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INFORMATIONAL
Network Working Group T. Berners-Lee
Request for Comments: 1630 CERN
Category: Informational June 1994
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
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
IESG Note:
Note that the work contained in this memo does not describe an
Internet standard. An Internet standard for general Resource
Identifiers is under development within the IETF.
Introduction
This document defines the syntax used by the World-Wide Web
initiative to encode the names and addresses of objects on the
Internet. The web is considered to include objects accessed using an
extendable number of protocols, existing, invented for the web
itself, or to be invented in the future. Access instructions for an
individual object under a given protocol are encoded into forms of
address string. Other protocols allow the use of object names of
various forms. In order to abstract the idea of a generic object,
the web needs the concepts of the universal set of objects, and of
the universal set of names or addresses of objects.
A Universal Resource Identifier (URI) is a member of this universal
set of names in registered name spaces and addresses referring to
registered protocols or name spaces. A Uniform Resource Locator
(URL), defined elsewhere, is a form of URI which expresses an address
which maps onto an access algorithm using network protocols. Existing
URI schemes which correspond to the (still mutating) concept of IETF
URLs are listed here. The Uniform Resource Name (URN) debate attempts
to define a name space (and presumably resolution protocols) for
persistent object names. This area is not addressed by this document,
which is written in order to document existing practice and provide a
reference point for URL and URN discussions.
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The world-wide web protocols are discussed on the mailing list www-
talk-request@info.cern.ch and the newsgroup comp.infosystems.www is
preferable for beginner's questions. The mailing list uri-
request@bunyip.com has discussion related particularly to the URI
issue. The author may be contacted as timbl@info.cern.ch.
This document is available in hypertext form at:
http://info.cern.ch/hypertext/WWW/Addressing/URL/URI_Overview.html
The Need For a Universal Syntax
This section describes the concept of the URI and does not form part
of the specification.
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
different existing and proposed systems. Therefore, the concept of
the universal set of all objects, and hence the universal set of
names and addresses, in all name spaces, becomes important. This
allows 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.
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URIs
This document defines a way to encapsulate a name in any
registered name space, and label it with the the name space,
producing a member of the universal set. Such an encoded and
labelled member of this set is known as a Universal Resource
Identifier, or URI.
The universal syntax allows access of objects available using
existing protocols, and may be extended with technology.
The specification of the URI syntax does not imply anything about
the properties of names and addresses in the various name spaces
which are mapped onto the set of URI strings. The properties
follow from the specifications of the protocols and the associated
usage conventions for each scheme.
URLs
For existing Internet access protocols, it is necessary in most
cases to define the encoding of the access algorithm into
something concise enough to be termed address. URIs which refer
to objects accessed with existing protocols are known as "Uniform
Resource Locators" (URLs) and are listed here as used in WWW, but
to be formally defined in a separate document.
URNs
There is currently a drive to define a space of more persistent
names than any URLs. These "Uniform Resource Names" are the
subject of an IETF working group's discussions. (See Sollins and
Masinter, Functional Specifications for URNs, circulated
informally.)
The URI syntax and URL forms have been in widespread use by
World-Wide Web software since 1990.
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Design Criteria and Choices
This section is not part of the specification: it is simply an
explanation of the way in which the specification was derived.
Design criteria
The syntax was designed to be:
Extensible New naming schemes may be added later.
Complete It is possible to encode any naming
scheme.
Printable It is possible to express any URI using
7-bit ASCII characters so that URIs may,
if necessary, be passed using pen and ink.
Choices for a universal syntax
For the syntax itself there is little choice except for the order
and punctuation of the elements, and the acceptable characters and
escaping rules.
The extensibility requirement is met by allowing an arbitrary (but
registered) string to be used as a prefix. A prefix is chosen as
left to right parsing is more common than right to left. The
choice of a colon as separator of the prefix from the rest of the
URI was arbitrary.
