Internet Draft                                          Karen R. Sollins
draft-ietf-urn-req-frame-03.txt                                  MIT/LCS
Expires January 30, 1998                                   July 30, 1997

       Architectural Principles of Uniform Resource Name Resolution


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Abstract:

This document addresses the issues of the discovery of URN (Uniform
Resource Name) resolver services that in turn will directly translate
URNs into URLs (Uniform Resource Locators) and URCs (Uniform Resource
Characteristics).  The document falls into three major parts, the
assumptions underlying the work, the guidelines in order to be a
viable Resolver Discovery Service or RDS, and a framework for
designing RDSs.  The guidelines fall into three principle areas:
evolvability, usability, and security and privacy.  An RDS that is
compliant with the framework will not necessarily be compliant with
the guidelines.  Compliance with the guidelines will need to be
validated separately.

. Introduction

The purpose of this document is to lay out the engineering criteria
for what we will call here a Resolver Discovery Service (RDS), a
service to help in the learning about URN (Uniform Resource Name)
resolvers.  The term "resolver" is used in this document to indicate a
service that translates URNs to URLs (Uniform Resource Locators) or
URCs (Uniform Resource Characteristics).  Some resolvers may provide
direct access to resources as well.  An RDS helps in finding a
resolver to contact for further resolution.  It is worth noting that
some RDS designs may also incorporate resolver functionality.  This
function of URN resolution is a component of the realization of an
information infrastructure.  In the case of this work, that
infrastructure is to be available, "in the Internet" or globally, and
hence the solutions to the problems we are addressing must be globally

                                 - 1 -

scalable.  In this document, we are focussing specifically on the
design of RDS schemes.

The Uniform Resource Identifier Working Group defined a naming
architecture, as demonstrated in a series of three RFCs 1736[1],
1737[2], and 1738[3].  Although several further documents are needed
to complete the description of that architecture, it incorporates
three core functions often associated with "naming": identification,
location, and mnemonics or semantics.  By location, we mean
fully-qualified Domain Names or IP addresses, possibly extended with
TCP ports and/or local identifiers, such as pathnames.  Names may
provide the ability to distinguish one resource from another, by
distinguishing their "names".  Names may help to provide access to a
resource by including "location" information.  Lastly, names may have
other semantic or mnemonic information that either helps human users
remember or figure out the names, or include other semantic
information about the resource being named.  The URI working group
concluded that there was need for persistent, globally unique
identifiers, distinct from location or other semantic information;
these were called URNs.  These "names" provide identity, in that if
two of them are "the same" (under some simple rule of
canonicalization), they identify the same resource.  Furthermore, the
group decided that these "names" were generally to be for machine,
rather than human, consumption.

In contrast to URNs, one can imagine a variety human-friendly naming
(HFN) schemes supporting different suites of applications and user
communities.  These will need to provide mappings to URNs in tighter
or looser couplings, depending on the namespace.  It is these HFNs
that will be mnemonic, content-full, and perhaps mutable, to track
changes in use and semantics.  They may provide nicknaming and other
aliasing, relative or short names, context sensitive names,
descriptive names, etc.  Their definition is not part of this effort,
but will clearly play an important role in the long run.

URNs as described in RFC 1737 are defined globally; they are
ubiquitous in that a URN anywhere in any context identifies the same
resource.  Given this requirement on URNs, one must ask about its
implication for an RDS.  Does ubiquity imply a guarantee of RDS
resolution everywhere?  Does ubiquity imply resolution to the same
information about resolution everywhere?  In both cases the answer is
probably not.  One cannot make global, systemic guarantees, except at
an expense beyond reason.  In addition there may be policy reasons for
not resolving in the same way everywhere.  It is quite possible that
the resolution of a URN to an instance of a resource may reach
different instances or copies under different conditions.  Thus,
although a URN anywhere refers to the same resource, in some
environments under some conditions, and at different times, due to
either the vagaries of network conditions or policy controls a URN may
sometimes be resolvable and other times or places not.  Ubiquitous
resolution cannot be assumed simply because naming is ubiquitous.  On
the other hand wide deployment and usage will be an important feature
of any RDS design.

Within the URI community there has been a concept used frequently that
for lack of a better term we will call a _hint_.  A hint is something

                                 - 2 -

that helps in the resolution of a URN; in theory we map URNs to hints
as an interim stage in accessing a resource.  In practice, an RDS may
map a URN directly into the resource itself if it chooses to.  It is
very likely that there will be hints that are applicable to large sets
of URNs, for example, a hint that indicates that all URNs with a
certain prefix or suffix can be resolved by a particular resolver.  A
hint may also have meta-information associated with it, such as an
expiration_time or certification of authenticity.  We expect that
these will stay with a hint rather than being managed elsewhere.  We
will assume in all further discussion of hints that they include any
necessary meta-information as well as the hint information itself.
Examples of hints are: 1) the URN of a resolver service that may
further resolve the URN, 2) the address of such a service, 3) a
location at which the resource was previously found.  The defining
feature of hints is that they are only hints; they may be out of date,
temporarily invalid, or only applicable within a specific locality.
They do not provide a guarantee of access, but they probably will help
in the resolution process.  We must assume that most resolutions of
URNs will be provided by the use of locally stored hints, because
maintaining a database of globally available, completely up-to-date
location information is infeasible for performance reasons.  There are
a number of circumstances in which one can imagine that hints become
invalid, either because a resource has moved or because a different
URN resolver service has taken over the responsibility for resolution
of the URN.  Hints may be found in a variety of places.  It is
generally assumed that a well engineered system will maintain or cache
a set of hints for each URN at each location where that URN is found.
These may have been acquired from the owner of the resources, a
recommendation of the resource, or one of many other sources.  In
addition, for those situations in which those hints found locally
fail, a well engineered system will provide a fall-back mechanism for
discovering further hints.  It is this fall-back mechanism, an RDS,
that is being addressed in this document.  As with all hints, there
can never be a guarantee that access to a resource will be available
to all clients, even if the resource is accessible to some.  However,
an RDS is expected to work with reasonably high reliability, and,
hence, may result in increased response time.

