Network Working Group                                          C. Weider
Request for Comments: 1308                                           ANS
FYI: 13                                                      J. Reynolds
                                                              March 1992

              Executive Introduction to Directory Services
                        Using the X.500 Protocol

Status of this Memo

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


   This document is an Executive Introduction to Directory Services
   using the X.500 protocol. It briefly discusses the deficiencies in
   currently deployed Internet Directory Services, and then illustrates
   the solutions provided by X.500.

   This FYI RFC is a product of the Directory Information Services
   (pilot) Infrastructure Working Group (DISI).  A combined effort of
   the User Services and the OSI Integration Areas of the Internet
   Engineering Task Force (IETF).


   The Internet is growing at a phenomenal rate, with no deceleration in
   sight.  Every month thousands of new users are added. New networks
   are added literally almost every day. In fact, it is entirely
   conceivable that in the future every human with access to a computer
   will be able to interact with every other over the Internet and her
   sister networks. However, the ability to interact with everyone is
   only useful if one can locate the people with whom they need to work.
   Thus, as the Internet grows, one of the limitations imposed on the
   effective use of the network will be determined by the quality and
   coverage of Directory Services available.

   Directory Services in this paper refers not only to the types of
   services provided by the telephone companies' White Pages, but to
   resource location, Yellow Pages services, mail address lookup, etc.
   We will take a brief look at the services available today, and at the
   problems they have, and then we will show how the X.500 standard
   solves those problems.

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RFC 1308                Executive Intro to X.500              March 1992


   In the interests of brevity, we will only look at the WHOIS service,
   and at the DNS. Each will illustrate a particular philosophy, if you
   will, of Directory Services.

   The WHOIS service is maintained by the Defense Data Network Network
   Information Center, or DDN NIC.  It is currently maintained at GSI
   for the IP portion of the Internet. It contains information about IP
   networks, IP network managers, a scattering of well-known personages
   in the Internet, and a large amount of information related
   specifically to the MILNET systems. As the NIC is responsible for
   assigning new networks out of the pool of IP addresses, it is very
   easily able to collect this information when a new network is
   registered. However, the WHOIS database is big enough and
   comprehensive enough to exhibit many of the flaws of a large
   centralized database. First, centralized location of the WHOIS
   database causes slow response during times of peak querying activity,
   storage limitations, and also causes the entire service to be
   unavailable if the link to GSI is broken. Second, centralized
   administration of the database, where any changes to the database
   have to be mailed off to GSI for human transcription into the
   database, increases the turnaround time before the changes are
   propagated, and also introduces another source of potential error in
   the accuracy of the information. These particular problems affect to
   different degrees any system which attempts to provide Directory
   Services through a centralized database.

   The Domain Name Service, or DNS, contains information about the
   mapping of host and domain names, such as, "", to IP
   addresses. This is done so that humans can use easily remembered
   names for machines rather than strings of numbers. It is maintained
   in a distributed fashion, with each DNS server providing nameservice
   for a limited number of domains.  Also, secondary nameservers can be
   identified for each domain, so that one unreachable network will not
   necessarily cut off nameservice. However, even though the DNS is
   superlative at providing these services, there are some problems when
   we attempt to provide other Directory Services in the DNS. First, the
   DNS has very limited search capabilities. Second, the DNS supports
   only a small number of data types. Adding new data types, such as
   photographs, would involve very extensive implementation changes.


   X.500 is a CCITT protocol which is designed to build a distributed,
   global directory. It offers the following features:

   * Decentralized Maintenance:

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RFC 1308                Executive Intro to X.500              March 1992

     Each site running X.500 is responsible ONLY for its local part of
     the Directory, so updates and maintenance can be done instantly.

   * Powerful Searching Capabilities:
     X.500 provides powerful searching facilities that allow users to
     construct arbitrarily complex queries.

   * Single Global Namespace:
     Much like the DNS, X.500 provides a single homogeneous namespace
     to users. The X.500 namespace is more flexible and expandable
     than the DNS.

   * Structured Information Framework:
     X.500 defines the information framework used in the Directory,
     allowing local extensions.

   * Standards-Based Directory Services:
     As X.500 can be used to build a standards-based directory,
     applications which require directory information (e-mail,
     automated resources locators, special-purpose directory tools)
     can access a planet's worth of information in a uniform manner,
     no matter where they are based or currently running.

   With these features alone, X.500 is being used today to provide the
   backbone of a global White Pages service. There is almost 3 years of
   operational experience with X.500, and it is being used widely in
   Europe and Australia in addition to North America. In addition, the
   various X.500 implementations add some other features, such as
   photographs in G3-FAX format, and color photos in JPEG format.
   However, as X.500 is standards based, there are very few
   incompatibilities between the various versions of X.500, and as the
   namespace is consistent, the information in the Directory can be
   accessed by any implementation. Also, work is being done in providing
   Yellow Pages services and other information resource location tasks
   in the Directory.

   However, there are some limitations to the X.500 technology as it is
   currently implemented. One price that is paid for the flexibility in
   searching is a decline in the speed of the searching. This is because
   a) searches over a part of the distributed namespace may have to
   traverse the network, and some implementations cache all the
   responses before giving them to the user, and b) some early
   implementations performed search slowly anyway. A second problem with
   the implementations is that for security reasons only a limited
   amount of information is returned to the user; for example, if a
   search turns up 1000 hits, only 20 or so are returned to the user.
   Although this number is tunable, it does mean that someone with a big
   search will have to do a lot of work. The performance of the

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RFC 1308                Executive Intro to X.500              March 1992

   Directory, while increasing rapidly in the last two years, is still
   not able to provide real-time directory services for such things as
   routing protocols.  However, work is being done to speed up service.

   The X.500 Directory is taking us closer to the day when we will
   indeed have the entire world on our desktops, and X.500 will help
   insure that we can find whom and what we need.


   For a more detailed technical introduction to X.500 and an extensive
   bibliography, see "Technical Overview of Directory Services Using the
   X.500 Protocol", by Weider, Reynolds, and Heker. This is available
   from the NIC as FYI 14, RFC 1309.  For a catalogue of X.500
   implementations, see "A Catalog of Available X.500 Implementations",
   ed. Lang and Wright.  This is available from the NIC as FYI 11, RFC


   Security issues are not discussed in this paper.


   Chris Weider
   Advanced Network and Services, Inc.
   2901 Hubbard, G-1
   Ann Arbor, MI 48105-2437

   Phone (313) 663-2482

   Joyce K. Reynolds
   Information Sciences Institute
   University of Southern California
   4676 Admirality Way
   Marina del Rey, CA 90292

   Phone: (310) 822-1511

DISI Working Group                                              [Page 4]