The decoding of the rest of the string is defined as a function of
the prefix. New prefixed are introduced for new schemes as
necessary, in agreement with the registration authority. The
registration of a new scheme clearly requires the definition of
the decoding of the URI into a given name space, and a definition
of the properties and, where applicable, resolution protocols, for
the name space.
The completeness requirement is easily met by allowing
particularly strange or plain binary names to be encoded in base
16 or 64 using the acceptable characters.
The printability requirement could have been met by requiring all
schemes to encode characters not part of a basic set. This led to
many discussions of what the basic set should be. A difficult
case, for example, is when an ISO latin 1 string appears in a URL,
and within an application with ISO Latin-1 capability, it can be
handled intact. However, for transport in general, the non-ASCII
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characters need to be escaped.
The solution to this was to specify a safe set of characters, and
a general escaping scheme which may be used for encoding "unsafe"
characters. This "safe" set is suitable, for example, for use in
electronic mail. This is the canonical form of a URI.
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. It is also a
character which is not available on certain keyboards. The equals
sign is commonly used in the encoding of names having
attribute=value pairs. The percent sign was eventually chosen as
a suitable escape character.
There is a conflict between the need to be able to represent many
characters including spaces within a URI directly, and the need to
be able to use a URI 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.
The use of white space characters is risky in URIs to be printed
or sent by electronic mail, and the use of multiple white space
characters is very risky. 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. This is why the canonical
form for URIs has all white spaces encoded.
Reommendations
This section describes the syntax for URIs as used in the WorldWide
Web initiative. The generic syntax provides a framework for new
schemes for names to be resolved using as yet undefined protocols.
URI syntax
A complete URI 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
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address part. A BNF description of the URL syntax is given in an a
later section. The components are as follows. Fragment identifiers
and relative URIs are not involved in the basic URL definition.
SCHEME
Within the URI of a object, the first element is the name of the
scheme, separated from the rest of the object by a colon.
PATH
The rest of the URI follows the colon in a format depending on the
scheme. The path is interpreted in a manner dependent on the
protocol being used. However, when it contains slashes, these
must imply a hierarchical structure.
Reserved characters
The path in the URI has a significance defined by the particular
scheme. Typically, it is used to encode a name in a given name
space, or an algorithm for accessing an object. In either case, the
encoding may use those characters allowed by the BNF syntax, or
hexadecimal encoding of other characters.
Some of the reserved characters have special uses as defined here.
THE PERCENT SIGN
The percent sign ("%", ASCII 25 hex) is used as the escape
character in the encoding scheme and is never allowed for anything
else.
HIERARCHICAL FORMS
The slash ("/", ASCII 2F hex) character is reserved for the
delimiting of substrings whose relationship is hierarchical. This
enables partial forms of the URI. Substrings consisting of single
or double dots ("." or "..") are similarly reserved.
The significance of the slash between two segments is that the
segment of the path to the left is more significant than the
segment of the path to the right. ("Significance" in this case
refers solely to closeness to the root of the hierarchical
structure and makes no value judgement!)
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Note
The similarity to unix and other disk operating system filename
conventions should be taken as purely coincidental, and should
not be taken to indicate that URIs should be interpreted as
file names.
HASH FOR FRAGMENT IDENTIFIERS
The hash ("#", ASCII 23 hex) character is reserved as a delimiter
to separate the URI of an object from a fragment identifier .
QUERY STRINGS
The question mark ("?", ASCII 3F hex) is used to delimit the
boundary between the URI of a queryable object, and a set of words
used to express a query on that object. When this form is used,
the combined URI stands for the object which results from the
query being applied to the original object.
Within the query string, the plus sign is reserved as shorthand
notation for a space. Therefore, real plus signs must be encoded.
This method was used to make query URIs easier to pass in systems
which did not allow spaces.
The query string represents some operation applied to the object,
but this specification gives no common syntax or semantics for it.
In practice the syntax and sematics may depend on the scheme and
may even on the base URI.