The remainder of this document falls into three sections.  The first
identifies several sets of assumptions underlying this work.  There are
three general assumptions:
   * URNs are persistent;
   * URN assignment can be delegated;
   * Decisions can be made independently, enabling isolation from decisions
     of one's peers.

The next section lays out three central principles Resolver Discovery
Service design.  For each of these, we have identified a number of
more specific guidelines that further define and refine the general
principle.  This section is probably the most critical of the
document, because one must hold any proposed RDS scheme up against
these principles and corollary guidelines to learn whether or not it
is adequate.  The three central principles can be summarized as:
   1) An RDS must allow for evolution and evolvability;
   2) Usability of an RDS with regard to each of the sets of constituents
       involved in the identification and location or resources is paramount;

                                 - 3 -

   3) It is centrally important that the security and privacy needs of all
       constituents be feasibly supported, to the degree possible.
Each of the three major subsections of the guidelines section begins
with a summary list of the more detailed guidelines identified in that
section.

The final section of the document lays out a framework for such RDSs.
The purpose of this last section is to bound the search space for RDS
schemes.  The RDS designer should be aware that meeting the guidelines
is of primary importance; it is possible to meet them without
conforming to the framework.  As will be discussed further in this
last section, designing within the framework does not guarantee
compliance, so compliance evaluation must also be part of the process
of evaluation of a scheme.

. Assumptions

Based on previous internet drafts and discussion in both the URN BOFs
and on the URN WG mailing list, three major areas of assumptions are
apparent: longevity, delegation, and independence.  Each will be
discussed separately.

The URN requirements[2] state that a URN is to be a "persistent
identifier".  It is probably the case that nothing will last forever,
but in the time frame of resources, users of those resources, and the
systems to support the resources, the identifier should be considered
to be persistent or have a longer lifetime than those other entities.
There are two assumptions that are implied by longevity of URNs:
mobility and evolution.  Mobility will occur because resources may
move from one machine to another, owners of resources may move among
organizations, or the organizations themselves may merge, partition,
or otherwise transforms themselves.  The Internet is continually
evolving; protocols are being revised, new ones created, while
security policies and mechanisms evolve as well.  These are only
examples.  In general, we must assume that almost any piece of the
supporting infrastructure of URN resolution will evolve.  In order to
deal with both the mobility and evolution assumptions that derive from
the assumption of longevity, we must assume that users and their
applications can remain independent of these mutating details of the
supporting infrastructure.

The second assumption is that naming and resolution authorities may
delegate some of their authority or responsibility; in both cases, the
delegation of such authority is the only known method of allowing for
the kind of scaling expected.  It is important to note that a
significant feature of this work is the potential to separate name
assignment, the job of labelling a resource with a URN, from name
resolution, the job of discovering the resource given the URN.  In
both cases, we expect multi-tiered delegation.  There may be RDS
schemes that merge these two sets of responsibilities and delegation
relationships; by doing so, they bind together or overload two
distinctly different activities, thus probably impeding growth.

The third assumption is independence or isolation of one authority
from another and, at least to some extent, from its parent.  When one
authority delegates some of its rights and responsibilities to another

                                 - 4 -

authority, the delegatee can operate in that domain independently of
its peers and within bounds specified by the delegation, independently
of the delegator.  This isolation is critically important in order to
allow for independence of policy and mechanism.

This third assumption has several corollaries.  First, we assume that
the publisher of a resource can choose resolver services,
independently of choices made by others.  At any given time, the owner
of a namespace may choose a particular URN resolver service for that
delegated namespace.  Such a URN resolver service may be outside the
RDS service model, and only identified or located by the RDS service.
Second, it must be possible to make a choice among RDS services.  The
existence of multiple RDS services may arise from the evolution of an
RDS service, or development of new ones.  Although at any given time
there is likely to be only one or a small set of such services, the
number is likely to increase during a transition period from one
architecture to another.  Thus, it must be assumed that clients can
make a choice among a probably very small set of RDSs.  Third, there
must be independence in the choice about levels and models of security
and authenticity required.  This choice may be made by the owner of a
naming subspace, in controlling who can modify hints in that subspace.
A naming authority may delegate this choice to the owners of the
resources named by the names it has assigned.  There may be
limitations on this freedom of choice in order to allow other
participants to have the level of security and authenticity they
require, for example, in order to maintain the integrity of the RDS
infrastructure as a whole.  Fourth, there is an assumption of
independence of choice of the rule of canonicalization of URNs within
a namespace, limited by any restrictions or constraints that may have
been set by its parent namespace.  This is a choice held by naming
authorities over their own subnamespaces.  Rules for canonicalization
will be discussed further in the framework section below.  Thus, there
are assumptions of independence and isolation to allow for delegated,
independent authority in a variety of domains.

