Uniform Resource Locators (URL) Tim Berners-Lee
Internet Draft CERN
Expires May 1994 October 1993
Uniform Resource Locators (URL)
A Unifying Syntax for the Expression of
Names and addresses of Objects on the Network
Status of this memo
This document is an Internet Draft. Internet Drafts are working
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Distribution of this document is unlimited. Please send comments
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Abstract
Many protocols and systems for document search and retrieval are
currently in use, and many more protocols or refinements of
existing protocols are to be expected in a field whose expansion is
explosive.
These systems are aiming to achieve global search and readership of
documents across differing computing platforms, and despite a
plethora of protocols and data formats. As protocols evolve,
gateways can allow global access to remain possible. As data
formats evolve, format conversion programs can preserve global
access. There is one area, however, in which it is impractical to
make conversions, and that is in the names and addresses used to
identify objects. This is because names and addresses of objects
are passed on in so many ways, from the backs of envelopes to
hypertext objects, and may have a long life.
A common feature of almost all the data models of past and proposed
systems is something which can be mapped onto a concept of "object"
and some kind of name, address, or identifier for that object. One
can therefore define a set of name spaces in which these objects
can be said to exist.
Practical systems need to access and mix objects which are part of
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Practical systems need to access and mix objects which are part of
different existing and proposed systems.
This paper discusses the requirements on a universal syntax which
can be used to encapsulate a name in any registered name space.
This will allow names in different spaces to be treated in a common
way, even though names in different spaces have differing
characteristics, as do the objects to which they refer
The universal syntax to objects available using existing protocols,
and may be extended with technology. It makes a recommendation for
a generic syntax, and for specific forms for "Uniform Resource
Locators" (URLs)of objects accessible using existing Internet
protocols.
The syntax has been in widespread use by World-Wide Web software
since 1990.
Terms
The objects on the network which are to be named and addressed
include typically objects which can be retrieved, and objects which
can be searched. There is a great variety of other objects which
may support other operations. We imply nothing about the contents
of objects in this document. Whereas human-readable documents are
currently the center of interest of the field, we envisage all
aspects discussed in this paper applying to generalized objects
when systems to handle them become available. The "object" is the
unit of reference and need not correspond to any unit of storage.
We refer to objects which can be searched as "indexes". We
emphasize that this is the abstract view of the client, and these
objects need not correspond to physical files on computers. We
refer to the person who does the retrieval or searching as the
user.
Within this document, we use the terms "name" very generally for a
string of characters describing an object, whatever its
combination of properties mentioned below. (The term usually has a
narrower meaning but we needed some term for the universal set.).
This uniform syntax applied to a generic name is known as a Uniform
Resource Identifier (URI). The term "address" is reserved for an
string which specifies a more or less physical location. The term
"locator" refers to a URL as here defined. URIs which have a
greater persistence than URLs are referred to as URNs.
Characteristics
This section characteristics of various naming schemes,
requirements which some ofexisting schemes meet, and requirements
for the URL scheme itself. URLs, as an introduction of and
background for the Recommendations section.
USES OF NAMES AND ADDRESSES
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A name allows a user, with the help of a "client" program, to
retrieve or operate on objects via a "server" program. A name may
be passed for example:
In communication of any form between two people, to refer to a
document, or part of a document;
As part of the description of a link associated with a hypertext
document;
As part of the result of searching an index.
Some typical requirements on a name which are met to a varying
degree by various schemes are for example that the name is
Persistent A given name will remain valid as long as it
is needed;
Extensible A given naming syntax will remain valid
through the introduction of new protocols and
directory technologies;
Resolvable A name will contain enough information to
allow the document or index to which it
refers to be accessed, perhaps via resolution
into an intermediate, more physical, name.
Unique Each object can only have one such name.
The fact that two such names are different
implies that the objects to which they refer
are different (in some way).
Unambiguous The fact that two names are identical
implies that the objects named are the same
(in some way).
The syntax discussed is the syntax of one name, be it a lasting
name or a physical address. When a directory server or hypertext
link contains a set of alternative names, then that is beyond the
scope of this syntax. Similarly, a syntax for describing a
compound object is outside the scope of this syntax. The specific
locator name spaces (defined under the umbrella of the general
syntax) each meet the requirements above to a greater or lesser
extent.