OTHER RESERVED CHARACTERS
The astersik ("*", ASCII 2A hex) and exclamation mark ("!" , ASCII
21 hex) are reserved for use as having special signifiance within
specific schemes.
Unsafe characters
In canonical form, certain characters such as spaces, control
characters, some characters whose ASCII code is used differently in
different national character variant 7 bit sets, and all 8bit
characters beyond DEL (7F hex) of the ISO Latin-1 set, shall not be
used unencoded. This is a recommendation for trouble-free
interchange, and as indicated below, the encoded set may be extended
or reduced.
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Encoding reserved characters
When a system uses a local addressing scheme, it is useful to provide
a mapping from local addresses into URIs so that references to
objects within the addressing scheme may be referred to globally, and
possibly accessed through gateway servers.
For a new naming scheme, any mapping scheme may be defined provided
it is unambiguous, reversible, and provides valid URIs. It is
recommended that where hierarchical aspects to the local naming
scheme exist, they be mapped onto the hierarchical URL path syntax in
order to allow the partial form to be used.
It is also recommended that the conventional scheme below be used in
all cases except for any scheme which encodes binary data as opposed
to text, in which case a more compact encoding such as pure
hexadecimal or base 64 might be more appropriate. For example, the
conventional URI encoding method is used for mapping WAIS, FTP,
Prospero and Gopher addresses in the URI specification.
CONVENTIONAL URI ENCODING SCHEME
Where the local naming scheme uses ASCII characters which are not
allowed in the URI, these may be represented in the URL by a
percent sign "%" immediately 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 unencoded in a URI.
REDUCED OR INCREASED SAFE CHARACTER SETS
The same encoding method may be used for encoding characters whose
use, although technically allowed in a URI, 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. Because a % sign always indicates
an encoded character, a URI may be made "safer" simply by encoding
any characters considered unsafe, while leaving already encoded
characters still encoded. Similarly, in cases where a larger set
of characters is acceptable, % signs can be selectively and
reversibly expanded.
Before two URIs can be compared, it is therefore necessary to
bring them to the same encoding level.
However, the reserved characters mentioned above have a quite
different significance when encoded, and so may NEVER be encoded
and unencoded in this way.
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The percent sign intended as such must always be encoded, as its
presence otherwise always indicates an encoding. Sequences which
start with a percent sign but are not followed by two hexadecimal
characters are reserved for future extension. (See Example 3.)
Example 1
The URIs
http://info.cern.ch/albert/bertram/marie-claude
and
http://info.cern.ch/albert/bertram/marie%2Dclaude
are identical, as the %2D encodes a hyphen character.
Example 2
The URIs
http://info.cern.ch/albert/bertram/marie-claude
and
http://info.cern.ch/albert/bertram%2Fmarie-claude
are NOT identical, as in the second case the encoded slash does not
have hierarchical significance.
Example 3
The URIs
fxqn:/us/va/reston/cnri/ietf/24/asdf%*.fred
and
news:12345667123%asdghfh@info.cern.ch
are illegal, as all % characters imply encodings, and there is no
decoding defined for "%*" or "%as" in this recommendation.
Partial (relative) form
Within a object whose URI is well defined, the URI of another object
may be given in abbreviated form, where parts of the two URIs are the
same. This allows objects within a group to refer to each other
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without requiring the space for a complete reference, and it
incidentally allows the group of objects to be moved without changing
any references. It must be emphasized that when a reference is
passed in anything other than a well controlled context, the full
form must always be used.
In the World-Wide Web applications, the context URI is that of the
document or object containing a reference. In this case partial URIs
can be generated in virtual objects or stored in real objects,
without the need for dramatic change if the higher-order parts of a
hierarchical naming system are modified. Apart from terseness, this
gives greater robustness to practical systems, by enabling
information hiding between system components.
The partial form relies on a property of the URI syntax that certain
characters ("/") and certain path elements ("..", ".") have a
significance reserved for representing a hierarchical space, and must
be recognized as such by both clients and servers.
