Network Working Group                                S. Hardcastle-Kille
Request for Comments: 1430                              ISODE-Consortium
                                                               E. Huizer
                                                              SURFnet bv
                                                                 V. Cerf
                           Corporation for National Research Initiatives
                                                                R. Hobby
                                         University of California, Davis
                                                                 S. Kent
                                                Bolt, Beranek and Newman
                                                           February 1993

                   A Strategic Plan for Deploying an
                    Internet X.500 Directory Service

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is


   There are a number of reasons why a new Internet Directory Service is
   required.  This document describes an overall strategy for deploying
   a Directory Service on the Internet, based on the OSI X.500 Directory
   Service.  It then describes in more detail the initial steps which
   need to be taken in order to achieve these goals, and how work
   already undertaken by Internet Engineering Task Force Working Groups
   (IETF WGs) is working towards these goals.

Table of Contents

   1.    REQUIREMENTS                                                  2
   2.    SUMMARY OF SOLUTION                                           3
   3.    INFORMATION FRAMEWORK                                         3
   3.1   The Technical Model                                           3
   3.2   Extending the Technical Model                                 4
   3.3   The Operational Model                                         5
   4.    NAME ASSIGNMENT                                               5
   5.    DIRECTORY INFRASTRUCTURE                                      6
   5.1   Short Term Requirements                                       7
   5.2   Medium Term Requirements                                      9
   5.3   Long Term Requirements                                        9
   6.    DATAMANAGEMENT                                                9
   6.1   Legal Issues                                                 10
   7.    TECHNICAL ISSUES                                             10

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RFC 1430                     X.500 Strategy                February 1993

   7.1   Schema                                                       11
   7.2   Use on the Internet                                          11
   7.3   Replication of Knowledge and Data                            12
   7.4   Presentation of Directory Names                              13
   7.5   DSA Naming and MD Structure                                  13
   8.    SECURITY                                                     13
   8.1   Directory Provision of Authentication                        14
   8.2   Directory Security                                           15
   9.    RELATION TO DNS                                              16
   10.   EXTERNAL CONNECTIONS                                         16
   11.   REFERENCES                                                   17
   12.   Security Considerations                                      19
   13.   Authors' Addresses                                           20


   There is substantial interest in establishing a new Directory Service
   on the Internet. In the short term, there is pressure to establish
   two new services:

   -  White Pages lookup of users;

   -  Support for X.509 Authentication for a range of applications in
      particular for Privacy Enhanced mail [Lin89].

   In the medium term, there are likely to be many requirements for
   Directory Services, including:

   - General resource lookup, for information ranging from committee
     structures to bibliographic data;

   - Support of management of the Internet infrastructure, and
     integration of configuration information into the higher level

   - Support of applications on the Internet. For example:

      o  Electronic distribution lists;
      o  Capability information on advanced user agents;
      o  Location of files and archive services.

   - Support for Mail Handling Systems; Be they RFC-822 based or X.400
     based (IETF MHS-DS WG), e.g.,:

      o  Support for routing;
      o  Info on User agent capabilities; essential for a usage of
         Multimedia mail like MIME (Multipurpose Internet Mail

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   For the longer term, more sophisticated usages of X.500 are possible
   extending it into a useful and fast yellow pages service.


   In principle, the current Internet Domain Name System (DNS) could be
   used for many of these functions, with appropriate extensions.
   However, it is suggested that a higher level of directory service is
   needed. It is proposed to establish an Internet Directory Service
   based on X.500.  This provides appropriate functionality for the
   services envisaged and gives flexibility for future extension. This
   extension could be achieved either by tracking the evolution of the
   OSI Standard or by work specific to the Internet. In practice, it is
   likely to be a mixture of both.

   By deploying X.500 in some form on the Internet, a truly global and
   universal Directory Service can be built that will provide Internet
   users with fast access to all kinds of data. The X.500 Directory
   Service in this case may range from a simple white pages service
   (information on people and services) to coupling various existing
   databases and information repositories in a universal way.

   Currently, several different but cooperating X.500 Directory Services
   pilots are taking place on the Internet. These pilots form an
   important base for experimenting with this new service. Starting with
   these pilots, with the X.500 products arriving on the market today,
   and given sufficient funding for the central services described in
   this paper an operational X.500 Directory Service can be deployed.

   The final goal of the strategy described in this paper is to deploy a
   fully operational Directory Service on the Internet, providing the
   functions mentioned in the previous section.