CURRENT PRACTICE
Current protocols use many different standards for names. For some
protocols, such as ISO-10163 Search and Retrieve protocol[16], the
names returned in a search are only valid during the session. For
others, such as FTP[9], they are lasting names which may be used
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for object retrieval at a later time. Typically, however, they are
not long-lasting names which are independent of the location of the
object. Such names may be provided using directory servers such as
x.500. They will refer to the registration, however formal or
informal, of a object with a particular organisation or person.
Both hypertext and manual references rely on long- lasting names.
Current names are basically location specifiers (addresses). These
may be known as Uniform Resource Locators (URLs). They give the
necessary parts of an address for a reader to access an information
provider using the given protocol, and ask for the object required.
Examples of names used by various protocols include
File Transfer Protocol (Postel 1985):
Host name or IP-address
[TCP port]
[user name, password]
Filename
W.A.I.S. (Kahle 1990)
Host name or IP-address
[TCP port]
local document id
Gopher (Alberti 1991)
Host name or IP-address
[TCP port]
database name
selector string
HTTP (Berners-Lee 1991)
Host name or IP-address
[TCP port]
local object id
NNTP (Kantor 1986) group
Group name
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NNTP article
Host name
unique message identifier
Prospero links (Neuman 1992)
Host name or IP address
[UDP port]
Host specific object name
[version]
[identifier]*
x.500 distinguished name
Country
Organisation
Organisational unit
Person
Local object identifier
Other systems with their own naming schemes include BITNET
"LISTSERV" application, FTAM file retrieval, SQLnetTM remote
database search, proprietary distributed file systems, etc.
Conventional syntax for writing these addresses involve various
forms of punctuation to separate these parts. This sometimes, but
not always, allows the naming scheme to be deduced from the
punctuation. For example, a name of the form
xxx.yyy.zz.edu:/pub.aa.bb.cc often implies anonymous FTP access.
However, there is no well-defined algorithm for parsing an
arbitrary name, as there is no common syntax.
EXPANDABILITY
There will necessarily be a phase during which lasting names will
become more common, as the deployment of directory services
increases to the point where every user has direct or indirect
access to one. Even then, however, one can envisage more than one
competing directory system, and cases in which physical names are
still required. A directory service takes a lasting name and
reduces it to a physical address (or set of addresses) which,
though less useful for lasting reference, is the only way to
actually retrieve the object. An addressing syntax is required
which will be able to encompass existing physical address spaces,
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and be extendible to any future protocols. This requires that it
contain an identifier for the protocol in use. The format of the
rest of the address will necessarily depend to a certain extent on
the protocol.
RELEVANCE
The life of a name is limited by any information contained within
it which may become prematurely invalid. It is therefore necessary
to limit the contents of a name to the information required for the
operations above. Other extraneous information about the object
(its size, data format, authorisation details, etc.) may in general
change with time and should not be part of the name. One might
expect such information to be part of the "header" of a object, and
for protocols to allow the header information to be retrieved
independently of the objects themselves. Any physical address may
be subject to change with time: hence we encourage the move to
lasting names and directory services.
UNIQUENESS
Clearly one requires unambiguous names in the sense that one name
should refer to only one logical object. This is the case with all
the addressing schemes in use, whether they are directory systems
or physical addresses. (The internet addresses all rely on the
domain name (Mockapetris 1987) of the host to achieve this).
However, given that names can be translated, many apparently
different names may lead to the same object. Any object may
therefore be referred to by many names. One needs to be able to
know whether two objects, retrieved through different paths, are
in fact the same object. It is suggested that each object have a
unique "official" name. This name could be stored in the object in
some representations, or stored in a database accessible to the
server, for example. Any references within that object should be
parsed in the context of the official name. In the presence of a
directory service, the official name will normally be the
registered name of the object. However, a name in any scheme will
do, so long as it is completely specified. On systems which do not
allow the name to be stored (such as anonymous FTP archive sites),
a possible ambiguity will always exist as to whether two similarly
named objects are in fact the same. Note that Internet newsgroup
names are unique world-wide, and news articles carry a unique
message id. In most other cases, however, there is no guarantee
that dereferencing a URL will work, or that if it does the object
it refers to will in fact be the object intended. URLs such as FTP
addresses are transient in that files may be moved and even
replaced by different files of the same name. This disorganisation
may be limited by good server management, but a naming scheme which
is independent also of internet host name is obviously preferable.