A partial form can be distinguished from an absolute form in that the
latter must have a colon and that colon must occur before any slash
characters. Systems not requiring partial forms should not use any
unencoded slashes in their naming schemes. If they do, absolute URIs
will still work, but confusion may result. (See note on Gopher
below.)
The rules for the use of a partial name relative to the URI of the
context are:
If the scheme parts are different, the whole absolute URI must
be given. Otherwise, the scheme is omitted, and:
If the partial URI starts with a non-zero number of consecutive
slashes, then everything from the context URI up to (but not
including) the first occurrence of exactly the same number of
consecutive slashes which has no greater number of consecutive
slashes anywhere to the right of it is taken to be the same and
so prepended to the partial URL to form the full URL. Otherwise:
The last part of the path of the context URI (anything following
the rightmost slash) is removed, and the given partial URI
appended in its place, and then:
Within the result, all occurrences of "xxx/../" or "/." are
recursively removed, where xxx, ".." and "." are complete path
elements.
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Note: Trailing slashes
If a path of the context locator ends in slash, partial URIs are
treated differently to the URI with the same path but without a
trailing slash. The trailing slash indicates a void segment of the
path.
Note: Gopher
The gopher system does not have the concept of relative URIs, and the
gopher community currently allows / as data characters in gopher URIs
without escaping them to %2F. Relative forms may not in general be
used for documents served by gopher servers. If they are used, then
WWW software assumes, normally correctly, that in fact they do have
hierarchical significance despite the specifications. The use of HTTP
rather than gopher protocol is however recommended.
Examples
In the context of URI
magic://a/b/c//d/e/f
the partial URIs would expand as follows:
g magic://a/b/c//d/e/g
/g magic://a/g
//g magic://g
../g magic://a/b/c//d/g
g:h g:h
and in the context of the URI
magic://a/b/c//d/e/
the results would be exactly the same.
Fragment-id
This represents a part of, fragment of, or a sub-function within, an
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.
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Specific syntaxes for representing fragments in text documents by
line and character range, or in graphics by coordinates, or in
structured documents using ladders, are suitable for standardization
but not defined here.
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 continued and containing objects, is
not addressed by this document.
Fragment identifiers do 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.
There is no implication that a fragment identifier refers to anything
which can be extracted as an object in its own right. It may, for
example, refer to an indivisible point within an object.
Specific Schemes
The mapping for URIs onto some existing standard and experimental
protocols is outlined in the BNF syntax definition. Notes on
particular protocols follow. These URIs are frequently referred to
as URLs, though the exact definition of the term URL is still under
discussion (March 1993). The schemes covered are:
http Hypertext Transfer Protocol (examples)
ftp File Transfer protocol
gopher Gopher protocol
mailto Electronic mail address
news Usenet news
telnet, rlogin and tn3270
Reference to interactive sessions
wais Wide Area Information Servers
file Local file access
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The following schemes are proposed as essential to the unification of
the web with electronic mail, but not currently (to the author's
knowledge) implemented:
mid Message identifiers for electronic mail
cid Content identifiers for MIME body part
The schemes for X.500, network management database, and Whois++ have
not been specified and may be the subject of further study. Schemes
for Prospero, and restricted NNTP use are not currently implemented
as far as the author is aware.
The "urn" prefix is reserved for use in encoding a Uniform Resource
Name when that has been developed by the IETF working group.
New schemes may be registered at a later time.
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 host details are not passed on to the client when the URL is an
HTTP URL which refers to the server in question. In this case the
string sent starts with the slash which follows the host details.
However, when an HTTP server is being used as a gateway (or "proxy")
then the entire URI, whether HTTP or some other scheme, is passed on
the HTTP command line. The search part, if present, is sent as part
of the HTTP command, and may in this respect be treated as part of
the path. No fragmentid part of a WWW URI (the hash sign and
following) is sent with the request. Spaces and control characters
in URLs must be escaped for transmission in HTTP, as must other
disallowed characters.