   The most critical aspect of the Directory Service is to establish an
   Internet Information Framework. When establishing a sophisticated
   distributed directory with a coherent information framework, it
   involves substantial effort to map data onto this framework. This
   effort is an operational effort and far outweighs the technical
   effort of establishing servers and user agents.

3.1   The Technical Model

   By choosing the X.500 model as a basis for the information framework,
   it will also be part of a (future) global information framework. The
   key aspects of this model are:

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RFC 1430                     X.500 Strategy                February 1993

   - A hierarchical navigational system that couples distributed
     databases (of various kinds), which allows for management of the
     data by the organization/person responsible for the data;

   - Each object in this information structure (called the Directory
     Information Tree, DIT) is represented as an entry;

   - Objects are typed by an "object class", which permits multiple

   - An object is described by a set of attributes;

   - Each attribute is typed. Attribute types are hierarchical;

   - Each attribute type has an associated attribute syntax, which may
     be generic or shared with other attributes (e.g., Integer Syntax;
     Distinguished name Syntax); This allows for representation of
     simple attributes (e.g., strings or bitmaps) or complex ones with
     detailed structures.

   - Each entry has an unambiguous and unique global name;

   - Alternate hierarchies may be built by use of aliases or pointers of
     distinguished name syntax.

   This framework allows for representation of basic objects such as
   users within organizations. It is also highly extensible, and so can
   be used for a range of other applications.

3.2   Extending the Technical Model

   In the longer term, the model could be extended to deal with a number
   of other requirements which potentially must be met by an Internet
   Directory Service. Possible extensions include:

   - Support of ordered attributes (needed by some applications such as
     message storage);

   - Extensions to allow unification with management information,
     associated with SNMP (Simple Network Management Protocol) [CFSD90]
     or other management protocols;

   - Handling of non-hierarchical data in a better manner for searching
     and retrieval, whilst retaining the basic hierarchy for management
     purposes.  This is essentially building a general purpose resource
     location service on top of the basic infrastructure. It will need
     work on the information model, and not just the access protocols.

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   It is noted that although X.500 may not provide the ultimate solution
   to information retrieval, it has good potential for solving a lot of
   information service related problems.

3.3   The Operational Model

   To make the Directory Service with a coherent information framework
   really operational requires a lot of effort. The most probable
   operational model is one where larger organizations on the Internet
   maintain their part of the DIT on their own DSA (Directory System
   Agent). Smaller organizations will "rent" DSA space from regional
   networks or other service providers. Together these DSAs will form
   the Internet Directory Service Infrastructure. To couple the various
   parts of the DIT that are contained on these Internet DSAs, a special
   DSA containing the Root for the naming hierarchy within the DIT has
   to be established and maintained.

   The following tasks can be foreseen:

   -  Defining the naming hierarchy; See section 4.
   -  Creating the Directory Infrastructure; See section 5.
   -  Getting the Data into the directory; and
   -  Managing the data in the Directory. See section 6.


   In order to deploy the Internet Directory Service, it is important to
   define how the naming hierarchy will be structured. Although the
   basic model suggests a simple monolithic "database" containing all of
   the Internet's information infrastructure, with a namespace divided
   along geographic boundaries, this may not be the definite model that
   turns out to be the most appropriate to the Internet. Different
   models may evolve according to the needs of the Internet and the
   applications used on the Internet (i.e., some parts of the DIT may be
   assigned at the root for the Internet). Below this one can envisage
   several loosely coupled namespaces each with their own area of
   applicability. This should be handled as a part of the general
   operation of a directory service. An example of this might be
   assignment of a representation of the Domain Namespace under the root
   of the DIT. This is further discussed in [BHK91a].

   However, the core DIT information will be nationally assigned. The
   parts of the DIT below country level will be managed differently in
   each country. In many countries, registration authorities will be
   established according to the OSI Standard [ISO]. This has been done
   in some countries by the national ISO member body representative (for
   example in the UK by BSI).

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   The lower parts of the hierarchy will, in general, be delegated to
   organizations who will have control over Name Assignment in that part
   of the tree. There is no reason to mandate how to assign this
   hierarchy, although it is appropriate to give guidelines. Proposed
   solutions to assignment of namespace are given in [BHK92].

