Network Working Group Keith Moore
Internet-Draft University of Tennessee
Expires: 7 January 1996
Location-Independent URLs
or
URNs considered harmful
draft-ietf-uri-urns-harmful-00.txt
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Abstract
This document describes a means by which location-independent access to
resources can be provided, without creating a new class of resource
names. Instead, a resolution service is proposed for existing URLs,
which allows information providers to advertise meta-information about a
resource named by a URL, and/or alternate locations from which that par-
ticular resource might be accessed.
Depending on your point of view, the approach described in this document
might be taken as one or more of: (a) an alternative solution to the
"URL problem", (b) a strategy for gradual transition from URLs to URNs,
or (c) a worst-case scenario in the event that URN adoption takes too
long.
1. The Problem
URLs are widely used today, but they have three basic problems: (1) they
are tied to host names, (2) they are tied to filenames on a particular
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host, and (3) they are tied to access protocols. Since all of these are
likely to change over time, URLs are not stable names.
Host names impose two problems: stability and scalability. Host names
based on DNS are frequently unstable because they (by design) reflect
administrative hierarchies, which tend to change fairly frequently rela-
tive to the lifetime of a resource. Host names also reflect the names
of organizations, which also tend to change over time. Finally, a
resource "owned" or maintained by one organization can migrate to
another "organization", thus necessitating a change in the host name at
which a resource is located. All of these changes invalidate old URLs.
So long as the only way to access an object named by a URL, is to con-
tact the host whose name appears in a URL, it is difficult to provide
scalable access to those objects. Popular world wide web sites assign
several server machines to a single host name (perhaps using a modified
name server which randomizes responses), but this requires that the
entire collection of files made available using that host name, be con-
sistently mirrored to each server machine. As a practical consequence,
all of those server machines are usually maintained at a single loca-
tion, even though better service and conservation of bandwidth would
result from distributing the servers around the network.
Filenames embedded in URLs serve two purposes which are in conflict.
They need to be stable so that a reference to a named resource will con-
tinue to be valid for as long as needed. On the other hand, files are
usually organized into hierarchies to help humans browse through the
file system. The hierarchies need to change from time to time as new
files are added and old ones are removed. When the file system hierar-
chy is changed, the old URLs become invalid.
Having access protocols implicit in a resource name imposes an addi-
tional barrier: The client must support the access protocol (perhaps via
a proxy) in order to access the resource. This is effectively true even
if the resource is available via other protocols, because other URLs for
those protocols may not be available. Even if several URLs are avail-
able, the user must be knowledgable enough to choose one which is sup-
ported by his client, will pass through his security firewall, etc.
The issue of stability deserves careful scrutiny. Some people might be
happy with URLs if only they did not become stale during their useful
lifetime. Others have suggested that we need resource names to support
enforcement of copyright laws, and that resource names should therefore
be stable for the duration of the copyright (up to 150 years or more).
2. The "official future" scenario: URNs and URCs
The widely expected solution to these problems is something like the
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following: Instead of using URLs as resource names, we will migrate to
using "uniform resource *names*" or URNs. URNs will not be tied to
locations, but there will be resolution services available which will
allow a user to obtain the "characteristics" of the resource (also known
as the URC). Locations (URLs) of resources will also be obtainable via
the URN, either as part of the URC, indirectly (through a location-
independent file name or LIFN which appears in a URC), or perhaps via
some separate resolution service.
To solve the problem of stale links, users will have to upgrade to
browsers that support URNs directly, or access the web via proxy servers
which convert URNs to URLs. References to existing objects (several
million of them) will need to be converted to provide URNs instead of or
in addition to the URLs.
3. Cost of Transition to URNs
We have resource names now; they're called URLs. URLs have all kinds of
undesirable properties, but one nice thing is that they're cheap. All
you need is a host on the Internet and a DNS name, and you have your own
very large chunk of URL-space. Since DNS is a well-established part of
the Internet infrastructure, it's "free".
There's a cost to providing another level of indirection. New protocols
have to be defined and tested, new (and more complex) clients and
servers must be debugged. The new services have to be managed; the new
clients have to be configured correctly. Information providers (which
potentially includes nearly everybody) will have to learn how to use the
new tools.