EXAMPLES
These examples are not part of the specification: they are
provided as illustations only. The URI of the "welcome" page to a
server is conventionally
http://www.my.work.com/
As the rest of the URL (after the hostname an port) is opaque
to the client, it shows great variety but the following are all
fairly typical.
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http://www.my.uni.edu/info/matriculation/enroling.html
http://info.my.org/AboutUs/Phonebook
http://www.library.my.town.va.us/Catalogue/76523471236%2Fwen44--4.98
http://www.my.org/462F4F2D4241522A314159265358979323846
A URL for a server on a different port to 80 looks like
http://info.cern.ch:8000/imaginary/test
A reference to a particular part of a document may, including the
fragment identifier, look like
http://www.myu.edu/org/admin/people#andy
in which case the string "#andy" is not sent to the server, but is
retained by the client and used when the whole object had been
retrieved.
A search on a text database might look like
http://info.my.org/AboutUs/Index/Phonebook?dobbins
and on another database
http://info.cern.ch/RDB/EMP?*%20where%20name%%3Ddobbins
In all cases the client passes the path string to the server
uninterpreted, and for the client to deduce anything from
FTP
The ftp: prefix indicates that the FTP protocol is used, as defined
in STD 9, RFC 959 or any successor. The port number, if present,
gives the port of the FTP server if not the FTP default.
User name and password
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 Internet-style mail
address.
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Where possible, this mail address should correspond to a usable
mail address for the user, and preferably give a DNS host name
which resolves to the IP address of the client. Note that servers
currently vary in their treatment of the anonymous password.
Path
The FTP protocol allows for a sequence of CWD commands (change
working directory) and a TYPE command prior to service commands
such as RETR (retrieve) or NLIST (etc.) which actually access a
file.
The arguments of any CWD commands are successive segment parts of
the URL delimited by slash, and the final segment is suitable as
the filename argument to the RETR command for retrieval or the
directory argument to NLIST.
For some file systems (Unix in particular), 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 the same as the URL path. This does NOT mean that the
URL is a Unix filename.
Note: Retrieving subsequent URLs from the same host
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.
Data type
The data content type 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. A suitable FTP transfer type (for
example binary "I" or text "A") must in turn be deduced from the
data content type. It is recommended that conventions for
suffixes of public archives be established, but it is outside the
scope of this standard.
An FTP URL may optionally specify the FTP data transfer type by
which an object is to be retrieved. Most of the methods correspond
to the FTP "Data Types" ASCII and IMAGE for the retrieval of a
document, as specified in FTP by the TYPE command. One method
indicates directory access.
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The data type is specified by a suffix to the URL. Possible
suffixes are:
;type = <type-code> Use FTP type as given to perform data
transfer.
/ Use FTP directory list commands to read
directory
The type code is in the format defined in RFC 959 except that THE
SPACE IS OMITTED FROM THE URL.
Transfer Mode
Stream Mode is always used.
Gopher
The gopher URL specifies the host and optionally the port to which
the client should connect. This is followed by a slash and a single
gopher type code. This type code is used by the client to determine
how to interpret the server's reply and is is not for sending to
server. The command string to be sent to the server immediately
follows the gopher type character. It consists of the gopher
selector string followed by any "Gopher plus" syntax, but always
omitting the trainling CR LF pair.
When the gopher command string contains characters (such a embedded
CR LF and HT characters) not allowed in a URL, these are encoded
using the conventional encoding.
Note that some gopher selector strings begin with a copy of the
gopher type character, in which case that character will occur twice
consecutively. Also note that the gopher selector string may be an
empty string since this is how gopher clients refer to the top-level
directory on a gopher server.
If the encoded command string (with trailing CR LF stripped) would be
void then the gopher type character may be omiited and "1" (ASCII 31
hex) is assumed.
Note that slash "/" in gopher selector strings may not correspond to
a level in a hierarchical structure.