The modularity assumptions of delegation and isolation imply
independence of decision and implementation, leading to a
decentralization that provides a certain degree of safety from denial
of service.  Based on these these assumptions in conjunction with that
of longevity and those for URLs and URNs as detailed in RFCs 1736 and
1737, we can now turn to the guidelines for an RDS.

. Guidelines

The guidelines applying to an RDS center around three important design
principles in the areas of evolvability, usability, and security and
privacy.  At its core the function of an RDS is to provide hints for
accessing a resource given a URN for it.  These hints may range in
applicability from local to global, and from short-lived to
long-lived.  They also may vary in their degree of verifiable
authenticity.  While it may be neither feasible nor necessary that
initial implementations support every guideline, every implementation
must support evolution to systems that do support the guideline more
fully.


                                 - 5 -

It is important to note that there are requirements, not applicable
specifically to an RDS that must also be met.  A whole URN system will
consist of names in namespaces, the resolution information for them,
and the mapping from names in the namespaces to resolution information
(or hints).  URNs themselves must meet the requirements of RFC 1737.
In addition, namespaces themselves must meet certain requirements as
described in RFC NNNN[5].  Although all these requirements and
guidelines are not described here, they must be supported to provide
an acceptable system.

Each section below begins with a summary of the points made in that
section.  There is some degree of overlap among the areas, such as in
allowing for the evolution of security mechanisms, etc., and hence
issues may be addressed in more than one section.  It is also
important to recognize that conformance with the guidelines will often
be subjective.  As with most IETF guidelines and requirements, most of
these are not quantifiable and hence conformance is a judgment call
and a matter of degree.  Lastly, the reader may find that some of them
are those of general applicability to distributed systems and some are
specific to URN resolution.  Those of general applicability are
included for completeness and are not distinguished as such.

3.1 Evolution

The issues in the area of the first principle, that of evolvability,
are:

    1.1) An RDS must be able to support scaling in at least three
         dimensions: the number of resources for which URNs will be
         required, the number of publishers and users of those
         resources, and the complexity of the delegation, as authority
         for resolution grows and possibly reflects delegation in
         naming authority;
    1.2) A hint resolution environment must support evolution of
         mechanisms, specifically for:
         * a growing set of URN schemes;
         * new kinds local URN resolver services;
         * new authentication schemes;
         * alternative RDS schemes active simultaneously;
    1.3) An RDS must allow the development and deployment of
         administrative control mechanisms to manage human behavior with
         respect to limited resources.

One of the lessons of the Internet that we must incorporate into the
development of mechanisms for resolving URNs is that we must be prepared
for change.  Such changes may happen slowly enough to be considered
evolutionary modifications of existing services, or dramatically enough
to be considered revolutionary.  They may permeate the Internet universe
bit by bit, living side by side with earlier services or they may take
the Internet by storm, causing an apparent complete transformation over
a short period of time.  There are several directions in which we can
predict the need for evolution.  At the very least, the community and
the mechanisms proposed should be prepared for these.

First, scaling is a primary issue in conjunction with evolution.  The
number of users, both human and electronic, as well as the number of

                                 - 6 -

resources will continue to grow exponentially for the near term, at
least.  Hence the number of URNs will also increase similarly.  In
addition, with growth in sheer numbers is likely to come growth in the
delegation of both naming authority and resolution authority.  These
facts mean that an RDS design must be prepared to handle increasing
numbers of requests for inclusion, update and resolution, in a set of
RDS servers perhaps inter-related in more complex ways.  This is not
to say that there will necessarily be more updates or resolutions per
URN; we cannot predict that at this time.  But, even so, the
infrastructure may become more complex due to delegation, which may
(as can be seen in Section 4 on the framework) lead to more complex
rules for rewriting or extracting terms for staged resolution.  Any
design is likely to perform less well above some set of limits, so it
is worth considering the growth limitations of each design
alternative.

Second, we expect there to be additions and changes to the mechanisms.
The community already understands that there must be a capacity for
new URN schemes, as described in [5].  A URN scheme will define a set
of URNs that meet the URN requirements[2], but may have further
constraints on the internal structure of the URN. The intention is
that URN schemes can be free to specify parts of the URN that are left
opaque in the larger picture.  In fact, a URN scheme may choose to
make public or keep private the algorithms for any such "opaque" part
of the URN.  In any case, we must be prepared for a growing number of
URN schemes.