   In North America, there is an alternative approach being developed by
   the North American Directory Forum (NADF), which leverages existing
   registration mechanisms [For91]. It is not yet clear what form a
   final North American Directory Service will take. It is expected that
   similar initiatives will be taken in other places, such as Europe.
   For the Internet, the Internet Society (ISOC) has been suggested as a
   possible Naming Authority.

   A discussion of the main issues involved with representing the Real
   World in the Directory Service is part of the work undertaken by the
   IETF OSI DS Working Group.

   The core of the Internet Directory will therefore come to exist of a
   country based structure with different national naming schemes below
   the countries.  It is clearly desirable that the Internet Directory
   Service follows any evolving national and international hierarchies.
   However, this should not be allowed to cause undue delay. The
   strategy proposed is to proceed with name assignment as needed, and
   to establish interim registration authorities where necessary, taking
   practical steps to be aligned with emerging national authorities
   wherever possible.

   It is suggested that the Internet Directory Service does two things:

   First, each national part of the Internet DIT namespace should be
   delegated to an appropriate organization, which will usually be in
   the country of question.  Second, the delegated organization should
   assign names for that country as part of the Internet Directory
   Service. This should be done in a manner which is appropriately
   aligned with any emerging local or national service, but does not
   unduly delay the deployment of the Internet Directory Service.  For
   most countries, this will fit in as a natural evolution of the early
   directory piloting, where operators of pilots have acted as interim
   name registration authorities.


   To provide access to the Internet Directory Service, an
   infrastructure has to be built. Although the technical components of
   an X.500 infrastructure are clear: DSAs (that hold the actual data)
   and DUAs (that allow users and applications to access the data), a
   lot more is needed for deployment of an Internet Directory Service.

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RFC 1430                     X.500 Strategy                February 1993

   The Integrated Directory Services (IDS) Working Group of the IETF is
   playing a key role in solving most of the issues that are related to
   the building of an appropriate infrastructure.

   Many of the issues cited in this section have come forward out of
   interim pilots that have been established on the Internet:

   PSI White Pages Pilot
      This is a pilot service which is operating X.500 on the Internet.
      In many ways it is operating as an Internet wide pilot.

      Fielding Operational X.500, a project to explore the development
      and interoperability of X.500 implementations.

   Paradise (Piloting A ReseArch DIrectory Service in Europe)
      This project has been providing the necessary glue to hold the
      various national activities together [Par91].

5.1   Short Term Requirements

   -  Central Operations. There is a need for a number of operations
      to be managed as a service for the whole Internet. These services

      o A root DSA; containing the top-level of the DIT, has to be
        provided.  Currently, this root DSA is managed by the Paradise

      o Name assignment; Inserting names into the Directory, this has
        been discussed in section 4. This could be done in conjunction
        with the appropriate Registration Authority or by the
        Registration Authority.  In most cases it is likely to be the
        former, and mechanisms will need to be set up to allow
        organizations to get their names installed into the directory,
        either direct or through the registration authority.

      o Knowledge management; i.e., the information on "which DSA holds
        what part of the DIT, and how can that DSA be accessed". DSAs
        will be established by Organizations. There will be a need to
        centrally coordinate the management of the knowledge information
        associated with these DSAs. This is likely to be coupled to the
        name assignment.

      o Knowledge and Data replication; For the Directory to perform
        well, knowledge and data high up in the DIT must be
        significantly replicated. A service must be provided to make
        replicated information available to DSAs that need it.

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RFC 1430                     X.500 Strategy                February 1993

      It is suggested that for the time being, Paradise should be used
      as the initial basis for handling the top-level of the DIT and for
      provision of the central services. However, the services mentioned
      above need to be provided at a national level for every
      participating country in the Internet Directory Service. Whenever
      an organization starts a new country branch of the DIT in the
      Internet Directory Service the central operations will have to
      help out to make sure that these services will be properly
      installed on a national level.

   - An effective service will need to have sufficient implementations,
     in order to give full coverage over different hardware and software
     platforms, and to demonstrate openness. The recent Directory
     Information Services (pilot) Infrastructure Working Group's (DISI)
     Survey of Directory Implementations suggests that there will not be
     a problem here.  This provides a list of available X.500
     implementations and their capabilities [LW91].

   - An executive summary, necessary to convince the management of
     computer centers to invest manpower into setting up a X.500
     Directory Service.  This is provided by DISI [WR92].