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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.
News
The news locators refer to either news group names or article message
identifiers which must conform to the rules for a Message-Id of RFC
1036 (Horton 1987). 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 (RFC 977, Kantor 1986)
(The ARTICLE by message-id command ) or using any other protocol for
the conveyance of usenet news articles, or by reference to a body of
news articles already received.
Note 1:
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
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 working group.
Further extension possible may be to allow the naming of subject
threads as addressable objects.
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
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addressed individually or, in general, automatically retrieved, this
is a less desirable, though currently common, solution.
URN
The "Universal Resource Name" is currently (March 1993) under
development in the IETF. A requirements specification is in
preparation. It currently looks as though it will be a short string
suitable for encoding in URI syntax, for which case the "urn:" prefix
is reserved. The URN shall be encoded precisely as defined in the
(future) URN standard, except in that:
If the official description of the URN syntax includes any
constant wrapper characters, then they shall not be omitted from
the URI encoding of the URN;
If the URN has a hierarchical nature, then the slash delimiter
shall be used in the URI encoding;
If the URN has a hierarchical nature, the most significant part
shall be encoded on the left in the URI encoding;
Any characters with reserved meanings in the URI syntax shall be
escape encoded
These rules of course apply to any URI scheme. It is of course
possible that the URN syntax will be chosen such that the URI
encoding will be a 1-1 transcription.
An example might be a name such as
urn:/iana/dns/ch/cern/cn/techdoc/94/1642-3
but the reader should refer to the latest URN drafts or
specifications.
WAIS
The current WAIS implementation public domain requires that a client
know the "type" of a object prior to retrieval. This value is
returned along with the internal object identifier in the search
response. It has been encoded into the path part of the URL in order
to make the URL sufficient for the retrieval of the object.
Within the WAIS world, names do not of course need to be prefixed by
"wais:" (by the partial form rules).
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The wpath of a WAIS URL consists of encoded fields of the WAIS
identifier, in the same order as inthe WAIS identifier. For each
field, the identifier field number is the digits before the equals
sign, and the field contents follow, encoded in the conventional
encoding, terminated by ";".
file
The other URI schemes (except nntp) share the property that they are
equally valid at any geographical place.
There is however a real practical requirement to be able to generate
a URL for an object in a machine's local file system.
The syntax is similar to the ftp syntax, but in this case the slash
is used to donate boundaries between directory levels of a
hierarchical file system is used. The "client" software converts the
file URL into a file name in the local file name conventions. This
allows local files to be treated just as network objects without any
necessity to use a network server for access. This may be used for
example for defining a user's "home" document in WWW.
There is clearly a danger of confusion that a link made to a local
file should be followed by someone on a different system, with
unexpected and possibly harmful results. Therefore, the convention
is that even a "file" URL is provided with a host part. This allows
a client on another system to know that it cannot access the file
system, or perhaps to use some other local mecahnism to access the
file.
The special value "localhost" is used in the host field to indicate
that the filename should really be used on whatever host one is.
This for example allows links to be made to files which are
distribted on many machines, or to "your unix local password file"
subject of course to consistency across the users of the data.
A void host field is equivalent to "localhost".
Message-Id
For systems which include information transferred using mail
protocols, there is a need to be able to make cross-references
between different items of information, even though, by the nature of
mail, those items are only available to a restricted set of people.
Two schemes are defined. The first, "mid:", refers to the STD 11,
RFC 822 Message-Id of a mail message. This Identifier is already
used in RFC 822 in for example the References and In-Reply-to field.
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The rest of the URL after the "mid:" is the RFC822 msg-id with the
constant <> wrapper removed, leaving an identifier whose format in
fact happens to be the same as addr-spec format for mailboxes (though
the semantics are different).
The use of a "mid" URL implies access to a body of mail already
received. If a message has been distributed using NNTP or other
usenet protocols over the news system, then the "news:" form should
be used.
