RFC 1684

Network Working Group                                            P. Jurg
Request for Comments: 1684                                    SURFnet bv
Category: Informational                                      August 1994

          Introduction to White Pages Services based on X.500

Status of this Memo

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


   This document aims at organisations who are using local and global
   electronic communication on a day to day basis and for whom using an
   electronic White Pages Service is therefore indispensable.

   The document provides an introduction to the international ITU-T
   (formerly CCITT) X.500 and ISO 9594 standard, which is particularly
   suited for providing an integrated local and global electronic White
   Pages Service.

   In addition a short overview of the experience gained from the
   Paradise X.500 pilot is given. References to more detailed
   information are included.

   The document should be useful for managers of the above mentioned
   organisations who need to get the necessary executive commitment for
   making the address information of their organisation available by
   means of X.500.

Table Of Contents

   1. Introduction ................................................  2
   2. Concept of X.500 ............................................  3
     2.1  Directory Model .........................................  3
     2.2  Information Model .......................................  4
   3.  Benefits of X.500 ..........................................  5
   4.  Organisational aspects of X.500(experience from Paradise) ..  6
   5.  Applications of X.500 ......................................  8
   6.  References .................................................  9
   7.  Security Considerations .................................... 10
   8.  Author's Address ........................................... 10

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

1. Introduction

   Due to the tremendous growth and development of international
   computer networks we have nowadays the possibility to overcome -
   without having to travel - geographical distances when working
   together with other people. Besides the possibility of using the
   telephone we may use electronic data exchange to discuss working
   documents, new ideas, plans or whatsoever. One of the most popular
   means for this is electronic mail, which can be used to exchange
   all kinds of electronic data: from informal pure text messages to
   formatted and multi-media documents.

   As the number of people connected to computer networks grows (and
   it does continuously, it is at least doubling each year!), it
   becomes more difficult to track down people's electronic (mail)
   addresses. Hence, in order to make global communication over
   computer networks work, a global White Pages service is
   indispensable. Such a service should of course provide people's
   electronic mail addresses, but could also easily contain telephone
   and fax numbers and postal addresses.

   Currently, one technical solution for a globally distributed
   White Pages service is X.500 and there exists an international
   infrastructure based on X.500 technology called 'Paradise'
   (Piloting An inteRnationAl DIrectory SErvice), which contains about
   1.5 million entries belonging to persons and 3,000 belonging to
   organisations. Worldwide 35 countries are involved. Paradise is
   also a project of the EC. The project continues until September
   1994. Afterwards its operational tasks will be taken over by a
   European service provider for the R&D community (DANTE).

   The goal of Paradise and related national initiatives is to
   stimulate and extend the use of the X.500 White Pages service.
   Within the pilot attention is paid to technical and organisational
   aspects. The Paradise infrastructure is mainly based on the
   Internet Protocol. The specific issues that are related to the use
   of the Internet Protocol for X.500 can be found in [5].

   In the decision process of joining the international X.500
   infrastructure and opening (part) of the local (address)
   information to the outside world, it is important that an
   organisation fully understands the technical and organisational
   issues that are involved.

   This document tries to be of help in this matter first by
   explaining the main concepts of X.500 (section 2) and subsequently
   by pointing out its benefits (section 3), the organisational
   aspects that are involved (section 4), and for which other

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   applications the X.500 infrastructure may be used in the near
   future (section 5).

2. Concept of X.500

   The X.500 standard describes a so-called 'Directory Service', which
   can be used for all types of electronic directories. This document
   focusses on the use of X.500 for a global White Pages Directory.
   The concept of X.500 may roughly be divided in the 'Directory
   model' and the 'Information model'.

   2.1  Directory model

   X.500 uses a distributed approach to achieve the goal of a global
   Directory Service. The idea is that local (communication oriented)
   information of an organisation is maintained locally in one or more
   so called Directory System Agents (DSA's). 'Locally' is a flexible
   expression here: it is possible that one DSA keeps information of
   more than one organisation. A DSA essentially is a database:

      - in which the information is stored according to the X.500
        standard (see section 2.2),

      - that has the ability, where necessary, to exchange data
        with other DSA's.