READABILITY BY PEOPLE
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This requirement has been put forward by several people (Clifford
Lynch, Douglas Engelbart among others), and disputed by others.
The author's view is that it will be a while before technology and
standardisation have reached the point at which names and addresses
will be hidden from human beings. As long as they must be written
on the backs of envelopes and "cut and pasted" between workstation
windows, there is a strong need for names to be
Short
Composed of printable (preferably non-white) characters
To a certain extent, understadable by a human being.
STRUCTURE OF NAMES AND ADDRESSES
A physical address is required in order for:
The user's program to contact the server;
The server to perform the operation (e.g. search and index,
retrieve a object, or look up the name) and return a result;
The user's program to locate an individual position or element
within a returned object.
This suggests that a name be structured, such that the parts
necessary for these three operations be separate and only used by
those system elements which need those parts. This corresponds to
the basic principle of information hiding. In fact, four parts
are necessary, including the indicator of the naming scheme to be
used:
The naming scheme: a registered identifier for the protocol.
The name of a suitable server. The format of this part must be
well defined. It will depend on the lower-layer protocols in
use. Systems which use widely distributed information, such as
x.500 and NNTP, do not need this part as each client generally
contacts his nearest server (or a particular server).
Information to be passed to the server. This may be private to
the server, as all names may be generated and used by the same
server. This part of the name should be opaque to the client.
Information to be used by the application once the object has
been retrieved. This part is private to the application (or,
more strictly, the data format) and so cannot be defined here.
Both lasting names and physical addresses often share a
hierarchical structure. This follows often from the organisation of
the system. From the naming point of view, it has the advantage
that a reference in one object to another object need not include
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that part of the structure which is common to both names.
CHOICES FOR A UNIVERSAL SYNTAX
The requirements above leave little room for choice save for the
order and punctuation of the elements of an address. It is only
reasonable for the order of writing of the parts to be consistently
from left to right (or right to left) with increasing specificity.
Punctuation schemes fall into two categories (Huitema 1991): tagged
schemes in which field are given names, and fields which use
special characters and field order. The latter tend to be more
compact schemes.
protocol: aftp host: xxx.yyy.edu path:
/pub/doc/README
PR=aftp; H=xx.yy.edu; PA=/pub/doc/README;
PR:aftp/xx.yy.edu/pub/doc/README
/aftp/xx.yy.edu/pub/doc/README
Fig 1. Some alternative tagged and untagged representations
The choice of special symbols for punctuation tends to be a matter
of taste. It is easier to read addresses whose symbols correspond
to those of one's favourite operating system. A variety of symbols
is needed so that when a name is abbreviated it is possible to tell
which parts have been omitted.
The recommendation below uses special characters in order to
achieve a compact name, and uses where possible punctuation symbols
established in the internet or unix community.
The choice of escape character for introducing representations of
non-allowed characters also tends to be a matter of taste. An ANSI
standard exists in the C language, using the back-slash character
"\". The use of this character on unix command lines, however, can
be a problem as it is interpreted by many shell programs, and would
have itself to be escaped.
There is a conflict between the need to be able to represent many
characters including spaces within a URL directly, and the need to
be able to use a URL in environments which have limited character
sets or in which certain characters are prone to corruption. This
conflict has been resolved by use of an hexadecimal escaping method
which may be applied to any characters forbidden in a given
context. When URLs are moved between contexts, the set of
characters escaped may be enlarged or reduced unambiguously.
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The use of multiple white space characters is discouraged in URLs
to be printed or sent by electronic mail. This is because of the
frequent introduction of extraneous white space when lines are
wrapped by systems such as mail, or sheer necessity of narrow
column width, and because of the inter-conversion of various forms
of white space which occurs during character code conversion and
the transfer of text between applications.
Recommendations
This section describes the syntax for "Uniform Resource Locators"
(URLs): that is, basically physical addresses of objects which are
retrievable using protocols already deployed on the net. The
generic syntax provides a framework for new schemes for names to be
resolved using as yet undefined protocols.
The syntax is described in two parts. Firstly, we give the syntax
rules of a completely specified name; secondly, we give the rules
under which parts of the name may be omitted in a well-defined
context.
FULL FORM
A complete URL consists of a naming scheme specifier followed by a
string whose format is a function of the naming scheme. For
locators of information on the internet, a common syntax is used
for the IP address part. A BNF description of the URL syntax is
given in an a later section. The components are as follows.