There are also issues of serviceability and reliability. If the client
doesn't do what the user thinks it should, the system administrator has
one or two more possible culprits. Every additional level of lookup
imposes an overhead in network bandwidth and delay (as seen by the
user). When adding a layer to a service that millions of people depend
on, the new layer must integrate smoothly with those that already exist
so as not to adversely impact reliability.
Even after a URN lookup service exists and is available in clients, most
clients will still support only URLs for some time, and most references
to resources will still use URLs. During that period, there will be an
increased cost due not only to the need to maintain both sets of names,
but also to having multiple sources of failure. (Did the attempt to
access the resource fail because the URL was stale, or because the URN
server returned the wrong answer?). Due to the large number of
resources presently named by URLs, one can imagine needing a service to
map from URLs to URNs, in addition to the other way around.
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For any new protocol, naming scheme, or additional level of lookup, the
costs and benefits need to be analyzed to see whether the anticipated
gain is likely to be worth the cost. An important question is: when
will the user see a benefit from using a client that supports URNs?
Another is: how much investment is required to develop the URN infras-
tructure to the point that the user does see that benefit?
4. Current state of affairs
The Uniform Resource Identifiers working group was chartered in December
of 1992. Most of the problems identified in section I of this document
have been apparent for at least that long, yet there is no agreement on
an implementable solution for those problems.
A specification for URLs - which were already widely used and interoper-
able before URI was chartered - has been published. Some requirements
for URNs have been specified and published as RFC 1737. Requirements
for URCs are still being debated. Neither URNs, URCs, nor the resolu-
tion protocols have yet been specified.
To be fair, the URI working group was chartered only to develop resource
identifiers, not to design a complete system which solves problems.
Part of the reason for the long delay over URNs and URCs may be that the
discussion centers around the components of the system, without explic-
itly considering the system as a whole or the environment in which those
components must operate. While many of the proposals for URNs and URCs
were obviously designed to solve certain problems, there may still be a
lack of agreement about what problems need to be addressed, or the rela-
tive importance of those problems. There may also be a lack of under-
standing or agreement on the cost of providing a particular service ver-
sus the benefit that it would provide.
Frederick Brooks's _The Mythical Man-Month_ describes a phenomenon known
as "the second-system effect". He writes:
An architect's first work is apt to be spare and clean. He
knows he doesn't know what he's doing, so he does it carefully
and with great restraint.
As he designs his first work, frill after frill and
embellishment after embellishment occur to him. These
get stored away to be used "next time". Sooner or later
the first system is finished, and the architect, with firm
confidence and a demonstrated mastery of that class of
systems, is ready to build a second system.
This second is the most dangerous system a man ever designs...
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It is tempting to cite URNs and URCs as examples of the second-system
effect. Most of us are painfully aware of the deficiencies of URLs, and
also of the difficulties in changing from URLs to some other scheme.
There is therefore a tremendous incentive to do the job "right" this
time around, lest we end up with URNs and URCs that are somehow insuffi-
cient for the job within a few years. Some would even suggest that we
need to get URNs right "once and for all".
But it's at least possible that problems inherent in URLs and the world
wide web are not due not only to a lack of foresight in their design
(after all, who could have predicted the tremendous success of the
web?), but also to the lack of ability to control the ways in which the
web was used. To put it a different way, it takes time for tens of
thousands of new information providers to acquire disciplines and build
tools for consistent resource naming and keeping track of resource loca-
tions. This is true regardless of the scheme used for resource names.
We probably cannot anticipate needs of web users and information
providers for more than the next few years, and we are deluding our-
selves if we believe we can impose a discipline on its use simply by
creating a new space for resource names. We need to design a system
which is adaptable to future needs without knowing precisely what those
needs are. At the same time, if a solution for possible future problems
does not address today's needs, it is doomed to failure.
The functional requirements for URNs (from RFC 1737) include: global
scope, global uniqueness, persistence, scalability (of assignment),
legacy support, extensibility, independence, and resolution. Notably
absent from this list is scalability of access, even though the lack
thereof is one of our most pressing problems.
5. An alternate scenario: Location-Independent URLs
The usual explanation for why URLs are a Bad Thing (tm) is "URLs are
tied to locations". But the Internet has already had one successful
transition away from location-based names - in electronic mail. Once
upon a time, email addresses were of the form user@host, and were there-
fore tied to the network "locations" of those hosts. To send mail to a
user at a host, you connected to that host's SMTP server (before that,
it was the FTP server), and told it to deliver the message to that
user's mailbox.