   - Due to the possible different and rather independent structured
     namespaces that can be envisaged in the DIT for different purposes,
     DUAs will have to be "tuned intelligently" for the applications that
     they are used for.

   - To allow users easy access to the Internet Directory Service even
     from low powered workstations, a lightweight protocol has to be
     developed over TCP/IP. Already two private protocols that do this
     have been developed: The Michigan DIXIE protocol [HSB91] and the PSI
     Directory Assistance Service [Ros91]. The IETF OSI Directory
     Services Working Group (OSI-DS WG) is currently working on a
     standard lightweight protocol called LDAP.

   - Although the Internet Directory Service does not have to make any
     mandatory requirements about the use of lower layers, it is noted
     that the use of STD 35, RFC 1006 to allow use of OSI applications on
     top of TCP/IP is essential for deployment in the Internet. Other
     stacks like the ones using CLNS, CONS and X.25(80) will probably
     also be deployed in parts of the Internet. DSAs with different
     stacks will be linked through use of either application level relays
     (chaining) or Transport Service bridges.

   - There are multiple issues that are not dealt with (properly) in the
     X.500 standard and thus prevent the building of an Internet
     Directory service.  Intermediate solutions for these issues have to
     be established in an "open" way. The results will have to be

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     deployed as well as to be fed back into the relevant standard
     committees. The IETF OSI-DS WG deals with these issues. Section 7
     describes several of these issues.

   - Site support. The IETF IDS WG is looking at providing the necessary
     documentation to help with the provision of support for Directory
     users at participating sites.

5.2   Medium Term Requirements

   - Enhanced performance is necessary to allow for a real global usage;

   - The schema has to be extended to allow for various kinds of data,

      o  NIC data;
      o  Resource location;

   - Support for Internet Message Handling services (RFC-822, MIME and
     X.400).  This work is already undertaken by the IETF MHS-DS WG.

5.3   Long Term Requirements

   - To make sure that X.500 evolves into an operational service, it is
     essential to track its evolution, and to feed back into the
     evolution process.

   - Interface existing RDBMS into the Directory Service.

   - To increase the performance of the directory, and thereby making it
     useful for an even wider range of applications (e.g., policy based
     routing), a lightweight protocol for access and system usage is


   The whole of the Directory Infrastructure won't stand much chance
   without proper datamanagement of the data contained within the DIT.
   Procedures need to be established to assure a certain Level of
   Quality of the data contained in the DIT.

   Due to the very nature of X.500, the management of the data is
   distributed over various sources. This has the obvious advantage that
   the data will be maintained by the owner of the data. It does
   however, make it quite impossible to describe one single procedure
   for datamanagement.

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RFC 1430                     X.500 Strategy                February 1993

   For the Internet Directory Service, guidelines will have to be
   developed (by the IETF IDS WG), to help organizations that start with
   deployment of X.500 on how to manage data in their part of the DIT.
   The guidelines should describe a minimum level of quality that has to
   be supplied to make the service operational. The IETF OSI-DS WG will
   initiate a pilot on Quality of Service parameters in the Directory,
   that will be of use.

   Pilot datamanagement projects will have to be done (e.g., existing
   databases should be connected to the Internet Directory Service).
   Tools that are developed to achieve this should be made available to
   the Internet community for possible future use.

6.1   Legal Issues

   Most countries connected to the Internet have some sort of law that
   dictates how data on people can and cannot be made available. These
   laws deal with privacy and registration issues, and will differ from
   country to country.  It is suggested that each of the national
   organizations within the Internet that manages the Internet Directory
   Services master for that country, undertake some research as to the
   applicability of laws within that country on data made public through
   use of X.500.

   In the mean time, a general "User Bill of Rights" should be
   established to indicate what the proper use of the Internet Directory
   Service is. This "Bill of Rights" could be drafted by the IETF IDS
   WG.  As a basis, the NADF "User Bill of Rights" [For92] can be used.


   The IETF has established the OSI-DS WG. The major component of the
   initial work of this group is to establish a technical framework for
   deploying a Directory Service on the Internet, making use of the
   X.500 protocols and services [CCI88b].  This section describes the
   work already done by this working group, which has been implicitly
   focused on the technical infrastructure needed to deploy the Internet
   Directory service.

   The OSI Directory Standards do not yet contain sufficient specifics
   to enable the Internet Directory Service to be built. Full openness
   and interoperability are a key goal, so we may need Internet specific
   agreements, at least until the ISO standards are more complete. This
   section notes areas where the standards do not have sufficient
   coverage, and indicates the RFCs which have been written to overcome
   these problems.