Content-Id
The second scheme, "cid:", is similar to "mid:", but makes reference
to a body part of a MIME message by the value of its content-id
field. This allows, for example, a master document being the first
part of a multipart/related MIME message to refer to component parts
which are transferred in the same message.
Note
Beware however, that content identifiers are only required to be
unique within the context of a given MIME message, and so the cid:
URL is only meaningful with the context the same MIME message. For
a reference outside the message, it would need to be appended to
the message-id of the whole message. A syntax for this has not
been defined.
Schemes for Further Study
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
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.
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NETWORK MANAGEMENT DATABASE
This is a subject for study.
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. In all other cases the "news" scheme should
be used.
The news server name, newsgroup name, and index number of an
article within the newsgroup on that particular server are given.
The NNTP protocol must be used.
Note 1.
This form of URL is not of global accessability, as typically
NNTP servers only allow access from local clients. Note that
the article numbers within groups vary from server to server.
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. This is a local identifier
for a resource which is often available globally, and so is not
recommended except in the case in which incomplete NNTP
implementations on the local server force its adoption.
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 and an optional
version number. If present, the version number is separated from the
host specific object name by the characters "%00" (percent zero
zero), this being an escaped string terminator (null). External
Prospero links are represented as URLs of the underlying access
method and are not represented as Prospero URLs.
Registration of naming schemes
A new naming scheme may be introduced by defining a mapping onto a
conforming URL syntax, using a new prefix. Experimental prefixes may
be used by mutual agreement between parties, and must start with the
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characters "x-". The scheme name "urn:" is reserved for the work in
progress on a scheme for more persistent names.
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 URL, that the client software have provision for being
configured to use specific gateway locators for indirect access
through new naming schemes.
BNF of Generic URI Syntax
This is a BNF-like description of the URI syntax. at the level at
which specific schemes are not considered.
A vertical line "|" indicates alternatives, and [brackets] indicate
optional parts. Spaces are represented 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
URIs as the other productions. The "national" and "punctuation"
characters do not appear in any productions and therefore may not
appear in URIs.
fragmentaddress uri [ # fragmentid ]
uri scheme : path [ ? search ]
scheme ialpha
path void | xpalphas [ / path ]
search xalphas [ + search ]
fragmentid xalphas
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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 |
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
digit 0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
safe $ | - | _ | @ | . | &
extra ! | * | " | ' | ( | ) | ,
reserved = | ; | / | # | ? | : | space
escape % hex hex
hex digit | a | b | c | d | e | f | A | B | C |
D | E | F
national { | } | vline | [ | ] | \ | ^ | ~
punctuation < | >
void
(end of URI BNF)
BNF for specific URL schemes
This is a BNF-like description of the Uniform Resource Locator
syntax. A vertical line "|" indicates alternatives, and [brackets]
indicate optional parts. Spaces are represented 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.
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The current IETF URI Working Group preference is for the prefixedurl
production. (Nov 1993. July 93: url).
The "national" and "punctuation" characters do not appear in any
productions and therefore may not appear in URLs.
The "afsaddress" is left in as historical note, but is not a url
production.
prefixedurl u r l : url
url httpaddress | ftpaddress | newsaddress |
nntpaddress | prosperoaddress | telnetaddress
| gopheraddress | waisaddress |
mailtoaddress | midaddress | cidaddress
scheme ialpha
httpaddress h t t p : / / hostport [ / path ] [ ?