Along came DNS and the MX record. Addresses are now of the form
user@domain. Instead of connecting to the host associated with that
domain, one now connects to one of the mail exchangers for that domain
listed in the DNS. One result is that email domains are increasingly
decoupled from host names. It is not uncommon for a single email domain
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to serve hundreds of Internet hosts, which may not even be directly con-
nected to the Internet. Email addresses are now less likely to be tied
to individual host names. And since there can be multiple mail exchang-
ers for a domain, other results have been increased fault-tolerance via
redundancy, and better ability to handle load.
Imagine a new DNS record type called RCS (for "resource catalog server")
which performed an MX-like function for URLs. For example, the records:
www.netlib.org. RCS 10 netlib2.cs.utk.edu.
RCS 10 netlib1.epm.ornl.gov.
would inform a web client that meta-information for any URL containing
the domain www.netlib.org, could be found using the resource catalog
servers at netlib2.cs.utk.edu and netlib1.epm.ornl.gov.
These records would be obtained in a single DNS query for
www.netlib.org, along with the IP addresses of the RCS servers. If
there had been no RCS records, the same query would have returned the IP
addresses of www.netlib.org. (Existing TXT records could be used
instead of RCS records, but additional DNS queries would be required to
look up the IP addresses of any resource characteristics servers.)
Once the addresses of RCS servers were known for the domain in a URL,
the client would use a special-purpose resolution protocol to obtain
characteristics of the resource named by that URL, alternate locations
(i.e additional URLs) at which that URL could be accessed, or both.
New clients that supported this scheme would use the same URLs as the
old clients, but would gain immediate benefit from being able to dis-
cover alternate servers for their resources (with little penalty for
trying). Eventually they would be able to make use of URCs as well.
When coupled with a mechanism such as SONAR for finding network proxim-
ity information (see draft-moore-sonar-00.txt), the client would gain
the ability to automatically choose a nearby location for that resource
(thereby improving access times), and to recover from the failure of any
single resource server.
Legacy clients would still be able to access those resources as long as
there were a *host* with the same name as that used in the URL which
provided such access. Information providers would therefore continue to
maintain such servers until most legacy clients had been replaced.
It should be possible to reserve a few chunks of DNS for naming authori-
ties for long-term resource-names. Subdomains within these spaces would
be required to NOT be meaningful to humans; thus, the names themselves
need never be obsolete. While such a subdomain would initially be
assigned to a publisher, the responsibility for serving that subdomain
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could be transferred as necessary when that publisher or its intellec-
tual property assets changed hands. Finally, resource names would only
be assigned for any particular subdomain for a short time (perhaps a
year), after which a new resource name would be used. This would allow
the resolution service for older subdomains could be shifted from pri-
mary servers to "custodians".
One nice feature of this scheme is that it still works for ordinary
users and their home pages. Sites could set up their location and/or
URC servers with their existing DNS names; they need not obtain new nam-
ing authority names. If a user's web page becomes unexpectedly popular,
a resource catalog server and appropriate RCS records could be installed
to inform clients of alternate locations, even though no prior arrange-
ments were made.
But publishers and others who had an interest in making resources avail-
able over the long-term (including probably anybody who wanted to make
money selling access to his works), would see the benefit in using the
new name space. And new naming authority names could be distinguished
from ordinary DNS names.
Such a system would not provide scalable resolution of resource names
for several centuries in the future. But it probably would work for a
few decades, during which usage patterns are almost certainly to change
beyond what we can anticipate. It would also encourage building of an
infrastructure for maintaining meta-information about resources.
6. URN functional requirements as applied to URLs
RFC 1737 lists the following functional requirements for URNs:
a. Global scope: A URN is a name with global scope which does not
imply a location. It has the same meaning everywhere.
Non-relative URLs already have the same meaning everywhere. If an MX
like system is defined, a URL no longer implies a location.
b. Global uniqueness: The same URN will never be assigned to two dif-
ferent resources.
This is generally true in practice for URLs. It is only rarely that a
file name which is exported to the world, is re-used to name a com-
pletely different resource. To the extent that this is not true, it is
a matter of discipline and education on the part of information
providers. If providers do not understand the value of unique naming,
the introduction of URNs will not force them to apply the discipline.