Fragment-id
This represents a part of, fragment of, or a sub-function within,
an object or object. Its syntax and semantics are defined by the
application responsible for the object, or the specification of the
content type of the object. The only definition here is of the
allowed characters by which it may be represented in a URL.
The fragment-id follows the URL of the whole object from which it
is separated by a hash sign (#). If the fragment-id is void, the
hash sign may be omitted: A void fragment-id with or without the
hash sign means that the URL refers to the whole object.
While this hook is allowed for identification of fragments, the
question of addressing of parts of objects, or of the grouping of
objects and relationship between contined and containing objects,
is not addressed by this object.
This object does not address the question of objects which are
different versions of a "living" object, nor of expressing the
relationships between different versions and the living object.
SCHEME
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Within the URL of a object, the first element is the name of the
scheme, separated from the rest of the object by a colon. The rest
of the URL follows the colon in a format depending on the scheme.
Internet protocol parts
Those schemes which refer to internet protocols have a common
syntax for the rest of the object name. This starts with a double
slash "//" to indicate its presence, and continues until the
following slash "/". Within that section are
An optional user name,
if this must be quoted to the server,
followed by a commercial at sign "@". (Use
of this field is discouraged. Provision of
encoding a password after the user name,
delimited by a colon, could be made but
obviously is only useful when the password is
public, in which case it should not be
necessary, so that is also discouraged.)
The internet domain name
of the host in RFC1037 format (or,
optionally and less advisably, the IP address
as a set of four decimal digits)
The port number, if it is not the default number for the
protocol, is given in decimal notation after
a colon.
Path The rest of the locator is known as the
"path". It may define details of how the
client should communicate with the server,
including information to be passed
transparently to the server without any
processing by the client.
The path is interpreted in a manner dependent on the protocol being
used. However, when it contains slashes, these must imply a
hierarchical structure.
PARTIAL FORM
In a certain limited set of cases, generally within a certain
application, it may be useful to pass only a section of the URL.
Within a object whose URL is well defined, the URL of another
object may be given in abbreviated form, where parts of the two
URLs are the same. This allows objects within a group to refer to
each other without requiring the space for a complete reference,
and it incidentally allows the group of objects to be moved
without changing any references. This is not discussed in detail
here, it is only mentioned so that the characters required by the
technique be reserved for that purpose. It must be emphasised that
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RFC XXXX Uniform Resource Locators (URL) October 1993
when a reference is passed in anything other than a well controlled
context, the full form must always be used.
The partial form relies on a property of the URL syntax that
certain characters ("/") and certain path elements ("..", ".") have
a significance reserved for representing a hierarchical space, and
must be recognised as such by both clients and servers.
A partial form can be distinguished from a full form in that a full
form must have a colon and that colon must occur before any slash
characters.
The rules for the use of a partial name are:
If the scheme parts are different, the whole absolute locator
must be given. Otherwise, the scheme is omitted, and:
If the host and/or port parts are the different, the host, port
name and all the rest of the locator must be given.
If the access and host parts are the same, then the path may be
given in absolute (fully qualified) or relative form. Within the
path:
If a leading slash is present, the path is absolute. Otherwise,
a relative path is interpreted as follows:
The last part of the path of the context locator (anything
following the rightmost slash) is removed, and the given partial
URL appended in its place.
Within the result, all occurrences of "/xxx/.." or "/." are
recursively removed, where xxx, ".." and "." are complete path
elements.
Note: If a path of the context locator end in slash, partial URLs
will be treated differently to their treatment with respect to the
same path without a slash. Using a trailing slash on a directory
name is not therefore recommended. The signifcance of a trailing
slash may be considered as that of the locator of a file with void
name within that directory.
ENCODING PROHIBITED CHARACTERS
When a system uses a local addressing scheme, it is useful to
provide a mapping from local addresses into URLs so that references
to objects within the addressing scheme may be referred to
globally, and possibly accessed through gateway servers.
Any mapping scheme may be defined provided it is unambiguous,
reversible, and provides valid URLs. It is recommended that where
hierarchical aspects to the local naming scheme exist, they be
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RFC XXXX Uniform Resource Locators (URL) October 1993
mapped onto the hierarchical URL path syntax in order to allow the
partial form to be used.
