Network Working Group K. Sollins
Request for Comments: 2276 MIT/LCS
Category: Informational January 1998
Architectural Principles of Uniform Resource Name Resolution
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
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.
Table of Contents
1. Introduction..................................................22. Assumptions...................................................53. Guidelines....................................................73.1 Evolution.....................................................73.2 Usability....................................................103.2.1 The Publisher................................................113.2.2 The Client...................................................123.2.3 The Management...............................................133.3 Security and Privacy.........................................144. The Framework................................................185. Acknowledgements.............................................236. References...................................................237. Author's Address.............................................238. Full Copyright Statement.....................................24
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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 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. In
addition, names may have other semantic or mnemonic information that
either helps human users remember or figure out the names, or
includes 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. Finally, with these guidelines for
RDS's, this group has recognized the value of the separation of name
assignment management from name resolution management.
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
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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 as well as
technical 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 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. By
whatever means available, a set of hints may be discovered. Some
combination of software and human choice will determine which hints
will be tried and in what order.
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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) 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
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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.
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.
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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 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.
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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.
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
guidelines more fully.
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 by the URN Working Group [4]. 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 many IETF guidelines and requirements,
many 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.
The issues in the area of the first principle, that of evolvability,
are:
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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
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.
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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 [4]. 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
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.
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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.
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;
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2.3.2) The management of hints must allow for administrative
controls that encourage certain sorts of behavior
deemed 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
important to recognize that these may be sitautions in which
interests of some of the participants (for exampel a use and a
publisher) may be in conflict; some resolution will be needed.
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.
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
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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 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.
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
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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.
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
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the community as a whole. Thus, for example, a small one-time charge
for authoritatively storing a hint might 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.
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
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;
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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 [5] provide a useful catalog of potential threats.
Of these the passive threats to privacy or confidentiality and the
active threats to 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
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
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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 as possible 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
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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:
* 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 address. 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
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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 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.
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 [6]. 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
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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.
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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
The NID defines a top level syntax. This syntax will determine
whether 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
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depicts this process.
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 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 [7]). Lastly, it leaves open access to
the Global NID Registry. Is this distributed to every client, or
managed in widely distributed servers? It is important to note that
it is the intention here that a single RDS scheme is likely to
support names from many or all naming schemes, as embodied in their
NIDs.
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.
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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 may not meet all the guidelines, but
should be expected to itemized and justify any lack of compliance.
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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.
[1] Kunze, J., "Functional Recommendations for Internet Resource
Locators", RFC 1736, February 1995.
[2] Sollins, K., and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1738, December 1994.
[3] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[4] URN Working Group, "Namespace Identifier Requirements for URN
Services," Work in Progress.
[5] 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.
[6] Moats, R., "URN Syntax", RFC 2141, May 1997.
[7] 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>.
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