If providers do understand the value of unique naming, they can provide
unique names for URLs.
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c. Persistence: It is intended that the lifetime of a URN be perma-
nent. That is, the URN will be globally unique forever, and may
well be used as a reference to a resource well beyond the lifetime
of the resource it identifies or of any naming authority involved
in the assignment of its name.
The persistence of URLs depends on the persistence of their components -
file names and domain names.
Persistence of file names is also a matter of discipline on the part of
an information provider, since file systems and existing protocol
servers are capable of storing files under fairly arbitrary names. If
the "browsing" function that currently requires human-meaningful file
names and arranges them into human-meaningful hierarchies is replaced
with some other means of resource discovery, information providers are
free to begin using some other naming scheme that allows stable names,
*without* introducing a new kind of resource name. Similarly, persis-
tence of domain names can be achieved by using domain names that do not
correspond to organizational names or hierarchies, and which will never
be re-assigned.
On the other hand, for persistent resource names to be useful, requires
a long-term commitment to maintain the characteristics and location data
and to provide the resolution and/or access services. Few organizations
(and not all countries) can reliably commit to providing such support
for several generations, yet large centralized databases would provide a
dangerous means for control of networked information.
d. Scalability: URNs can be assigned to any resource that might con-
ceivably be available on the network, for hundreds of years.
URLs meet this requirement.
e. Legacy support: The scheme must permit the support of existing
legacy naming systems, insofar as they satisfy the other require-
ments described here. For example, ISBN numbers, ISO public identi-
fiers, and UPC product codes seem to satisfy the functional
requirements, and allow an embedding that satisfies the syntactic
requirements described here.
URLs were designed to support legacy protocols, and therefore are very
adaptable to legacy naming systems.
f. Extensibility: Any scheme for URNs must permit future extensions to
the scheme.
URLs have proven to be extensible to several protocols other than those
for which they were designed.
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g. Independence: It is solely the responsibility of a name issuing
authority to determine the conditions under which it will issue a
name.
URLs meet this requirement.
h. Resolution: A URN will not impede resolution (translation into a
URL, q.v.). To be more specific, for URNs that have corresponding
URLs, there must be some feasible mechanism to translate a URN to a
URL.
By introducing a resolution service which uses URLs as resource names
and allows location of resolution servers using DNS, URLs meet this
requirement.
7. Limitations
If we provide the means for the continued use of URLs, some users and/or
information providers may delay migrating to new protocols (URC records,
resolution servers) or to better resource naming schemes (URLs based on
stable domain names and file names, or a new URL type named "URN").
Nevertheless, the new proposal provides incentives to migrate for both
users and providers.
DNS names are not ideally suited as naming authority names. The problem
is not so much due to the names themselves (since new DNS subtrees can
be reserved for naming authorities) but to the way the lookup hierarchy
is exposed in the name. If a particular branch of the tree grows too
big (as in the .COM domain), it is difficult to sub-divide it. This can
be solved by limiting the number of branches at any node within the por-
tion of DNS space reserved for naming authority names. Alternatively, a
special-purpose server (which understand the DNS query protocol) could
be constructed to provide resolution for any large and flat subspace
within the DNS tree.
8. Summary
The difficulty in transition from URLs to URNs, combined with the delay
by IETF in defining URNs and resolution services, encourages solutions
to the existing problems of the web which use existing URLs. Such solu-
tions could possibly be implemented and deployed without the cooperation
of the IETF.
On the other hand, IETF could use this mechanism to provide a transition
from present-day URLs to names which (while syntactically identical to
URLs), had all of the characteristics desired for URNs. The transition
to such a scheme would be easier than a transition to an entirely new
naming scheme.
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Many of the difficulties with the present URL scheme can be seen as a
lack of knowledge and discipline on the part of network information
providers. This is to be expected, considering the rapid growth of the
web and the fact that many of its information providers are ordinary
users. These problems would exist regardless of the naming scheme used,
and will only be solved through education, new tools, and time.
9. Security Considerations
Security Considerations are not addressed in this memo.
10. Author's Address
Keith Moore
Department of Computer Science
University of Tennessee
107 Ayres Hall
Knoxville TN 37996-1301
moore@cs.utk.edu
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