Often in conjunction with a new URN scheme, but possibly independently
of any particular URN scheme, new kinds of resolver services may
evolve.  For example, one can imagine a specialized resolver service
based on the particular structure of ISBNs that improves the
efficiency of finding documents given their ISBNs.  Alternatively, one
can also imagine a general purpose resolver service that trades
performance for generality; although it exhibits only average
performance resolving ISBNs, it makes up for this weakness by
understanding all existing URN schemes, so that its clients can use
the same service to resolve URNs regardless of naming scheme.  In this
context, there will always be room for improvement of services,
through improved performance, better adaptability to new URN schemes,
or lower cost, for example.  New models for URN resolution will evolve
and we must be prepared to allow for their participation in the
overall resolution of URNs.

If we begin with one overall plan for URN resolution, into which the
enhancements described above may fit, we must also be prepared for an
evolution in the authentication schemes that will be considered either
useful or necessary in the future.  There is no single globally
accepted authentication scheme, and there may never be one.  Even if
one does exist at some point in time, we must always be prepared to
move on to newer and better schemes, as the old ones become too easily
spoofed or guessed.

In terms of mechanism, although we may develop and deploy a single RDS
scheme initially, we must be prepared for that top level model to
evolve.  Thus, if the RDS model supports an apparently centralized
(from a policy standpoint) scheme for inserting and modifying

                                 - 7 -

authoritative information, over time we must be prepared to evolve to
a different model, perhaps one that has a more distributed model of
authority and authenticity.  If the model has no core but rather a
cascaded partial discovery of information, we may find that this
becomes unmanageable with an increase in scaling.  Whatever the model,
we must be prepared for it to evolve with changes in scaling,
performance, and policy constraints such as security and cost.

The third evolutionary issue is even more mechanical than the others.
At any point in time, the community is likely to be supporting a
compromise position with respect to resolution.  We will probably be
operating in a situation balanced between feasibility and the ideal,
perhaps with policy controls used to help stabilize use of the
service.  Ideally, the service would be providing exactly what the
customers wanted and they in turn would not request more support than
they need, but it seems extremely unlikely.  Since we will almost
always be in a situation in which some service provision resources
will be in short supply, some form of policy controls will generally
be necessary.  Some policy controls may be realized as mechanisms
within the servers or in the details of protocols, while others may
only be realized externally to the system.  For example, suppose hint
entries are being submitted in such volume that the hint servers are
using up their excess capacity and need more disk space.  Two
suggestions for policy control are pricing and administrative.  As
technology changes and the balance of which resources are in short
supply changes, the mechanisms and policies for controlling their use
must evolve as well.

3.2 Usability

To summarize, the usability guidelines fall into three areas based on
participation in hint management and discovery:

    2.1) The publisher
       2.1.1) URN to hint resolution must be correct and efficient with
              very high probability;
       2.1.2) Publishers must be able to select and move among URN
              resolver services to locate their resources;
       2.1.3) Publishers must be able to arrange for multiple access
              points for their location information;
       2.1.4) Publishers should be able to provide hints with varying
              lifetimes;
       2.1.5) It must be relatively easy for publishers to specify to
              the management and observe their hint information as well
              as any security constraints they need for their hints.
    2.2) The client
       2.2.1) The interface to the RDS must be simple, effective, and
              efficient;
       2.2.2) The client and client applications must be able to
              understand the information stored in and provided by the
              RDS easily, in order to be able to make informed choices.
    2.3) The management
       2.3.1) The management of hints must be as unobtrusive as
              possible, avoiding using too many network resources;
       2.3.2) The management of hints must allow for administrative
              controls that encourage certain sorts of behavior deemed

                                 - 8 -

              necessary to meet other requirements;
       2.3.3) The configuration and verification of configuration of
              individual RDS servers must be simple enough not to
              discourage configuration and verification.

Usability can be evaluated from three distinct perspectives: those of a
publisher wishing to make a piece of information public, those of a
client requesting URN resolution, and those of the provider or manager
of resolution information.  We will separately address the usability
issues from each of these three perspectives.

It is worth noting that there are two additional sorts of participants
in the whole naming process, as discussed in the URN WG.  They are the
naming authorities which choose and assign names, and the authors who
include URNs in their resources.  These two are not relevant to the
design of an RDS and hence are not discussed further here.

3.2.1 The Publisher

The publisher must be able to make URNs known to potential customers.
From the perspective of a publisher, it is of primary importance that
URNs be correctly and efficiently resolvable by potential clients with
very high probability.  Publishers stand to gain from long-lived URNs,
since they increase the chance that references continue to point to
their published resources.

The publisher must also be able to choose easily among a variety of
potential services that might translate URNs to location information.
In order to allow for this mobility among resolvers, the RDS
architecture must support such transitions, within policy control
bounds.  It is worth noting that although multiple listing services
are available in telephone books, they are generally accompanied by a
fee.  There is nothing preventing there being fees collected for
similar sorts of services with respect to URNs.

The publisher must be able to arrange for multiple access points to a
published resource.  For this to be useful, resolver services should
be prepared to provide different resolution or hint information to
different clients, based on a variety of information including
location and the various access privileges the client might have.  It
is important to note that this may have serious implications for
caching this information.  For example, companies might arrange for
locally replicated copies of popular resources, and would like to
provide access to the local copies only for their own employees.  This
is distinct from access control on the resource as a whole, and may be
applied differently to different copies.