   Through the communication among each other the DSA's form the
   Directory Information Tree (DIT). The DIT is a virtual hierarchical
   datastructure consisting of a 'root', below which 'countries' are
   defined. Below the countries (usually) 'organisations' are defined,
   and below an organisation 'persons', or first additional
   'organisational units', are defined (see the simplified illustration
   below where only three countries and no organisational units are
   presented). The DIT is a representation of the global Directory.

             root                      o
                                     / | \
                                    /  |  \
             countries            uk   de  fr
                                 / |   /\   |\
                                /  |  /  \  | \
             organisations     a   b c    d e  f
                               |   | |    | |  |
             persons          ..  .. ...  .... ...

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   Each DSA holds a part of the global Directory and is able to find
   out, through the hierarchical DIT structure, which DSA holds which
   parts of the Directory.

   The standard does not describe how to distribute different part of
   the Directory among DSA's. However, the information corresponding to
   a single node of the DIT (i.e., a country, organisation, person)
   cannot be distributed over several DSA's. In practice a large
   organisation will maintain one or more DSA's that hold its part of
   the Directory. Smaller organisations may share a DSA with other
   organisations.The distribution among the DSA's is totally transparent
   to the users of the Directory.

   A user of the Directory can be a person or a computer. A user
   accesses the Directory through a so-called Directory User Agent
   (DUA). The DUA automatically contacts a nearby DSA by means of which
   the user may search or browse through the DIT and retrieve
   corresponding information. A DUA can be implemented in all sorts of
   user interfaces. Therefore users may access the Directory through
   dedicated DUA interfaces or for example e-mail applications.
   Currently most DUA nterfaces to be used by persons are dedicated, but
   it is expected that in the near future a lot of DUA interfaces will
   be integrated with other applications.

2.2 Information Model

   Besides the Directory model, the X.500 standard also defines the
   information model used in the Directory Service.

   All information in the Directory is stored in 'entries', each of
   which belongs to at least one so-called 'object class'. In the White
   Pages application of X.500, on which we focus here, object classes
   are defined such as 'country', 'organisation', 'organisational unit'
   and 'person'.

   The actual information in an entry is determined by so-called
   'attributes' which are contained in that entry. The object classes to
   which an entry belongs define what types of attributes an entry may
   use and hence what information is specific for entries belonging to
   that object class. The object class 'person' for example allows
   attribute types like 'common name', 'telephone number', and 'e-mail
   address' to be used and the object class 'organisation' allows for
   attribute types like 'organisation name' and 'business category'.
   Dependent on its type an attribute can take one or more values.

   To specify the name of an entry in the DIT, at least one attribute
   value of the entry is used. The entry of a person is usually named
   after the value of the attribute type 'common name'. The name of an

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   entry must be unique on the same level in the subtree of the DIT to
   which the entry belongs.

   An example of an entry belonging to the object class 'person' is:

       Attribute type              Attribute value
       --------------              --------------

       Object Class:               top
       Common Name:                Thomas Lenggenhager
                                   T. Lenggenhager
       Surname:                    Lenggenhager
       Postal Address:             SWITCH
                                   Limmatquai 138
                                   CH-8001 Zuerich
       Telephone Number:           +41 1 268 1540
       Facsimile Telephone Number: +41 1 268 1568
       Mail:                       lenggenhager@switch.ch

   This entry corresponds to the node in the DIT that occurs below the
   node of the organisation 'SWITCH' and is named after the first value
   of the attribute type 'common name': 'Thomas Lenggenhager'.

3.  Benefits of X.500

   Why should one use X.500 for a local White Pages service? Here are
   some good arguments:

      - The distributed character of the service. A large
        organisation may distribute the responsibility for the
        management of the information it presents through X.500 by
        distributing this information over several DSA's (without
        losing the overall structure)

      - The flexibility of the service. Besides for public purposes,
        X.500 may also be used for specific private Directory Service
        applications. Whereas the definitions of the DIT, object
        classes and attribute types of the public White Pages
        information within an organisation have to conform to those
        of the rest of world, the internal applications may use their
        own DIT structure and their own definitions of object classes
        and attributes (the values being only visible within (a part)
        of the organisation). Nevertheless one local infrastructure
        can be used for both the public and private computers.