   The work is being limited to (reasonably well) understood issues.

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RFC 1430                     X.500 Strategy                February 1993

   This means that whilst we will attempt to solve a wider range of
   problems, not all potential requirements will necessarily be met.

   The technical work is done in conjunction with the RARE WG on Network
   Application Support WG (formerly RARE WG3). The IETF WGs and the RARE
   WG have a common technical mailing list. It is intended that this
   will lead to a common European and North American technical approach.

7.1   Schema

   A Directory needs to be used in the context of an Information
   Framework. The standard directory provides a number of a attributes
   and object classes to enable basic operation. It is certain that the
   Internet community will have requirements for additional attributes
   and object classes. There is a need to establish a mechanism to
   register such information.

   Pilots in the European RARE Community and the US PSI White Pages
   Pilot have based their information framework on the THORN and RARE
   Naming Architecture. This architecture should be used for the
   Internet Directory Service, in conjunction with COSINE based services
   in Europe. A revised version of the Naming Architecture, with a
   mechanism for registration of new attributes and object classes, has
   been released as RFC 1274 [BHK91a].

7.2   Use on the Internet

   It is a recognized policy on the Internet to deploy OSI Applications
   over non-OSI lower layers (such as STD 35, RFC 1006) [RC87]. This
   policy allows deployment of OSI Applications before an OSI lower
   layer infrastructure has been deployed. Thus, the Internet Directory
   Service will decouple deployment of the OSI Directory from deployment
   of the OSI lower layers. As the Internet Directory service will
   extend into the far corners of the Internet namespace, where the
   underlying technology is not always TCP/IP, the Internet Directory
   Service will not make any mandatory requirements about use of lower
   layers. When configuring the Internet Directory Services, variations
   in the lower layers must be considered. The following options are

   - Operation on top of TCP/IP using a lightweight protocol.

   - Operation over TCP/IP using STD 35, RFC 1006. This is a practical
     requirement of deployment at very many Internet sites, and is the
     basis of the existing services. It is highly recommended that all
     participating DSAs support this stack.

   - Use of OSI Network Service (Connection Oriented or Connectionless).

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RFC 1430                     X.500 Strategy                February 1993

   - X.25(80) will probably not be used in the core infrastructure of
     the Internet Directory Service, but is the basis of some European
     activities.  It may be needed later to interconnect with US
     commercial systems not on the Internet. There will be a practical
     need to interwork with DSAs which only support this stack.

   This approach has the following implications:

   1. There is a need to represent TCP/IP addresses within OSI Network
      Addresses. This is specified in RFC 1277 [HK91a].

   2. It will be desirable to have a uniform method to present Network
      Addresses of this style. Therefore, a string representation of
      presentation addresses is specified in RFC 1278 [HK91d].

   3. This approach leads to the situation where not all DSAs can
      communicate directly due the different choice of lower layers.
      This is already a practical result of many European sites operating
      DSAs over X.25.  When the Internet Directory Service is deployed,
      the issue of which DSAs operate which stacks must be considered in
      order to achieve a coherent service.  In particular, there may be a
      need to require DSAs that serve parts higher up in the DIT to serve
      multiple stacks. This will be tackled as an operational issue.

   4. There may be a requirement to extend the distributed operations, so
      that there is no requirement for full connectivity (i.e., each DSA
      supports each stack). A solution to this problem, by defining
      "relay DSAs" is specified in RFC 1276 [HK91b].

7.3   Replication of Knowledge and Data

   There are a number of requirements on replication, both of data (the
   actual information on objects in the directory) and knowledge (the
   information on where do I find what data) information, which must be
   met before an Internet Directory can be deployed. The 1988 standard
   cannot be used as is, because it does not deal with replication or
   caching. This leads to serious problems with performance. There is a
   partial solution available in the 1992 version of the standard,
   however there are no products available yet that implement this
   solution.  These issues are discussed in more detail in RFC 1275

   As it took too long for 1992 implementations to arrive to be of any
   help to the already rapidly growing pilot that urgently needed a
   solution, an option was chosen to use a simple interim approach as
   defined in RFC 1276.  It will be clearly emphasized that this is an
   interim approach, which will be phased out as soon as the appropriate
   standards are available and stable implementations are deployed. The

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RFC 1430                     X.500 Strategy                February 1993

   interim approach is based on the approach used in the QUIPU
   Implementation and it is widely deployed in the existing pilots.

