1 URI Partitioning
There is some confusion in the web community over the partitioning of URI space,
specifically, the relationship among the concepts of URL, URN, and URI. The
confusion owes to the incompatibility between two different views of URI partitioning,
which we call the "classical" and "contemporary" views.
1.1 Classical View
During the early years of discussion of web identifiers (early to mid 90s),
people assumed that an identifer type would be cast into one of two (or possibly
more) classes. An identifier might specify the location of a resource (a URL)
or its name (a URN) independent of location. Thus a URI was either a URL or
a URN. There was discussion about generalizing this by addition of a discrete
number of additional classes; for example, a URI might point to metadata rather
than the resource itself, in which case the URI would be a URC (citation). URI
space was thus viewed as partitioned into subspaces: URL and URN, and additional
subspaces, to be defined. The only such additional space ever proposed was URC
and there never was any buy-in; so without loss of generality it's reasonable
to say that URI space was thought to be partitioned into two classes: URL and
URN. Thus for example, "http:" was a URL scheme, and "isbn:"
would (someday) be a URN scheme. Any new scheme would be cast into one or the
other of these two classes.
1.2 Contemporary View
Over time, the importance of this additional level of hierarchy seemed to lessen;
the view became that an individual scheme does not need to be cast into one
of a discrete set of URI types such as "URL", "URN", "URC",
etc. Web-identifer schemes are in general URI schemes; a given URI scheme may
define subspaces. Thus "http:" is a URI scheme. "urn:" is
also a URI scheme; it defines subspaces, called "namespaces". For
example, the set of URNs of the form "urn:isbn:n-nn-nnnnnn-n" is a
URN namespace. ("isbn" is an URN namespace identifier. It is not a
"URN scheme" nor a "URI scheme").
Further according to the contemporary view, the term "URL" does not
refer to a formal partition of URI space; rather, URL is a useful but informal
concept: a URL is a type of URI that identifies a resource via a representation
of its primary access mechanism (e.g., its network "location"), rather
than by some other attributes it may have. Thus as we noted, "http:"
is a URI scheme. An http URI is a URL. The phrase "URL scheme" is
now used infrequently, usually to refer to some subclass of URI schemes which
exclude URNs.
1.3 Confusion
The body of documents (RFCs, etc) covering URI architecture, syntax, registration,
etc., spans both the classical and contemporary periods. People who are well-versed
in URI matters tend to use "URL" and "URI" in ways that
seem to be interchangable. Among these experts, this isn't a problem. But among
the Internet community at large, it is. People are not convinced that URI and
URL mean the same thing, in documents where they (apparently) do. When one sees
an RFC that talks about URI schemes (e.g. [RFC 2396]), another that talks about
URL schemes (e.g. [RFC 2717]), and yet another that talks of URN schemes ([RFC
2276]) it is natural to wonder what's the difference, and how they relate to
one another. While RFC 2396 1.2 attempts to address the distinction between
URIs, URLs and URNs, it has not been successful in clearing up the confusion.
http://www.w3.org/TR/uri-clarification/
When you change a URI on your server, you can never completely
tell who will have links to the old URI. They might have made links from regular
web pages. They might have bookmarked your page. They might have scrawled the
URI in the margin of a letter to a friend.
When someone follows a link and it breaks, they generally lose confidence in
the owner of the server. They also are frustrated - emotionally and practically
from accomplishing their goal.
Enough people complain all the time about dangling links that I hope the damage
is obvious. I hope it also obvious that the reputation damage is to the maintainer
of the server whose document vanished.
http://www.w3.org/Provider/Style/URI