The following encoding method shall be used for mapping WAIS, FTP,
Prospero and Gopher addresses onto URLs. Where the local naming
scheme uses ASCII characters which are not allowed in the URL,
these may be represented in the URL by a percent sign "%" followed
by two hexadecimal digits (0-9, A-F) giving the ISO Latin 1 code
for that character. Character codes other than those allowed by
the syntax shall not be used in a URL.
The same encoding method may be used for encoding characters whose
use, although technically allowed in a URL, would be unwise due to
problems of corruption by imperfect gateways or misrepresentation
due to the use of variant character sets, or which would simply be
awkward in a given environment. As a % sign always indicates an
encoded character, a URL may be made safer simply by encoding any
characters considered unsafe, while leaving already encoded
characters still encoded.
(Note: If a new naming scheme is introduced which encodes binary
data as opposed to text, then a more compact encoding such as pure
hex or base 64 would be more appropriate.)
The same considerations apply to mapping local fragment identifiers
onto the fragmentid part of a URL.
Specific Schemes
The mapping for some existing standard and experimental protocols
is outlined in the BNF syntax definition. Notes on particular
protocols follow.
HTTP
The HTTP protocol specifies that the path is handled transparently
by those who handle URLs, except for the servers which de-reference
them. The path is passed by the client to the server with any
request, but is not otherwise understood by the client. The
fragmentid part is not sent with the request. The search part, if
present, is sent. Spaces in URLs should be escaped for transmission
in HTTP.
FTP
The ftp: prefix indicates a file which is to be picked up from the
file system of the given host. The FTP protocol is used. The port
number if given gives the port of the FTP server if not the FTP
default. (A client may in practice use local file access to
retrieve objects which are available though more efficient means
such as local file open or NFS mounting, where this is available
and equivalent).
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The syntax allows for the inclusion of a user name and even a
password for those systems which do not use the anonymous FTP
convention. The default, however, if no user or password is
supplied, will be to use that convention, viz. that the user name
is "anonymous" and the password the user's mail address.
The adoption of a unix-style syntax involves the conversion into
non-unix local forms by either the client or server. Some non-unix
servers do this, but clients wishing to access sites which do not
have unix-style naming will need certain algorithms to enable
other file systems to be identified and treated. Client software
may also have to be flexible in terms of the sequence of FTP
commands used with different varieties of server. In view of a
tendency for file systems to look increasingly similar, it was felt
that the URL convention should not be weighed down by extra
mechanisms for identifying these cases.
The data format of a file can only, in the general FTP case, be
deduced from the name, normally the suffix of the name. This is not
standardized. An alternative is for it to be transferred in
information outside the URL. The transfer mode (binary or text)
must in turn be deduced from the data format. It is recommended
that conventions for suffixes of public archives be established,
but it outside the scope of this paper.
NEWS
The news locators refer to either news group names or article
message identifiers which must conform to the rules of RFC 850. A
message identifier may be distinguished from a news group name by
the presence of the commercial at "@" character. These rules imply
that within an article, a reference to a news group or to another
article will be a valid URL (in the partial form).
A news URL may be dereferenced using NNTP or using any other
protocol for the conveyance of usenet news articles.
Note1:
Among URLs the news: URLs are anomalous in that they are
location-independent. They are unsuitable as URN candidates because
the NNTP architecture relies on the expiry of articles and
therefore a small number of articles being available at any time.
When a news: URL is quoted, the assumption is that the reader will
fetch the article or group from his or her local news host. News
host names are NOT part of news URLs.
Note 2:
An outstanding problem is that the message identifier is
insufficient to allow the retrieval of an expired article, as no
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algorithm exists for deriving an archive site and file name. The
addition of the date and news group set to the article's URL would
allow this if a directory existed of archive sites by news group.
Suggested subject of study in conjunction with NNTP WG. Further
extension possible may be to allow the naming of subject threads as
addressable objects.
NNTP
This is an alternative form of reference for news articles,
specifically to be used with NNTP servers, and particularly those
incomplete server implementations which do not allow retrieval by
message identifier.
The news server name, newsgroup name, and index number of an
article within the newsgroup on that particular server are given.
Note1.
This form of URL is not of global accessiablity, as typically NNTP
servers only allow access from local clients. This form or URL
should not be quoted outside this local area. It should not be
used within news articles for wider circulation than the one
server.
WAIS
The current WAIS implementation public domain requires that a
client know the "type" and length of a object prior to retrieval.
