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.
Abstract
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 ................................................ 22. Concept of X.500 ............................................ 32.1 Directory Model ......................................... 32.2 Information Model ....................................... 43. Benefits of X.500 .......................................... 54. Organisational aspects of X.500(experience from Paradise) .. 6
5. Applications of X.500 ...................................... 86. References ................................................. 97. Security Considerations .................................... 108. Author's Address ........................................... 10
RARE Working Group on Network Applications Support [Page 1]
RFC 1684 Introduction to X.500 White Pages Services August 1994
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).
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.
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
person
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'.
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
applications.
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.
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
issues.
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
issues.
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).
[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
recommend:
[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