7.4   Presentation of Directory Names

   The standard does not specify a means to present directory names to
   the user. This is seen as a serious deficiency, and a standard for
   presenting directory names is required. For Distinguished Names, a
   string representation is defined in [HK92a]. However, as the
   distinguished name is not very friendly for the user, a more user
   oriented specification of a standard format for representing names,
   and procedures to resolve them is chosen on the Internet, and is
   specified in [HK92b].

7.5   DSA Naming and MD Structure

   There are some critical issues related to naming of DSAs and the
   structure of Directory Management Domains. The main issues are:

   - It is hard to achieve very high replication of knowledge
     information as this is very widely spread;

   - There is a need to give DSAs more reasonable names, which will
     contain an indication on the role of the DSA; This is necessary for
     DSAs high up the DIT.

   - There is too much DIT clutter in the current pilots;

   - There is no real concept of a DMD (Directory Management Domain)

   These will be significant as the directory increases in size by
   orders of magnitude. The IETF OSI-DS WG is working to develop a
   solution in this area.


   A Directory can be an important component in the overall provision of
   security in a distributed system environment, especially when
   public-key cryptographic technology is employed. The directory can
   serve as a repository for authentication information, which, in turn,
   forms the basis of a number of OSI Authentication Services (e.g.,
   X.400) and non-OSI Services (e.g., privacy-enhanced mail, PEM). The
   directory may also use this and other stored authentication
   information to provide a wide range of security Services used by the
   Directory system itself.

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RFC 1430                     X.500 Strategy                February 1993

8.1   Directory Provision of Authentication

   The directory will be used to provide X.509 strong authentication.
   This places minimal requirements on the directory. To use this
   infrastructure, users of authentication services must have access to
   the directory. In practice, this type of authentication can be
   deployed only on a limited scale without use of a directory, and so
   this provision is critical for applications such as Privacy Enhanced
   Mail [Lin93]. The PEM development is considering issues relating to
   deploying Certification Authorities, and this discussion is not
   duplicated here.

   PEM defines a key management architecture based on the use of
   public-key certificates, in support of the message encipherment and
   authentication procedures defined in [Lin93]. The PEM certificate
   management design [Ken93] makes use of the authentication framework
   defined by X.509. In this framework, as adopted by PEM, a
   "certification authority" representing an organization applies a
   digital signature to a collection of data consisting of a user's
   public component, various information that serves to identify the
   user, and the identity of the organization whose signature is
   affixed.  This establishes a binding between these user credentials,
   the user's public component and the organization which vouches for
   this binding. The resulting, signed, data item is called a
   certificate. The organization identified as the certifying authority
   for the certificate is the "issuer" of that certificate. The format
   of the certificate is defined in X.509.

   In signing the certificate, the certification authority vouches for
   the user's identification, in the context specified by the identity
   of the issuer. Various types of organization may issue certificates,
   including corporate, educational, professional, or governmental
   entities. Moreover, these issuers may operate under different
   certification policies, so that not all certificates may be equally
   credible (i.e., some certificates may be more trustworthy as accurate
   identifiers of users, organizations, mailing lists, etc). The PEM
   certificate management design allows for this diversity of
   certification policies, while ensuring that any certificate can be
   traced unambiguously to the policy under which it was issued.

   The digital signature is affixed on behalf of that organization and
   is in a form which can be recognized by all members of the privacy-
   enhanced electronic mail community. This ability to universally
   verify any PEM certificate results because the PEM certification
   design is a singly rooted tree, in which the Internet Society acts as
   the root. Once generated, certificates can be stored in directory
   servers, transmitted via unsecure message exchanges, or distributed
   via any other means that make certificates easily accessible to

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   message originators, without regard for the security of the
   transmission medium.

8.2   Directory Security

   A number of security services are possible with the directory:

   Peer Authentication at Bind
      Authentication (one or two way) between DUA/DSA and DSA/DSA,
      established during the bind operation. This authentication may be
      provided using simple passwords (not recommended), one-way hashed
      passwords (more secure), or via public key cryptography (most
      secure). The various authentication options are specified in
      X.500(88), but most existent implementations implement only simple
      password authentication.