The publisher should be able to provide both long and short term
location information about accessing the resource.  Long term
information is likely to be such information as the long term address
of a resource itself or the location or identity of a resolver service
with which the publisher has a long term relationship.  One can
imagine that the arrangement with such a long term "authoritative"
resolver service might be a guarantee of reliability, resiliency to
failure, and atomic updates.  Shorter term information is useful for
short term changes in services or to avoid short lived congestion or

                                 - 9 -

failure problems.  For example, if the actual repository of the
resource is temporarily inaccessible, the resource might be made
available from another repository.  This short term information can be
viewed as temporary refinements of the longer term information, and as
such should be more easily and quickly made available, but may be less
reliable.  Some RDS designs may not distinguish between these two
extremes.

Lastly, the publishers will be the source of much hint information
that will be stored and served by the manager of the infrastructure.
Despite the fact that many publishers will not understand the details
of the RDS mechanism, it must be easy and straightforward for them to
install hint information.  This means that in general any one who
wishes to publish and to whom the privilege of resolution has been
extended through delegation, can do so.  The publisher must be able
not only to express hints, but also to verify that what is being
served by the manager is correct.  Furthermore, to the extent that
there are security constraints on hint information, the publisher must
be able to both express them and verify compliance with them easily.

3.2.2 The Client

From the perspective of the client, simplicity and usability are
paramount.  Of critical importance to serving clients effectively is
that there be an efficient protocol through which the client can acquire
hint information.  Since resolving the name is only the first step on
the way to getting access to a resource, the amount of time spent on it
must be minimized.

Furthermore, it will be important to be able to build simple, standard
interfaces to the RDS so that both the client and applications on the
client's behalf can interpret hints and subsequently make informed
choices.  The client, perhaps with the assistance of the application,
must be able to specify preferences and priorities and then apply them.
If the ordering of hints is only partial, the client may become directly
involved in the choice and interpretation of them and hence they must be
understandable to that client.  On the other hand, in general it should
be possible to configure default preferences, with individual
preferences viewed as overriding any defaults.

From the client's perspective, although URNs will provide important
functionality, the client is most likely to interact directly only with
human friendly names (HFNs).  As in direct human interaction (not
computer mediated), the sharing of names will be on a small, private, or
domain specific scale.  HFNs will be the sorts of references and names
that are easy to remember, type, choose among, assign, etc.  There will
also need to be a number of mechanisms for mapping HFNs to URNs.  Such
services as "yellow pages" or "search tools" fall into this category.
Although we are mentioning HFNs here, it is important to recognize that
HFNs and the mappings from HFNs to URNs is and must remain a separate
functionality from an RDS.  Hence, although HFNs will be critical to
clients, they do not fall into the domain of this document.


                                 - 10 -

3.2.3 The Management

Finally, we must address the usability concerns with respect to the
management of the hint infrastructure itself.  What we are terming
"management" is a service that is distinct from publishing; it is the
core of an RDS.  It involves the storage and provision of hints to the
clients, so that they can find published resources.  It also provides
security with respect to name resolution to the extent that there is a
commitment for provision of such security; this is addressed in
Section 3.3 below.

The management of hints must be as unobtrusive as possible. First, its
infrastructure (hint storage servers and distribution protocols) must
have as little impact as possible on other network activities.  It must
be remembered that this is an auxiliary activity and must remain in the
background.

Second, in order to make hint management feasible, there may need to
be a system for administrative incentives and disincentives such as
pricing or legal restrictions.  Recovering the cost of running the
system is only one reason for levying charges.  The introduction of
payments often has an impact on social behavior.  It may be necessary
to discourage certain forms of behavior that when out of control have
serious negative impact on the whole community.  At the same time, any
administrative policies should encourage behavior that benefits the
community as a whole.  Thus, for example, a small one-time charge for
authoritatively storing a hint will encourage conservative use of
hints.  If we assume that there is a fixed cost for managing a hint,
then the broader its applicability across the URN space, the more cost
effective it is.  That is, when one hint can serve for a whole
collection of URNs, there will be an incentive to submit one general
hint over a large number of more specific hints.  Similar policies can
be instituted to discourage the frequent changing of hints.  In these
ways and others, behavior benefitting the community as a whole can be
encouraged.

Lastly, symmetric to issues of usability for publishers, it must also be
simple for the management to configure the mapping of URNs to hints.  It
must be easy both to understand the configuration and to verify that
configuration is correct.  With respect to management, this issue may
have an impact not only on the information itself but also on how it is
partitioned among network servers that collaboratively provide the
management service or RDS.  For example, it should be straightforward to
bring up a server and verify that the data it is managing is correct.
Although this is not a guideline, it is worth nothing that since we are
discussing a global and probably growing service, encouraging volunteer
participants suggests that, as with the DNS, such volunteers can feel
confident about the service they are providing and its benefit to both
themselves and the rest of the community.

3.3 Security and Privacy

In summary, security and privacy guidelines can be identified as some
degree of protection from threats.  The guidelines that fall under
this third principle, that of security, are all stated in terms of
possibilities or options for users of the service to require and

                                 - 11 -

utilize.  Hence they address the availability of functionality, but
not for the use of it.  We recognize that all security is a matter of
degree and compromise.  These may not satisfy all potential customers,
and there is no intention here to prevent the building of more secure
servers with more secure protocols to suit their needs.  These are
intended to satisfy the needs of the general public.