These values are returned along with the internal object identifier
in the search response. They have been encoded into the path part
of the URL in order to make the URL sufficient for the retrieval of
the object. If changes to WAIS specifications make the internal
id something which is sufficient for later retrieval then this will
not be necessary. Within the WAIS world, names do not of course
not need to be prefixed by "wais:" (by the partial form rules).
The length not now being strictly necessary is kept for historical
reasons.
PROSPERO
The Prospero (Neuman, 1991) directory service is used to resolve
the URL yielding an access method for the object (which can then
itself be represented as a URL if translated). The host part
contains a host name or internet address. The port part is
optional. The path part contains a host specific object name, an
optional version number, and an optional list of attributes. If
these latter fields are present thy are separated from the host
specific object name and from each other by the characters "%00"
(percent, zero, zero), this being and escaped string terminator
(null). If the optional list of attributes is provided, the
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RFC XXXX Uniform Resource Locators (URL) October 1993
version number must be present, but may be the empty string (i.e.
the first attribute would be separated from the host specific name
by "%00%00"). External Prospero links are represented directly as
URLs of the underlying access method and are not represented as
Prospero URLs.
GOPHER
The first character of the URL path part (after the initial single
slash) is a single-character "type" field which is that used by the
Gopher protocol. The rest of the path is the "selector string",
with disallowed characters encoded. Note that some selector strings
begin with a copy of the gopher type character, in which case that
character will occur twice consecutively in the URL. If the type
character and selector are omitted, the type defaults to "1".
Gopher links which refer to non-Gopher protocols are represented
directly as URLs of the underlying access method and are not
represented as Gopher URLs.
MAILTO
This allows a URL to specify an RFC822 addr-spec mail address.
Note that use of % , for example as used in forming a gatewayed
mail address, requires conversion to %25 in a URL.
This semantics may be considered to be that the object referred to
by the mailto: URL is the set of messages sent to or from that
address. There is no algorithm to retrieve this set, but the SMTP
protocol allows messages to be added to it, and any given user may
be aware of a subset of its members.
TELNET, RLOGIN, TN3270
The use of URLs to represent interactive sessions is a convenient
extension to their uses for objects. This allows access to
information systems which only provide an interactive service, and
no information server. As information within the service cannot be
addressed individually or, in general, automatically retrieved,
this is a less desirable, though currently common, solution.
X500
The mapping of x500 names onto URLs is not defined here. A decision
is required as to whether "distinguished names" or "user friendly
names" (ufn), or both, should be allowed. If any punctuation
conversions are needed from the adopted x500 representation (such
as the use of slashes between parts of a ufn) they must be defined.
This is a subject for study.
WHOIS
This prefix describes the access using the "whois++" scheme in the
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RFC XXXX Uniform Resource Locators (URL) October 1993
process of definition. The host name part is the same as for other
IP based schemes. The path part can be either a whois handle for a
whois object, or it can be a valid whois query string. This is a
subject for further study.
NETWORK MANAGEMENT DATABASE
This is a subject for study.
REGISTRATION OF NAMING SCHEMES
A new naming scheme may be introduced by defining a mapping onto a
conforming URL syntax, using a new scheme identifier. Experimental
scheme identifiers may be used by mutual agreement between parties,
and must start with the characters "x-". The scheme name "urn:" is
reserved for the work in progress on a scheme for more persistent
names. Therefore URNs (Names) and URLs (Locators) be
distinguishable. An object which is either a URL or a URN is known
as a URI (Identifier).
It is proposed that the Internet Assigned Numbers Authority (IANA)
perform the function of registration of new schemes. Any submission
of a new URI scheme must include a definition of an algorithm for
the retrieval of any object within that scheme. The algorithm must
take the URI and produce either a set of URL(s) which will lead to
the desired object, or the object itself, in a well-defined or
determinable format.
It is recommended that those proposing a new scheme demonstrate its
utility and operability by the provision of a gateway which will
provide images of objects in the new scheme for clients using an
existing protocol. If the new scheme is not a locator scheme, then
the properties of names in the new space should be clearly defined.
It is likewise recommended that, where a protocol allows for
retrieval by URI, that the client software have provision for being
configured to use specific gateway locators for indirect access
through new naming schemes.