   Per-operation Authentication and Integrity
      This is usually used to identify the DUA originating an operation
      to the Directory (e.g., to authenticate prior to data
      modification). It may also be used to verify the identity of the
      DSA which provided data in a response to the user. In both
      examples, the integrity of the data also is ensured through the
      use of digital signatures. This is specified in X.500(88), but not
      yet widely implemented.

   Single Entry Access Control
      This is used to control which users (DUAs) can access and modify
      data within an entry. This is specified in X.500(92) and most DSA
      implementations provide this function.

   Multiple Entry Access Control
      This is used to control search and list operations, in order to
      allow location of information by searching, but to deter
      "trawling" of information and organizational structure. Usually,
      these access controls are limited in their ability to prevent
      trawling because of the conflicting goal of allowing a certain
      level of legitimate browsing in support of "white pages"

   Service Authorization
      This allows DSAs to control service in a data independent manner,
      based on peer authentication. For example, one might prevent
      students from making non-local queries, while permitting such
      queries by faculty and staff.

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RFC 1430                     X.500 Strategy                February 1993

   Security Policy
      This term encompasses the security goals for which data access
      control, service authorization, and authentication mechanisms are
      used to implement. For example, a local security policy might
      require that all directory database modifications employ strong
      authentication and originate from a computer at a known (local)

   Data Confidentiality
      The directory does not include explicit features to protect the
      confidentiality of data while in transit (e.g., between a DUA and
      DSA or between DSAs). Instead, it is assured that lower layer
      security protocols or other local security facilities will be
      employed to provide this security service. Ongoing work on
      adaptation of the Network Layer Security Protocol (NLSP) is a
      candidate for provision of this security service with directories.

   There is no specification of any Internet-wide security policy for
   directories, nor are there currently any security mechanisms required
   of all directories. Deployment of a directory could be based on a
   variety of policies:

   - Read only system, containing only public data and restricted to
     local modification.

   - Use of X.509 authentication, and private access control mechanisms
     (this will not allow open access control management, but this is not
     seen as a fundamental problem).

   It will be important to understand if global Internet requirements
   for minimum essential directory security mechanisms will be required
   to promote widespread use of directories. We recommend that an
   informational RFC be written to analyze this issue, with an
   operational policy guidelines or applicability statement RFC to


   It is important to establish the relationship between the proposed
   Internet Directory, and the existing Domain Name System. An
   Experimental Protocol RFC (RFC 1279) proposes a mapping of DNS
   information onto the Directory. Experiments should be conducted in
   this area [HK91e].


   It will be important for this activity to maintain suitable external
   liaisons. In particular to:

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RFC 1430                     X.500 Strategy                February 1993

   Other Directory Services and Directory Pilots

      To ensure a service which is coherent with other groups building
      X.500 services. e.g.,:

      -  Paradise
      -  NADF
      -  FOX
      -  PSI White Pages

   Standards Bodies

      To feed back experience gained from this activity, so that the
      next round of standards meets as many of the Internet requirements
      as possible. e.g.,:

      -  CCITT/ISO
      -  RARE WG-NAS
      -  EWOS/OIW
      -  ETSI


   [BHK91a]  Barker, P., and S. Hardcastle-Kille, "The COSINE and
             Internet X.500 Schema", RFC 1274, Department of Computer
             Science, University College London, November 1991.

   [BHK92]   Barker, P., and S. Hardcastle-Kille, "Naming Guidelines for
             Directory Pilots", RFC 1384, Department of Computer Science,
             University College London, ISODE Consortium, January 1993.

   [CCI88a]  The Directory --- authentication framework, December 1988.
             CCITT Recommendation X.509.

   [CCI88b]  The Directory --- overview of concepts, models and services,
             December 1988. CCITT X.500 Series Recommendations.

   [CCI90]   The Directory --- part 9 --- replication, October 1990.
             ISO/IEC CD 9594-9 Ottawa output.

   [CFSD90]  Case, J., Fedor, M., Schoffstall, M., and J. Davin, "A
             Simple Network Management Protocol", STD 15, RFC 1157,
             SNMP Research, Performance Systems International, MIT
             Laboratory for Computer Science, May 1990.

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RFC 1430                     X.500 Strategy                February 1993

   [For91]   The North American Directory Forum, "A Naming Scheme
             for C=US", RFC 1255, NADF, September 1991.
             Also NADF-175.  (See also RFC 1417.)