    3.1) It must be possible to create authoritative versions of a hint
         with access-to-modification privileges controlled;
    3.2) It must be possible to determine the identity of servers or
         avoid contact with unauthenticated servers;
    3.3) It must be possible to reduce the threat of denial of service
         by broad distribution of information across servers.
    3.4) It must be possible within the bounds of organizational
         policy criteria to provide at least some degree of privacy
         for traffic.
    3.5) It must be possible for publishers to keep private certain
         information such as an overall picture of the resources they
         are publishing and the identity of their clients;
    3.6) It must be possible for publishers to be able to restrict
         access to the resolution of the URNs for the resources they
         publish, if they wish.

When one discusses security, one of the primary issues is an enumeration
of the threats being considered for mitigation.  The tradeoffs often
include cost in money and computational and communications resources,
ease of use, likelihood of use, and effectiveness of the mechanisms
proposed.  With this in mind, let us consider a set of threats.

Voydock and Kent[7] provide a useful catalog of potential threats.  Of
these the passive threats to privacy or confidentiality and the active
threats of authenticity and integrity are probably the most important
to consider here.  To the extent that spurious association causes
threats to the privacy, authenticity, or integrity with respect to
information within servers managing data, it is also important.
Denial of service is probably the most difficult of these areas of
threats both to detect and to prevent, and we will therefore set it
aside for the present as well, although it will be seen that solutions
to other problems will also mitigate some of the problems of denial of
service.  Furthermore, because this is intended to be provide a global
service to meet the needs of a variety of communities, the engineering
tradeoffs will be different for different clients.  Hence the
guidelines are stated in terms of, "It must be possible..."  It is
important to note that the information of concern here is hint
information, which by nature is not guaranteed to be correct or
up-to-date; therefore, it is unlikely to be worth putting too much
expense into the correctness of hints, because there is no guarantee
that they are still correct anyway.  The exact choice of degree of
privacy, authenticity, and integrity must be determined by the needs
of the client and the availability of services from the server.

To avoid confusion it is valuable to highlight the meanings of terms
that have different meanings in other contexts.  In this case, the
term "authoritative" as it is used here connotes the taking of an
action or stamp of approval by a principal (again in the security
sense) that has the right to perform such an act of approval.  It has

                                 - 12 -

no implication of correctness of information, but only perhaps an
implication of who claimed it to be correct.  In contrast, the term is
often also used simply to refer to a primary copy of a piece of
information for which there may also be secondary or cached copies
available.  In this discussion of security we use the former meaning,
although it may also be important to be able to learn about whether a
piece of information is from a primary source or not and request that
it be primary.  This second meaning arises elsewhere in the document
and is so noted there.

It is also important to distinguish various possible meanings for
"access control".  There are two areas in which distinctions can be
made.  First, there is the question of the kind of access control that
is being addressed, for example, in terms of hints whether it is read
access, read and modify access, or read with verification for
authenticity.  Second, there is the question of to what access is being
controlled.  In the context of naming it might be the names themselves
(not the case for URNs), the mapping of URNs to hints (the business of
an RDS), the mapping of URNs to addresses (not the business of an RDS as
will be discussed below in terms of privacy), or the resource itself
(unrelated to naming or name resolution at all).  We attempt to be clear
about what is meant when using "access control".

There is one further issue to address at this point, the distinction
between mechanism and policy.  In general, a policy is realized by means
of a set of mechanisms.  In the case of an RDS there may be policies
internal to the RDS that it needs to have supported in order to do its
business as it sees fit.  Since, in general it is in the business of
storing and distributing information, most of its security policies may
have to do with maintaining its own integrity, and are rather limited.
Beyond that, to the degree possible, it should impose no policy on its
customers, the publishers and users.  It is they that may have policies
that they would like supported by the RDS.  To that end, an RDS should
provide a spectrum of "tools" or mechanisms that the customers can cause
to be deployed on their behalf to realize policies.  An RDS may not
provide all that is needed by a customer.  A customer may have different
requirements within his or her administrative bounds than outside.
Thus, "it must be possible..."  captures the idea that the RDS must
generally provide the tools to implement policies as needed by the
customers.

The first approach to URN resolution is to discover local hints.  In
order for hints to be discovered locally, they will need to be as
widely distributed to what is considered to be local for every locale.
The drawback of such wide distribution is the wide distribution of
updates, causing network traffic problems or delays in delivering
updates.  An alternative model would concentrate hint information in
servers, thus requiring that update information only be distributed to
these servers.  In such a model the vulnerable points are the sources
of the information and the distribution network among them.  Attackers
on the integrity of the information stored in a server may come in the
form of masquerading as the owner or the server of the information.
Wide replication of information among servers increases the difficult
of masquerading at all the locations of the information as well as
reducing the threat of denial service.  These lead us to three
identifiable guidelines for our security model:

                                 - 13 -

* ACCESS CONTROL ON HINTS: It must be possible to create an
  authoritative version of each hint with change control limited only
  to those principals with the right to modify it.  The choice of who
  those principals are or whether they are unlimited must be made by
  the publisher of a hint.