BNF syntax
This is a BNF-like description of the Uniform Resource Locator
syntax. A vertical line "|" indicates alternatives, and
[brackets] indicate optional parts. Spaces are representated by
the word "space", and the vertical line character by "vline".
Single letters stand for single letters. All words of more than one
letter below are entities described somewhere in this description.
The "generic" production gives a higher level parsing of the same
URLs as the other productions. The "national" and "punctuation"
characters fo not appear in any productions and therefore may not
appear in URLs.
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RFC XXXX Uniform Resource Locators (URL) October 1993
The "afsaddress" is left in as historical note, but is not a url
production
fragmentaddress uri [ # fragmentid ]
uri url
ur l generic | httpaddress | ftpaddress |
newsaddress | nntpaddress | prosperoaddress |
telnetaddress | gopheraddress | waisaddress
| mailtoaddress
generic scheme : path [ ? search ]
scheme ialpha
httpaddress h t t p : / / hostport [ / path ] [ ?
search ]
ftpaddress f t p : / / login / path
afsaddress a f s : / / cellname / path
newsaddress n e w s : groupart
nntpaddress n n t p : group / digits
mailtoaddress m a i l t o : : xalphas @ hostname
waisaddress waisindex | waisdoc
waisindex w a i s : / / hostport / database [ ? search
]
waisdoc w a i s : / / hostport / database / wtype /
path
groupart * | group | article
group ialpha [ . group ]
article xalphas @ host
database xalphas
wtype xalphas
prosperoaddress prosperolink
prosperolink p r o s p e r o : / / hostport / hsoname [ %
0 0 version [ attributes ] ]
hsoname path
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RFC XXXX Uniform Resource Locators (URL) October 1993
version digits
attributes attribute [ attributes ]
attribute alphanums
telnetaddress t e l n e t : / / login
gopheraddress g o p h e r : / / hostport [/ gtype [
selector ] ] [ ? search ]
login [ user [ : password ] @ ] hostport
hostport host [ : port ]
host hostname | hostnumber
cellname hostname
hostname ialpha [ . hostname ]
hostnumber digits . digits . digits . digits
port digits
selector path
path void | xpalphas [ / path ]
search xalphas [ + search ]
user xalphas
password xalphas
fragmentid xalphas
gtype xalpha
xalpha alpha | digit | safe | extra | escape
xalphas xalpha [ xalphas ]
xpalpha xalpha | +
xpalphas xpalpha [ xpalpha ]
ialpha alpha [ xalphas ]
alpha a | b | c | d | e | f | g | h | i | j | k |
l | m | n | o | p | q | r | s | t | u | v |
Berners-Lee 18
RFC XXXX Uniform Resource Locators (URL) October 1993
w | x | y | z | 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
0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
safe $ | - | _ | @ | . | &
extra ! | * | " | ' | ( | ) | : | ; | , | space
escape % hex hex
hex digit | a | b | c | d | e | f | A | B | C |
D | E | F
national { | } | vline | [ | ] | \ | ^ | ~
punctuation < | >
digits digit [ digits ]
alphanum alpha | digit
alphanums alphanum [ alphanums ]
void
Wrappers for URIs in plain text
This section does not formally form part of the URL specification.
URIs, including URLs, will ideally be transmitted though protocols
which accept them and data formats which define a context for them.
However, in practice nowadays there are many occasions when URLs
are included in plain ASCII non-marked-up text such as electronic
mail and usenet news messages.
In this case, it is convenient to have a separate wrapper syntax to
define delimiters which will enable the human or automated reader
to recognize that the URI is a URI.
The recommendation is that the angle brackets (less than and
greater than signs) of the ASCII set be used for this purpose.
These wrappers do not form part of the URL, are not mandatory, and
should not be used in contexts (such as SGML parameters, HTTP
requests, etc) in which delimiters are already specified.
Example
Yes, Jim, I found it under <ftp://info.cern.ch/pub> bu
t
Berners-Lee 19
RFC XXXX Uniform Resource Locators (URL) October 1993
you can probably pick it up from <ftp://ds.internic.ne
t/rfc>.
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.
The use of URLs containing passwords is clearly unwise.
Conclusion
A need has been demonstrated, and a number of requirements have
been stated for uniform resource locators (URLs). A scheme has been
proposed which builds on existing conventions to define a syntax
for URLs. This scheme has been in serious use by World-Wide Web
(W3) initiative since 1991. Adoption of the scheme in
correspondence, standards and software will ease the use of
references to on-line information in a flexible way as the coming
information age arrives.