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

      - Good alternative for paper Directories. The provision of
        White Pages services based on X.500 may be a good alternative
        for paper directories, because the latter directories are
        rarely up-to-date (due to the printing costs) and because
        X.500 not only can be used by humans but also by

   Some important arguments in favour of X.500 for global use are:

      - By its distributed nature X.500 is particularly suited for a
        large global White Pages directory. Maintenance can take
        place in a distributed way.

      - Good searching capabilities. X.500 offers the possibility to
        do searches in any level or in any subtree of the DIT. In
        order to do a search an attribute type together with a value
        have to be specified. Then the Directory searches for all
        entries that contain an attribute of that type with the given
        value. For example one can search for all persons in an
        organisation having a particular common name, or all
        organisations within a country that have telecommunications
        as their business category. It is up to the organisations
        that maintain the DSA's to decide who may perform which
        searches and also how many levels deep a search may be.

        Searches can be done on the basis of an exact or approximate
        match. It is worthwile to note that distributed searches
        (that need connections to a lot of DSA's) may be expensive
        and are generally not encouraged.

      - There are DUA interfaces for the White Pages service
        availablefor all types of workstations (DOS, Macintosh OS,
        Unix). For an overview of X.500 available software see
        RFC 1292 [2] or updates of this document.

      - X.500 is an international standard. Using a standard
        obviously means less problems with interoperability and
        interworking.Also the standard is updated according to
        practical experience.

4.  Organisational aspects of X.500 (experience from Paradise)

   The organisational aspects involved in operating a local X.500 (or
   any other electronic) Directory can roughly be divided in   three
   sub-aspects:datamanagement, legal issues and cost aspects. With
   respect to cost aspects there is no publicly known model or
   experience at the moment.

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   Therefore the focus in this document is on datamanagement and legal

   Data management refers to issues that are related to inserting
   appropriate information into the Directory and keeping it up to date.

   From the experience of participants in Paradise we obtain that the
   following items are of first importance:

      - Executive commitment. Without this it is almost impossible to
        create an organisation wide up-to-date electronic Directory.

      - Structure of the local DIT. In joining the international
        infrastructure an organisation has to conform to some rules
        for the local DIT structure as presented to the global X.500
        infrastructure. A recommendation on how to structure a local
        DIT and how to use the available attributes can be found in
        [7]. The most important recommendation in the latter document
        is to keep the local part of the DIT as simple (flat) as
        possible. The reason is that users from outside the
        organisation may otherwise have difficulties in finding
        entries of persons within the organisation (searches in the
        DIT are often only allowed one level deep).

      - Attributes to be used. For the existing infrastructure the
        objects and associated attributes that are globally used, are
        documented in [1].

      - Sources of the data. An organisation has to find out where to
        get what kind of data and develop procedures for uploading
        its DSA('s).

      - Delegating responsibilities for updates. Procedures have to
        bedeveloped for updates of the local Directory. These
        procedures have to include delegation of responsibilities.

      - Security procedures. Rules have to be set for access and
        security. Who may contact the DSA? Who will have access to
        which subtrees and what attributes?

   A study of the legal consequences of presenting (address) information
   via X.500 lead to the main conclusion that in Europe an organisation
   has to formally register its data collections.  Registration implies
   defining a goal for the application. This has to be done for the
   White Pages service as well as for any deviating local application of
   X.500. However, the different national laws may differ with respect
   to legal restrictions. For more information on this subject we refer
   to "Building a Directory Service, Final Report test phase SURFnet

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   X.500 pilot project", E.  Huizer, SURFnet B.V., Utrecht NL, 1994.
   (copies available from SURFnet B.V.)

   Among the Paradise members there are several pilots running at the
   moment with the goal to evaluate the organisational aspects. Case
   studies coming from these pilots will be documented.

   Small or medium size organisations that have not too many entries to
   insert in the Directory may use one of the different national
   initiatives concerning a 'central DSA'. These central DSA's are
   operated by national service providers and contain the White Pages
   information of a lot of small and medium size organisations. For
   organisations in countries without such a national service there is
   also a European central DSA (Paradise) and an American central DSA
   (InterNIC). It is worth noting that the central DSA services are only
   technical services, i.e., a participating organisation still has to
   cover the organisational issues. However, part of a central DSA
   service may be consultancy with respect to datamanagement and legal

5.  Applications of X.500

   Besides for White Pages, X.500 can be useful for all kinds of
   distributed information storage from which humans or machines can
   benefit. Examples that are likely to use X.500 in the near future
   are: distribution list mechanism, public key distribution for Privacy
   Enhanced Mail (PEM), routing of X.400 messages, distribution of EDI
   identifiers, etc. For more information we refer to [7]. Below the
   first three applications are briefly discussed.