* SERVER AUTHENTICITY: Servers and clients must be able to learn the
  identity of the servers with which they communicate.  This will be a
  matter of degree and it is possible that there will be more
  trustworthy, but less accessible servers, supported by a larger
  cluster of less authenticatable servers that are more widely
  available.  In the worst case, if the client receives what appears to
  be unvalidated information, the client should assume that the hint
  may be inaccurate and confirmation of the data might be sought from
  more reliable but less accessible sources.

* SERVER DISTRIBUTION: Broad availability will provide resistance to
  denial of service.  It is only to the extent that the services are
  available that they provide any degree of trustworthiness.  In
  addition, the distribution of services will reduce vulnerability
  of the whole community, by reducing the trust put in any single
  server.  This must be mitigated by the fact that to the extent trust
  is based on a linked set of servers, if any one fails, the whole
  chain of trust fails; the more elements there are in such a chain,
  the more vulnerable it may become.

Privacy can be a double-edged sword.  For example, on one hand, an
organization may consider it critically important that its competitors
not be able to read its traffic.  On the other hand, it may also
consider it important to be able to monitor exactly what its employees
are transmitting to and from whom, for a variety of reasons such as
reducing the probability that its employees are giving or selling the
company's secrets to verifying that employees are not using company
resources for private endeavor.  Thus, although there are likely to be
needs for privacy and confidentiality, what they are, who controls
them and how, and by what mechanisms vary widely enough that it is
difficult to say anything concrete about them here.

The privacy of publishers is much easier to safeguard.  Since they are
trying to publish something, in general privacy is probably not desired.
However, publishers do have information that they might like to keep
private: information about who their clients are, and information about
what names exist in their namespace.  The information about who their
clients are may be difficult to collect depending on the implementation
of the resolution system.  For example, if the resolution information
relating to a given publisher is widely replicated, the hits to _each_
replicated copy would need to be recorded.  Of course, determining if a
specific client is requesting a given name can be approached from the
other direction, by watching the client as we saw above.

There are likely to be some publishers publishing for a restricted
audience.  To the extent they want to restrict access to a resource,
that is the responsibility of the repository providing and restricting
access to the resource.  If they wish to keep the name and hints for a
resource private, a public RDS may be inadequate for their needs.  In

                                 - 14 -

general, it is intended for those who want customers to find their
resources in an unconstrained fashion.

The final privacy issue for publishers has to do with access control
over URN resolution.  This issue is dependent on the implementation of
the publisher's authoritative (in the sense of "primary) URN resolver
server.  URN resolver servers can be designed to require proof of
identity in order to be issued resolution information; if the client
does not have permission to access the URN requested, the service denies
that such a URN exists.  An encrypted protocol can also be used so that
both the request and the response are obscured.  Encryption is possible
in this case because the identity of the final recipient is known (i.e.
the URN server).  Thus, access control over URN resolution can and
should be provided by resolver servers rather than an RDS.

. The Framework

With these assumptions and guidelines in mind, we conclude with a
general framework within which RDS designs may fall.  As stated
earlier, although this framework is put forth as a suggested guide for
RDS designers, compliance with it will in no way guarantee compliance
with the guidelines.  Such an evaluation must be performed separately.
All such lack of compliance should be clearly documented.

The design of the framework is based on the syntax of a URN as
documented in RFC-2141[4].  This is:

        URN:<NID>:<NSS>

where URN: is a prefix on all URNs, NID is the namespace identifier, and
NSS is the namespace specific string.  The prefix identifies each URN as
such.  The NID determines the general syntax for all URNs within its
namespace.  The NSS is probably partitioned into a set of delegated and
subdelegated namespaces, and this is possibly reflected in further
syntax specifications.  In more complex environments, each delegated
namespace will be permitted to choose the syntax of the variable part of
the namespace that has been delegated to it.  In simpler namespaces, the
syntax will be restricted completely by the parent namespace.  For
example, although the DNS does not meet all the requirements for URNs,
it has a completely restricted syntax, such that any further structuring
must be done only by adding further refinements to the left, maintaining
the high order to low order, right to left structure.  A delegated
syntax might be one in which a host is named by the DNS, but to the
right of that and separated by an "@" is a string whose internal
ordering is defined by the file system on the host, which may be defined
high order to low order, left to right.  Of course, much more complex
and nested syntaxes should be possible, especially given the need to
grandfather namespaces.  In order to resolve URNs, rules will be needed
for two reasons.  One is simply to canonicalize those namespaces that do
not fall into a straightforward (probably right to left or left to
right) ordering of the components of a URN, as determined by the
delegated naming authorities involved.  It is also possible that rules
will be needed in order to derive from URNs the names of RDS servers to
be used in stages.

The NID defines a top level syntax.  This syntax will determine whether

                                 - 15 -

the NID alone or in conjunction with some extraction from the NSS (for
the top level naming authority name) is to be used to identify the first
level server to be contacted.  At each stage of the lookup either a new
rule for generating the strings used in yet another lookup (the strings
being the identity of another RDS server and possibly a string to be
resolved if it is different than the original URN) or a reference
outside the RDS to a URN resolver service, sidestepping any further use
of the RDS scheme.  Figure 1 depicts this process.


