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This chapter is from the book

This chapter is from the book

Brief History of Directory Services

There's an old saying that you can't get to where you're going unless you know where you've been. Before analyzing Active Directory, let's start with a look at the history of directory services in general. This is not an academic exercise. It's important to understand the reason behind the decisions made when directory services were formulated and who made those decisions.


The directory service story starts with a smallish document called X.500, “Data Networks and Open System Communications—Directory.” The cast of characters in this story includes a group of standards bodies and vendors from all over the world.

First and foremost is the International Telecommunication Union (ITU). The ITU is a United Nations agency that acts as a forum for governments that want to achieve consensus on global telecom issues. The ITU membership includes manufacturers and service providers from over 130 countries.

The branch of the ITU specifically tasked with making directory service recommendations is the Telecommunication Standardization Sector, or ITU-T. The ITU-T was formerly called the Comité Consultatif International Téléphonique et Télégraphique (CCITT).

The ITU-T issues recommendations in many areas, from broadcast requirements and measuring equipment to faxing. These recommendations are grouped into lettered series. For example, the V series covers data communication over telephone networks and includes such famous standards such as V.34, “Wideband Analog Modem Communication,” and V.90, “Connecting Analog to Digital Modems.”

The X series of recommendations, which includes the X.500 recommendations for directory services, covers a variety of data network and open system communication technologies, such as X.25 packet-switched networks and X.400 messaging systems. For a complete listing of ITU recommendations, see www.itu.int/publications/telecom.htm.


The ITU-T does not set standards; it only makes recommendations. Getting an international standard approved requires the consent of the International Organization for Standardization (ISO).

Source of the ISO Name

You may wonder why the initials ISO do not match the name, International Organization for Standardization. Actually, the letters are not initials at all. They come from the Greek word isos, meaning equal. These letters were used to avoid the hodgepodge of acronyms that would have resulted if the various member countries translated International Organization for Standardization into their own language with their own initials.

Unlike the ITU, whose membership comes from industry vendors, ISO members come from national standards bodies. The U.S. member is the American National Standards Institute (ANSI). The ISO web site is located at www.iso.ch. The ch indicates that the site is in Switzerland, just in case you are not up on your ISO 3166 two-letter country codes.

The ISO is responsible for standardization in just about every area, from the quality standards of ISO 9000 to the standard paper sizes of ISO 216. In the networking industry, it is most famous for ISO 7498, “Information Technology—Open System Interconnection—Basic Reference Model,” better known as the OSI Network Model.

ISO standards that affect data communication technology are often jointly published with the ITU-T. For example, the ISO standard that parallels the ITU-T X.500 recommendations for directory services is ISO 9594, “Information Technology—Open Systems Interconnection—The Directory.” Because the ISO issues standards and the ITU-T issues recommendations, it is actually a misnomer to refer to the X.500 Standard, but this is commonly done because the two documents are identical.


The ISO is the senior standards body in the world, but it certainly is not the only one. Many agencies dip their spoons in the standards soup bowl and they sometimes slosh on each other. In the data communications field, there is overlap between standards published by the ISO and standards published by the International Electrotechnical Commission (IEC).

The IEC deals with international standardization for electronics, magnetics, electromagnetics, electroacoustics, telecommunication, and energy production/distribution. They promulgate terminology, symbols, measurement standards, performance standards, dependability, design, development, safety, and environmental standards. The U.S. member of the IEC is also ANSI. The ISO and IEC joined with the ITU in publishing the directory service standards. The IEC web site is located at www.iec.ch.


In the United States, there is one senior standards body, ANSI. You are probably most familiar with ANSI for its work to standardize character-based data formats, although there are ANSI standards for just about anything. I used to work in the nuclear industry, where even the ballpoint pens were built to conform to an ANSI standard. The ANSI web site is www.ansi.org.


In a country where millions of people call television talk shows to give advice to total strangers about their sex lives, it should come as no surprise that many advisory bodies are eager to give input to ANSI. An advisory body with a great deal of influence over implementation of the X.500 standard is the Internet Engineering Task Force (IETF). Its web site is located at www.ietf.org.

The IETF is an amalgam of vendors, developers, researchers, designers, and architects of all stripes who have an interest in the workings of the Internet. Special working groups within the IETF ride herd on Internet workings in collaborative effort called the Internet Standards Process, a unique and somewhat lengthy operation that consists of thrashing a good idea mercilessly until it breaks into pieces that can be easily digested by the collective organism.

Request For Comments (RFC)

The Internet Standards Process is facilitated by documents called Request for Comments (RFCs) and Internet Drafts. To give you an idea of how long it takes to assimilate new ideas into Internet standards, out of the hundreds and hundreds of standards-track RFCs listed in RFC 2700, “Internet Official Protocol Standards,” there are only 59 standards. The rest of the documents squirm somewhere in the approval process.

Copies of RFCs, Standards, Standards Track documents, Internet Drafts, and other working papers can be found at the IETF site and at various mirrored sites around the Internet. I prefer the search engine at the Internet Engineering Standards Repository, www.normos.org.

The IETF can bypass ISO/IEC standards and ITU recommendations if they deem it necessary to get useful protocols out into the world. An example of this is the Lightweight Directory Access Protocol (LDAP). LDAP is a pared-down version of the X.500 directory service that forms the basis of Active Directory, Netscape Directory Services, and other products.

There is no LDAP standard from ISO and no LDAP recommendation from the ITU. LDAP is purely an Internet concoction. Active Directory implements the most current version of LDAP, version 3, as documented in RFC 2251, “Lightweight Directory Access Protocol v3.” This RFC expands and augments the original LDAP Standards Track document, RFC 1777, “Lightweight Directory Access Protocol.” There is a long list of RFCs that expand various LDAP features.

Although LDAP is not precisely an X.500 implementation, a great deal of the design basis of LDAP comes from X.500. So before going through LDAP in detail, let's take a quick look at its parent.

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