Home > Articles > Operating Systems, Server

This chapter is from the book

This chapter is from the book

X.500 Overview

A directory service is a distributed store of information about the users of a computer system and the infrastructure that supports that system.

The goal of X.500 was to cut through the babble of competing information repositories to define a single place where users from all nations could go to locate each other, learn about each other, discover common likes and dislikes, and eventually communicate freely to find a path to universal peace and brotherhood and the dawning of the Age of Aquarius. The key features of an X.500 directory service are as follows:

  • The information is distributed among many different servers.

  • Users can submit queries to any server to find information anywhere in the system.

  • Servers can find information on other servers because they share common knowledge about each other.

X.500 Components

The magic of X.500 comes from the flexible way it compartmentalizes and distributes information. This flexibility comes at the cost of complexity, though—not the least of which is a thicket of nomenclature rife with obscure computing jargon and Three Letter Acronyms (TLAs). These X.500 acronyms crop up quite a bit in Active Directory documentation, so it pays to give them a Quick Run Through (QRT). Refer to Figure 6.2 for a roadmap. Here are the X/500 TLAs:

  • Information in an X.500 Directory is stored in a Directory Information Base (DIB).

  • The DIB is divided into pieces that are structured into a hierarchy called a Directory Information Tree (DIT).

  • Each piece of the DIB is stored on a server called a Directory Service Agent (DSA).

  • A user who needs information from Active Directory submits queries via an application interface called a Directory User Agent (DUA).

  • A DUA communicates with a DSA using the Directory Access Protocol (DAP).

  • One DSA communicates with another using the Directory System Protocol (DSP).

  • Administrative information exchanged between DSAs is controlled via policies defined by the Directory Operational Binding Management Protocol (DOP).

  • A single Directory Management Organization (DMO) takes charge of a Directory Management Domain (DMD) that contains one or more DSAs.

  • Information held by one DSA is replicated to other DSAs in the same DMD using the Directory Information Shadowing Protocol (DISP).

06fig02.gifFigure 6.2. X.500 components and their communication protocols.

DAP, DSP, DISP, and all other high-level communication protocols in X.500 use OSI networking as defined in ITU Recommendation X.200/OSI-EIU Standard 7498.

X.500 Transaction Example

Here's an example of how these X.500 components tie together (see Figure 6.3). Let's say that the secondhand car dealers in America get together and decide to form an association. They want a directory service to store information about vehicles available for sale at each member's showroom.

06fig03.gifFigure 6.3. Diagram of an example X.500 communication scheme.

The DIB for this dealership directory service includes makes, models, years, vehicle identification numbers, and unbeatable prices. Each dealer is assigned a DMO that controls a DMD. The DIB in each DMD is hosted by at least one DSA, which exchanges administrative information with DSAs in other DMDs using DOP. Dealerships in the same region have individual DSAs that replicate their copy of the DIB between each other via DISP. The pieces of the DIB are joined into a single DIT, the root of which is hosted by a DSA at headquarters.

Why go through all this trouble? Well, if a customer at a dealership in Kankakee wants a cherry-colored Cherokee, the salesperson can sit at a DUA and submit a query to a local DSA via DAP. The DSA would check its copy of the local DIB and if it failed to locate a record, it would use DSP to query other DSAs until it either found a match or exhausted all possibilities. The DUA could then be programmed to suggest alternatives, like a cream-colored Chevelle in Chicago.

The important point to remember about this transaction is that there is no central repository of information. Each local DSA holds its own copy of the DIB. Referral mechanisms are used to distribute queries around the system.

Why LDAP Instead of X.500?

Several pedigreed X.500 directory services are commercially available, but few have achieved widespread popularity. The problem with pristine X.500 implementations is the overhead represented by all those protocols. When you get an army of DUAs all talking DAP to DSAs that refer queries to other DSAs using DSP while at the same time mirroring their DIBs to other DSAs in their DMD via DISP, my friend, you've got a whole D* lot to go wrong.

In the early 90s, a few bright folks at the University of Michigan wanted to build a directory service to handle their 100,000+ students, staff, and faculty. They gave up on the complexities of X.500 and came up with a scheme that retained the X.500 directory structure but gave it a streamlined access protocol based on standard TCP/IP instead of ISO. They also came up with a pared-down referral mechanism, a more flexible security model, and no fixed replication protocol. They called the result the Lightweight Directory Access Protocol, or LDAP. The rest, as they say, is history. The Blue and Maize folks no longer control LDAP development. The current repository of LDAP knowledge is at www.openldap.org.

Active Directory and LDAP

When Microsoft decided to replace the clumsy Registry-based account management system in classic NT with a true directory service, rather than devise a proprietary directory service of their own, they chose to adopt LDAP. Even more importantly, from our perspective as administrators, Microsoft chose to deliver their LDAP directory service using two proven technologies.

Extensible Storage Engine (ESE)

At its heart, a directory service database is made up of tables with rows representing objects of interest and columns representing attributes of those objects. What sets different databases apart is the way the tables are managed. This table manager is often called a database engine.

The LDAP standards do not stipulate a particular table management technology. For the Active Directory table manager, Microsoft used a revved-up version of the Extensible Storage Engine (ESE) first introduced with Exchange. Microsoft chose ESE over the SQL Server database engine because a SQL engine does not work efficiently with the object-oriented structure of an LDAP directory. The ESE engine, on the other hand, was primarily designed as an object-oriented database.

DNS-Based Locator System

Users cannot take advantage of the information in a directory service if they cannot find the servers hosting the information. Microsoft chose to build its LDAP directory service around the Domain Name System (DNS). When an LDAP client needs to find a server hosting a directory service, it does so by querying DNS. This enabled Microsoft to use new features in DNS to simplify the search.

For example, Microsoft took advantage of the relatively new service locator (SRV) record type to put pointers in DNS to indicate the names of servers hosting LDAP and Kerberos services. SRV records have a relatively complex structure, but Microsoft was able to avoid typographical errors by registering them automatically using Dynamic DNS.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020