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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..................................................2
 2.    Assumptions...................................................5
 3.    Guidelines....................................................7
 3.1   Evolution.....................................................7
 3.2   Usability....................................................10
 3.2.1 The Publisher................................................11
 3.2.2 The Client...................................................12
 3.2.3 The Management...............................................13
 3.3   Security and Privacy.........................................14
 4.    The Framework................................................18
 5.    Acknowledgements.............................................23
 6.    References...................................................23
 7.    Author's Address.............................................23
 8.    Full Copyright Statement.....................................24

Sollins Informational [Page 1] RFC 2276 Uniform Resource Name Resolution January 1998

1. 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 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

Sollins Informational [Page 2] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

Sollins Informational [Page 3] RFC 2276 Uniform Resource Name Resolution January 1998

 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

Sollins Informational [Page 4] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

2. 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.

Sollins Informational [Page 5] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

Sollins Informational [Page 6] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

3. 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
 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.

3.1 Evolution

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

Sollins Informational [Page 7] RFC 2276 Uniform Resource Name Resolution January 1998

     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.

Sollins Informational [Page 8] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

Sollins Informational [Page 9] RFC 2276 Uniform Resource Name Resolution January 1998

 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;

Sollins Informational [Page 10] RFC 2276 Uniform Resource Name Resolution January 1998

        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.

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

Sollins Informational [Page 11] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

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

Sollins Informational [Page 12] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

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

Sollins Informational [Page 13] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

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
 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;

Sollins Informational [Page 14] RFC 2276 Uniform Resource Name Resolution January 1998

     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

Sollins Informational [Page 15] RFC 2276 Uniform Resource Name Resolution January 1998

 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

Sollins Informational [Page 16] RFC 2276 Uniform Resource Name Resolution January 1998

 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

Sollins Informational [Page 17] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

4. 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 [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

Sollins Informational [Page 18] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

Sollins Informational [Page 19] RFC 2276 Uniform Resource Name Resolution January 1998

                          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

Sollins Informational [Page 20] RFC 2276 Uniform Resource Name Resolution January 1998

 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.

Sollins Informational [Page 21] RFC 2276 Uniform Resource Name Resolution January 1998

                       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.

Sollins Informational [Page 22] RFC 2276 Uniform Resource Name Resolution January 1998

5. 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.

6. References

 [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>.

7. Author's Address

 Karen Sollins
 MIT Laboratory for Computer Science
 545 Technology Sq.
 Cambridge, MA 02139
 Phone: +1 617 253 6006
 EMail: sollins@lcs.mit.edu

Sollins Informational [Page 23] RFC 2276 Uniform Resource Name Resolution January 1998

8. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Sollins Informational [Page 24]

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