                            URN:<NID><NSS>
                                 |
                                 |
                                 |
                                 |
                                 v
                       +-------------------+
                       |Global NID registry|
                       +-------------------+
                                 |
                                 |
                                 |
              (return rule or URN resolver service reference)
                                 |
                                 +----------------------------------+
                                 |                                  |
                       +->(apply rule to determine RDS server)      |
                       |         |                                  |
                       |         |                                  |
                       |         |                                  |
                       |    +----------+                            |
                       |    |RDS server|          +-----------------+
                       |    +----------+          |
                       |      |   |               v
                       |      |   |   (set of choices)
                       |      |   +----+----------(...)--------+
                       |   (rule)      |                       |
                       |      |        |                       |
                       |      |        |                       |
                       +------+        |                       |
                                       v                       v
                                  +----------+            +----------+
                                  |URN       |            |URN       |
                                  |resolver  |            |resolver  |
                                  |service   |            |service   |
                                  +----------+            +----------+



        Figure 1: An RDS framework


There are several points worth noting about the RDS framework.  First,
it leaves open the determination of the protocols, data organization,
distribution and replication needed to support a particular RDS scheme.
Second, it leaves open the location of the computations engendered by

                                 - 16 -

the rules.  Third, it leaves open the possibility that partitioning
(distribution) of the RDS database need not be on the same boundaries as
the name delegation.  This may seem radical to some, but if the
information is stored in balanced B-trees for example, the partitioning
may not be along those naming authority delegation boundaries (see
[6]).  Lastly, it leaves open access to the Global NID Registry.  Is
this distributed to every client, or managed in widely distributed
servers?

One concept that has not been addressed in Figure 1 is that there may be
more than one RDS available at any given time, in order to allow for
evolution to new schemes.  Thus, the picture should probably look more
like Figure 2.


                         URN:<NID>:<NSS>
                               |
                               |
                   +-----------+-------(...)-------+
                   |                               |
                   |                               |
                   |                               |
                   v                               v
         +---------------------+        +---------------------+
         |Global NID registry 1|        |Global NID registry N|
         +---------------------+        +---------------------+
                   .                               .
                   .                               .
                   .                               .


        Figure 2: More than one co-existing RDS scheme


If we are to support more than one co-existing RDS scheme, there will
need to be coordination among them with respect to storage and
propagation of information and modifications.  The issue is that
generally it should be assumed that all information should be available
through any operational RDS scheme.  One cannot expect potential
publishers to submit updates to more than one RDS scheme.  Hence there
will need to be a straightforward mapping of information from one to the
other of these schemes.  It is possible that that transformation will
only go in one direction, because a newer RDS service is replacing an
older one, which is not kept up to date, in order to encourage transfer
to the newer one.  Thus, at some point, updates may be made only to the
newer one and not be made available to the older one, as is often done
with library catalogs.

This framework is presented in order to suggest to RDS scheme designers
a direction in which to start designing.  It should be obvious to the
reader that adherence to this framework will in no way guarantee
compliance with the guidelines or even the assumptions described in
Sections 2 and 3.  These must be reviewed independently as part of the
design process.  There is no single correct design that will conform to
these guidelines.  Furthermore, it is assumed that preliminary proposals

                                 - 17 -

may not meet all the guidelines, but should be expected to itemized and
justify any lack of compliance.

. Acknowledgments

Foremost acknowledgment for this document goes to Lewis Girod, as my
co-author on a preliminary URN requirements document and for his
insightful comments on this version of the document.  Thanks also go
to Ron Daniel especially for his many comments on my writing.  In
addition, I recognize the contributors to a previous URN framework
document, the "Knoxville" group.  There are too many of you to
acknowledge here individually, but thank you.  Finally, I must thank
the contributors to the URN working group and mailing list
(urn-ietf@bunyip.com), for your animated discussions on these and
related topics.

. References

[] Kunze, J., "Functional Recommendations for Internet Resource
Locators", RFC 1736, February, 1995.

[] Sollins, K. and Masinter, L., "Functional Requirements for Uniform
Resource Names", RFC 1738, December, 1994.

[] Berners-Lee, T., Masinter, L., McCahill, M., "Uniform Resource
Locators (URL)", RFC 1738, December, 1994.

[] Moats, R., "URN Syntax", RFC 2141, May 1997.

[] Iannella, R. and Faltstrom, P., "Namespace Identifier Requirements
for URN Services," currently draft-ietf-urn-nid-req-01.txt.  Intended
to become an information rfc by the URN working group.

[] Slottow, E.G., "Engineering a Global Resolution Service,"
MIT-LCS-TR712, June, 1997.  Currently available as
<http://ana.lcs.mit.edu/anaweb/ps-papers/tr-712.ps> or
<http://ana.lcs.mit.edu/anaweb/pdf-papers/tr712.pdf>.

[] Voydock, V. L., and Kent, S. T., "Security Mechanisms in
High-Level Protocols", ACM Computing Surveys, v. 15, No. 2, June,
1983, pp. 135-171.

. Contact information:

Karen Sollins
MIT Laboratory for Computer Science
 Technology Sq.
Cambridge, MA 02139

Tel: +1 617 253 6006
Email: sollins@lcs.mit.edu

This Internet Draft expires on December 4, 1997.




                                 - 18 -