   The distribution list mechanism uses X.500 for finding the e-mail
   addresses of the persons that have subscribed to a list. The
   distributed approach of X.500 makes it possible that people change
   their e-mail address without having to change their subscription to
   distribution lists.

   PEM (see a.o. [8] or [4]) uses a public key mechanism for exchanging
   secure e-mail messages. For example: one will be able to end a secure
   message by encrypting a message with the publicly known (public) key
   of the recipient. Only the recipient of the message can decipher the
   message using his/her private key. In order to make such a mechanism
   work one must have access to the public keys of all possible
   recipients. X.500 can be used for this purpose.

   At this moment a world-wide pilot is running in which X.400 routing
   is done by means of X.500. X.400 MTA's use special DUA's to find via
   the Directory the MTA's to which the recipients of a message want
   their mail to be delivered. The distributed approach of X.500 will

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

   mean much less routing management (currently tables are used that
   have to be updated/exchanged periodically).

6.  References

   [1] Barker, P., and S. Kille,"The COSINE and Internet X.500 Schema",
       RFC 1274, University College London, November 1991.

   [2] Getchell, A., and S. Sataluri, Editors, "A Revised Catalog of
       Available X.500 Implementations", FYI 11, RFC 1632, Lawrence
       Livermore National Laboratory, AT&T Bell Laboratories, May 1994.

   [3] Weider, C., and J. Reynolds, "Executive Introduction to Directory
       Services using the X.500 Protocol", FYI 13, RFC 1308, ANS,
       USC/Information Sciences Institute, March 1992.

   [4] Linn, J., "Privacy Enhancement for Internet Electronic Mail:Part
       I: Message Encryption and Authentication Procedures", RFC 1421,
       IAB IRTF PSRG, IETF PEM WGs, Feblruary 1993.

   [5] Hardcastle-Kille, S., Huizer, E., Cerf, V., Hobby, R., and S.
       Kent, "A Strategic Plan for Deploying an Internet X.500 Directory
       Service", RFC 1430, ISODE Consortium, SURFnet bv, Corporation for
       National Research Initiatives, University of California, Davis,
       Bolt, Beranek and Newman, February 1993.

   [6] Yeong, W., Howes, T., and S. Kille, "Lightweight Directory Access
       Protocol", RFC 1487, Performance Systems International,
       University of Michigan, ISODE Consortium, July 1993.

   [7] Weider, C., and R. Wright, R., "A Survey of Advanced Usages of
       X.500", FYI 21, RFC 1491, Merit Network, Inc, Lawrence Berkeley
       Laboratory, July 1993.

   [8] "Privacy Enhanced Mail in more detail", Zegwaart, E., Computer
       Networks for Research in Europe Vol. 2, pp.  63-71.

   [9] Barker, P., Kille, S., and T. Lenggenhager, T., "Naming and
       Structuring Guidelines for X.500 Directory Pilots", RTR 11/RFC
       1617, University College London, ISODE Consortium, SWITCH, May
       1994.   For a good technical introduction to X.500 we also

  [10] Rose, M., "The Little Black Book", PSI Inc., Prentice Hall Inc.,
       New Jersey, 1992.

  [11] Steedman, D., "The Directory standard and its application",
       Technology Appraisals, Twickenham (U.K.), 1993.

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RFC 1684       Introduction to X.500 White Pages Services    August 1994

7.  Security Considerations

   Security issues are not explicitly discussed in this memo.

8.  Author's Address

   Peter Jurg
   SURFnet bv
   Postbus 19035
   NL-3501 DA Utrecht
   The Netherlands

   Phone: +31 30 310290
   Fax: +31 20 340903
   RFC822: Peter.Jurg@surfnet.nl
   X.400: C=nl; ADMD=400net; PRMD=surf; O=surfnet; S=jurg

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