search ]
ftpaddress f t p : / / login / path [ ftptype ]
afsaddress a f s : / / cellname / path
newsaddress n e w s : groupart
nntpaddress n n t p : group / digits
midaddress m i d : addr-spec
cidaddress c i d : content-identifier
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 /
wpath
wpath digits = path ; [ wpath ]
groupart * | group | article
group ialpha [ . group ]
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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
version digits
attributes attribute [ attributes ]
attribute alphanums
telnetaddress t e l n e t : / / login
gopheraddress g o p h e r : / / hostport [/ gtype [
gcommand ] ]
login [ user [ : password ] @ ] hostport
hostport host [ : port ]
host hostname | hostnumber
ftptype A formcode | E formcode | I | L digits
formcode N | T | C
cellname hostname
hostname ialpha [ . hostname ]
hostnumber digits . digits . digits . digits
port digits
gcommand path
path void | segment [ / path ]
segment xpalphas
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search xalphas [ + search ]
user alphanum2 [ user ]
password alphanum2 [ password ]
fragmentid xalphas
gtype xalpha
alphanum2 alpha | digit | - | _ | . | +
xalpha alpha | digit | safe | extra | escape
xalphas xalpha [ xalphas ]
xpalpha xalpha | +
xpalphas xpalpha [ xpalphas ]
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 |
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
digit 0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
safe $ | - | _ | @ | . | & | + | -
extra ! | * | " | ' | ( | ) | ,
reserved = | ; | / | # | ? | : | space
escape % hex hex
hex digit | a | b | c | d | e | f | A | B | C |
D | E | F
national { | } | vline | [ | ] | \ | ^ | ~
punctuation < | >
digits digit [ digits ]
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alphanum alpha | digit
alphanums alphanum [ alphanums ]
void
(end of URL BNF)
References
Alberti, R., et.al., "Notes on the Internet Gopher Protocol",
University of Minnesota, December 1991,
<ftp://boombox.micro.umn.edu/pub/gopher/ gopher_protocol>. See also
<gopher://gopher.micro.umn.edu/00/Information About Gopher/About
Gopher>
Berners-Lee, T., "Hypertext Transfer Protocol (HTTP)", CERN, December
1991, as updated from time to time,
<ftp://info.cern.ch/pub/www/doc/http-spec.txt>
Crocker, D., "Standard for ARPA Internet Text Messages" STD 11, RFC
822, UDel, August 1982.
Davis, F, et al., "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, Information and Documentation -
Search and Retrieve Application Protocol Specification for open
Systems Interconnection, ISO-10163.
Horton, M., and R. Adams, "Standard for Interchange of USENET
messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic
Studies, December 1987.
Huitema, C., "Naming: strategies and techniques", Computer Networks
and ISDN Systems 23 (1991) 107-110.
Kahle, B., "Document Identifiers, or International Standard Book
Numbers for the Electronic Age", <ftp:
//quake.think.com/pub/wais/doc/doc-ids.txt>
Kantor, B., and P. Lapsley, Kantor, B., and P. Lapsley, "Network News
Transfer Protocol", RFC 977, UC San Diego & UC Berkeley, February
1986. <ftp://ds.internic.net/rfc/rfc977.txt>
Kunze, J., "Requirements for URLs", Work in Progress.
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Lynch, C., Coalition for Networked Information: "Workshop on ID and
Reference Structures for Networked Information", November 1991. See
<wais://quake.think.com/wais-discussion-archives?lynch>
Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC
1034, USC/Information Sciences Institute, November 1987,
<ftp://ds.internic.net/rfc/rfc1034.txt>
Neuman, B. Clifford, "Prospero: A Tool for Organizing Internet
Resources", Electronic Networking: Research, Applications and
Policy, Vol 1 No 2, Meckler Westport CT USA, 1992. See also
<ftp://prospero.isi.edu/pub/prospero/oir.ps>
Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD 9,
RFC 959, USC/Information Sciences Institute, October 1985.
<ftp://ds.internic.net/rfc/rfc959.txt>
Sollins, K., and L. Masinter, "Requiremnets for URNs", Work in
Progress.
Yeong, W., "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., "Representing Public Archives in the Directory", Work in
Progress, November 1991, now expired.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Tim Berners-Lee
World-Wide Web project
CERN
1211 Geneva 23,
Switzerland
Phone: +41 (22)767 3755
Fax: +41 (22)767 7155
EMail: timbl@info.cern.ch
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