Acknowledgements
This paper builds on the basic W3 design and much discussion of
these issues by many people on the network. The discussion was
particularly stimulated by articles by Clifford Lynch (1991),
Brewster Kahle (1991) and Wengyik Yeong (1991b). Contributions from
John Curran (NEARnet), Clifford Neuman (ISI) Ed Vielmetti (MSEN)
and later the IETF URL BOF and URI working group have been
incorporated into this issue of this paper.
The draft url4 (Internet Draft 00) was generated from url3
following discussion and overall approval of the URL working group
on 29 March 1993. The paper url3 had been generated from udi2 in
the light of discussion at the UDI BOF meeting at the Boston IETF
in July 1992. Draft url4 was Internet Draft 00. Draft url5
incorporated changes suggested by Clifford Neuman, and draft url6
(ID 01) incorporated character group changes and a few other fixes
defined by the IETF URI WG in submitting it as a proposed standard.
URL7 (Internet Draft 02) incorporated changes introduced at the
Amsterdam IETF and refined in net discussion.
References
Alberti, R., et.al. (1991)
"Notes on the Internet Gopher Protocol"
University of Minnesota, December 1991,
<ftp://boombox.micro.umn.edu/pub/gopher/
Berners-Lee 20
RFC XXXX Uniform Resource Locators (URL) October 1993
gopher_protocol> . See also
<gopher://gopher.micro.umn.edu/00/Information
About Gopher/About Gopher>
Berners-Lee, T ., (1991)
"Hypertext Transfer Protocol (HTTP)" , CERN,
December 1991,
<ftp://info.cer
n.ch/pub/www/doc/http-spec.txt>
Crocker "Standard for ARPA Internet Text Messages".
David H. Crocker, RFC822,
Davis, F, et al., (1990)
"WAIS Interface Protocol: Prototype
Functional Specification", Thinking Machines
Corporation, April 23, 1990
<ftp://quake.think.com/pub/wa
is/doc/protspec.txt>
International Standards Organization, (1991)
Information and Documentation - Search and
Retrieve Application Protocol Specification
for open Systems Interconnection, ISO-10163
Huitema, C., (1991) "Naming: strategies and techniques",
Computer Networks and ISDN Systems 23 (1991)
107-110.
Kahle, Brewster, (1991)
"Document Identifiers, or International
Standard Book Numbers for the Electronic
Age",
<ftp:
//quake.think.com/pub/wais/doc/doc-ids.txt>
Kantor, B., and Lapsley, P., (1986)
"A proposed standard for the stream-based
transmission of news", Internet RFC-977,
February 1986.
<ftp://ds.internic.net/rfc/rfc977.txt>
Lynch, C., Coallition for Networked Information: (1991)
"Workshop on ID and Reference Structures for
Networked Information", November 1991. See
<wais://quake.think.com/wais-discussion-ar
chives?lynch>
Mockapetris, P., (1987)
"Domain names + concepts and facilities",
RFC-1034, USC-ISI, November 1987,
<ftp://ds.internic.net/rfc/rfc1034.txt>
Berners-Lee 21
RFC XXXX Uniform Resource Locators (URL) October 1993
Neuman, B. Clifford, (1992)
"Prospero: A Tool for Organizing Internet
Resources", Electronic Networking: Research,
Applications and Policy, Vol 1 No 2, Meckler
Westport CT USA. See also
<ftp://prospero.isi.edu/pub/prospero/oir.ps>
Postel, J. and Reynolds, J. (1985)
"File Transfer Protocol (FTP)", Internet
RFC-959, October 1985.
<ftp://ds.internic.net/rfc/rfc959.txt>
Yeong, W., (1991a) "Towards Networked Information Retrieval",
Technical report 91-06-25-01, June 1991,
Performance Systems International, Inc.
<ftp://uu.psi.com/wp/nir.txt>
Yeong, W., (1991b), "Representing Public Archives in the
Directory", Internet Draft, November 1991,
now expired.
AUTHOR'S ADDRESS
Tim Berners-Lee
Address: World-Wide Web project
CERN,
1211 Geneva 23,
Switzerland
Telephone: +41 (22)767 3755
Fax: +41 (22)767 7155
Email: timbl@info.cern.ch
Berners-Lee 22