   [For92]   The North American directory Forum, "User Bill of Rights
             for Entries and Listing in the Public Directory", RFC 1295,
             NADF, January 1992.  (See also RFC 1417.)

   [HK91a]   Hardcastle-Kille, S., "Encoding network addresses to
             support operation over non-OSI lower layers", RFC 1277,
             Department of Computer Science, University College London,
             November 1991.

   [HK91b]   Hardcastle-Kille, S., "Replication and distributed
             operations extensions to provide an internet directory
             using X.500", RFC 1276, Department of Computer Science,
             University College London, November 1991.

   [HK91c]   Hardcastle-Kille, S., "Replication requirement to
             provide an internet directory using X.500", RFC 1275,
             Department of Computer Science, University College
             London, November 1991.

   [HK91d]   Hardcastle-Kille, S., "A string encoding of presentation
             address", RFC 1278, Department of Computer Science,
             University College London, November 1991.

   [HK91e]   Hardcastle-Kille, S., "X.500 and domains", RFC 1279,
             Department of Computer Science, University College
             London, November 1991.

   [HK92a]   Hardcastle-Kille, S., "A string representation of
             Distinguished Names", Department of Computer Science,
             University College London, Work in Progress.

   [HK92b]   Hardcastle-Kille, S., "Using the OSI directory to achieve
             user friendly naming", Department of Computer Science,
             University College London, Work in Progress.

   [HSB91]   Howes, R., Smith, M., and B. Beecher, "DIXIE Protocol
             Specification", RFC 1249, University of Michigan,
             July 1991.

   [ISO]     Procedures for the operation of OSI registration
             authorities --- part 1: general procedures. ISO/IEC 9834-1.

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RFC 1430                     X.500 Strategy                February 1993

   [Ken93]   Kent, S., "Privacy Enhancement for Internet Electronic
             Mail: Part II - Certificate-based Key Management, RFC 1422,
             BBN, February 1993.

   [Kil88]   Kille, S., "The QUIPU Directory Service", In IFIP WG 6.5
             Conference on Message Handling Systems and Distributed
             Applications, pages 173--186. North Holland Publishing,
             October 1988.

   [Kil89]   Kille, S., "The THORN and RARE Naming Architecture",
             Technical report, Department of Computer Science,
             University College London, June 1989. THORN Report UCL-64
             (version 2).

   [Lin93]   Linn, J., "Privacy Enhancement for Internet Electronic
             Mail: Part I - Message Encryption and Authentication
             Procedures", RFC 1421, February 1993.

   [LW91]    Lang, R., and R. Wright, "A Catalog of Available X.500
             Implementations", FYI 11, RFC 1292, SRI International,
             Lawrence Berkeley Laboratory, January 1992.

   [Lyn91]   Lynch, C., "The Z39.50 information retrieval protocol: An
             overview and status report", Computer Communication Review,
             21(1):58--70, January 1991.

   [Par91]   Paradise International Report, Cosine. Paradise project,
             Department of Computer Science, University College London.
             November 1991.

   [RC87]    Rose, M., and D. Cass, "ISO Transport Services on
             top of the TCP", STD 35, RFC 1006, Northrop Corporation
             Technology Center, May 1987.

   [Ros91]   Rose, M., "Directory Assistance Service", RFC 1202,
             Performance Systems International, February 1991.

   [WR92]    Weider, C., and J. Reynolds, "Executive Introduction to
             Directory Services Using the X.500 Protocol", FYI 13,
             RFC 1308, ANS, ISI, March 1992.

12.  Security Considerations

   Security issues are discussed in Section 8.

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RFC 1430                     X.500 Strategy                February 1993

13. Authors' Addresses

   Steve Hardcastle-Kille
   ISODE Consortium
   PO box 505
   SW11 1DX London
   Phone: +44-71-223-4062

   Erik Huizer
   SURFnet bv
   PO box 19035
   3501 DA Utrecht
   The Netherlands
   Phone: +31-30 310290

   Vinton Cerf
   Corporation for National Research Initiatives
   1895 Preston White Drive, Suite 100
   Reston, VA 22091
   Phone: (703) 620-8990

   Russ Hobby
   University of California, Davis
   Computing Services
   Surge II Room 1400
   Davis, CA 95616
   Phone: (916) 752-0236

   Steve Kent
   Bolt, Beranek, and Newman
   50 Moulton Street
   Cambridge, MA 02138
   Phone: (617) 873-3988

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