)        December 1994 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 Berners-Lee, Masinter & McCahill                                [Page 2] RFC 1738            Uniform Resource Locators (URL)        December 1994    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. Berners-Lee, Masinter & McCahill                                [Page 3] RFC 1738            Uniform Resource Locators (URL)        December 1994    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 "/". Berners-Lee, Masinter & McCahill                                [Page 4] RFC 1738            Uniform Resource Locators (URL)        December 1994 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. Berners-Lee, Masinter & McCahill                                [Page 5] RFC 1738            Uniform Resource Locators (URL)        December 1994    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:> has an empty    user name and no password, <URL:> has no user name,    while <URL:> 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. Berners-Lee, Masinter & McCahill                                [Page 6] RFC 1738            Uniform Resource Locators (URL)        December 1994 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 Berners-Lee, Masinter & McCahill                                [Page 7] RFC 1738            Uniform Resource Locators (URL)        December 1994    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. Berners-Lee, Masinter & McCahill                                [Page 8] RFC 1738            Uniform Resource Locators (URL)        December 1994 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> Berners-Lee, Masinter & McCahill                                [Page 9] RFC 1738            Uniform Resource Locators (URL)        December 1994    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. Berners-Lee, Masinter & McCahill                               [Page 10] RFC 1738            Uniform Resource Locators (URL)        December 1994 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: Berners-Lee, Masinter & McCahill                               [Page 11] RFC 1738            Uniform Resource Locators (URL)        December 1994         +<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. Berners-Lee, Masinter & McCahill                               [Page 12] RFC 1738            Uniform Resource Locators (URL)        December 1994 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. Berners-Lee, Masinter & McCahill                               [Page 13] RFC 1738            Uniform Resource Locators (URL)        December 1994 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. Berners-Lee, Masinter & McCahill                               [Page 14] RFC 1738            Uniform Resource Locators (URL)        December 1994    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$USER:[MY.NOTES]NOTE123456.TXT    might become      <URL:file://$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 Berners-Lee, Masinter & McCahill                               [Page 15] RFC 1738            Uniform Resource Locators (URL)        December 1994    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 Berners-Lee, Masinter & McCahill                               [Page 16] RFC 1738            Uniform Resource Locators (URL)        December 1994    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 | "+" | "-" | "." ] Berners-Lee, Masinter & McCahill                               [Page 17] RFC 1738            Uniform Resource Locators (URL)        December 1994 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 Berners-Lee, Masinter & McCahill                               [Page 18] RFC 1738            Uniform Resource Locators (URL)        December 1994 ; 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" Berners-Lee, Masinter & McCahill                               [Page 19] RFC 1738            Uniform Resource Locators (URL)        December 1994 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. Berners-Lee, Masinter & McCahill                               [Page 20] RFC 1738            Uniform Resource Locators (URL)        December 1994    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. Berners-Lee, Masinter & McCahill                               [Page 21] RFC 1738            Uniform Resource Locators (URL)        December 1994 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:;       type=d> but you can probably pick it up from <URL:>.  Note the warning in <URL:http://ds.internic.       net/instructions/overview.html#WARNING>. Berners-Lee, Masinter & McCahill                               [Page 22] RFC 1738            Uniform Resource Locators (URL)        December 1994 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:;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:        /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:>    [4] Berners-Lee, T., "Hypertext Transfer Protocol (HTTP)",        CERN, November 1993.        <URL:>    [5] Braden, R., Editor, "Requirements for Internet Hosts --        Application and Support", STD 3, RFC 1123, IETF, October 1989.        <URL:>    [6] Crocker, D. "Standard for the Format of ARPA Internet Text        Messages", STD 11, RFC 822, UDEL, April 1982.        <URL:>    [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:>    [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:>    [9] Huitema, C., "Naming: Strategies and Techniques", Computer        Networks and ISDN Systems 23 (1991) 107-110. Berners-Lee, Masinter & McCahill                               [Page 23] RFC 1738            Uniform Resource Locators (URL)        December 1994   [10] Kahle, B., "Document Identifiers, or International Standard        Book Numbers for the Electronic Age", 1991.        <URL:>   [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:>   [12] Kunze, J., "Functional Requirements for Internet Resource        Locators", Work in Progress, December 1994.        <URL:        /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:>   [14] Neuman, B., and S. Augart, "The Prospero Protocol",        USC/Information Sciences Institute, June 1993.        <URL:        /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:>   [16] Sollins, K. and L. Masinter, "Functional Requirements for        Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,        December 1994.        <URL:>   [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:>   [18] Yeong, W. "Towards Networked Information Retrieval", Technical        report 91-06-25-01, Performance Systems International, Inc.        <URL:>, June 1991.   [19] Yeong, W., "Representing Public Archives in the Directory",        Work in Progress, November 1991. Berners-Lee, Masinter & McCahill                               [Page 24] RFC 1738            Uniform Resource Locators (URL)        December 1994   [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: Larry Masinter Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94034 Phone: (415) 812-4365 Fax: (415) 812-4333 EMail: 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: Berners-Lee, Masinter & McCahill                               [Page 25]