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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.

DISI Working Group [Page 1] 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:

DISI Working Group [Page 2] 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

DISI Working Group [Page 3] 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]

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