Network Working Group C. Weider
Request for Comments: 1308 ANS
FYI: 13 J. Reynolds
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.
2. CURRENT SERVICES AND THEIR LIMITATIONS
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, "home.ans.net", 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.
3. THE X.500 SOLUTION
X.500 is a CCITT protocol which is designed to build a distributed,
global directory. It offers the following features:
* Decentralized Maintenance:
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
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.
4: FOR FURTHER INFORMATION
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
5: SECURITY CONSIDERATIONS
Security issues are not discussed in this paper.
6: AUTHORS' ADDRESSES
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