Classes and Attributes

The world of Active Directory does not end right after understanding the classes and attributes there are several other components which need to be understood before saying that you really understand what Active Directory is.
In the implementation of directory service the designers had to take care to reduce down the complexities yet a brilliant performance. To achieve this they had to consider that the classes which are used to define an object with directory service need to be less in numbers.

For example, in a library card catalog, it would be a mistake to create a class called Somewhat-Less-Than-Riveting-Early-20th-Century-American-Novels, even though it seems like quite a few objects would fit in that class. In relation to the overall scope of a library, this classification would be too narrow. It would be better to have an attribute called Boring with a Boolean value. You could assign this attribute to the Book class so that objects derived from that class would get a Boring attribute that could be given a value of Yes or No or left empty. You could also assign the Boring attribute to the Periodical, Tape, and Video classes.

A directory can have hundreds of classes and many hundreds of attributes. If the attributes for each class had to be separately defined, the sheer number of perturbations would make the directory look less like a tree and more like an example of German expressionism.
Fortunately, attributes associated with a particular class often overlap those of other classes. For example, the attribute list for the Mailbox class includes all the attributes associated with the Mail-Recipient class with one addition, the Delivery-Mechanism attribute. So, instead of separately defining all the attributes in Mailbox class, LDAP allows the class to be defined as a child of the Mail-Recipient class. This permits it to inherit the attributes of its parent. The designer need only stipulate the new additional attribute or attributes that make the subordinate class unique.
Flow of the these attributes is like the flow of genes in the family tree. This figure (Seems like there is some problem with scripts running on the create post page so could not upload the relevant image, I will upload the same when the problem is fixed.) may help you to understand the flow.

Object Instances
Each object in Active Directory is derived from a specific object class. Another way of saying this is that an object represents an instance of a class. Each instance of an object class differs from another instance by having different values for its attributes.

Let’s assume that Merrick stands in front of a curious mob and exclaims,”I am not an elephant. I am a human being.” Had Mr. Merrick been a directory services designer, he could have clarified his point by adding, “I am an instance of the Human Being class, not the Elephant class. And the only difference between you and me is a relatively minor attribute of mine that has a different value from yours. So lay off, will you?”

A database schema defines the content and structure of the database. In a library card catalog, the schema would be a set of procedures and rules set down by the librarian.” Books go on green cards,” she tells you.” Videos go on red cards. File the cards alphabetically by Title in this cabinet and by Subject in that cabinet.” So on and so on. The schema for an LDAP directory service defines these items:

The attributes associated with each object class
The permissible object classes
The parent-child relationship of object classes, which in turn determines attribute inheritance
The data type associated with each attribute
The physical representation of the object in the user interface

Previous two posts were purely intended to help understanding LDAP information model. If you are really interested to understand the LDAP information model in depth then you can certainly have a look at RFC and then , there are several actually which explain the standards for the LDAP information model.

LDAP Information Model

Continuing the matter of the previous post, this post will contain the information about the LDAP namespace and the LDAP information mode.
For a newbie who always finds it difficult to understand the abbreviations and the terms used in Active Directory explanations ever since, I have tried to explain it in the simplest form with an example that everyone has come across at least once when we were in our school and university days.
As I said in the previous post, the directory database is nothing more than an object oriented database which contains information stored in the form of tables.

To narrow down the scope of the explanation and to simplify the example lets recall that Oak wood cupboard which used to store the books in it (in library off course. Well, being very honest I never used to be a loyal member of any library, so apart from observing and co relating those oak wood cupboards with directory database I never looked at them from any other perspective).
In other words the LDAP information model is somewhat like that old Oak wood cupboard. How? Here is an example.

A directory service may be a bit fancier than the database you use to tally the overtime pay you’ve lost since taking your salaried administrator position a few years back, but the principles of operation are pretty much the same.

Object-Oriented Database

In X.500 terminology, the directory service database is called a Directory Information Base (DIB). If you think of an old-style library card catalog system as a kind of directory service, one of those big oak cabinets with rows of drawers would be a DIB.

The X.500 directory service structure was developed at a time when object-oriented databases represented leading-edge technology. If your only exposure to database technology has been more modern relational databases, the design constraints of an object database can look a little strange.

In an object-oriented database, each record (object) occupies a unique position in a hierarchical namespace. The object’s name and path traces its origins to the top of the namespace, in much the same way that a Daughter of the American Revolution traces her forebears back to the Mayflower. A file system is an example of an object-oriented database.

Object databases consist of big, structured sequential files connected by a set of indexes that are themselves nothing more than big, structured sequential files. This underlying database technology is called Indexed Sequential Access Method, or ISAM. You’ll see this term in the Event log and other reports.

The ESE database engine exposes the flat ISAM structure as a hierarchy of objects. In addition, Microsoft makes extensive use of COM technology by representing Active Directory objects as COM objects via the Active Directory Services Interface (ADSI).

Classes and Attributes
a directory service contains information about specific types of objects, such as User objects, Computer objects, and so forth. These are called object classes. A class is a bundle of attributes with a name.
Attributes and Properties
Attributes are also often called properties. There is a difference between these two terms, but it is so subtle that most reference manuals, including this one, use them interchangeably.

The attributes associated with a particular object class differentiate it from other object classes. For example, User objects have different attributes than Computer objects or IP Security objects. Using a library card catalog as an example, different card formats represent different classes of items. A certain card format is used to record entries for Books. Another format is used for Tapes. The card format for Books would have spaces for Title, Author, ISBN, and so forth. A card for Tapes would have spaces for those entries plus additional spaces for Read-By and Play-Time.

An object class, then, is really nothing more than a bundle of attributes with a name. RFC 2256, “A Summary of the X.500(96) User Schema for use with LDAPv3,” defines 21 classes and 55 attributes for use in a standard LDAP directory service. Active Directory adds quite a few more for a total of about 200 object classes and 1500 attributes.

Classes also define the scope of a directory service database. You would not expect to find cards in a library card catalog representing Off-The-Road Vehicles or Double- Meat Hamburgers. Microsoft engineers defined the initial scope of Active Directory by including a certain set of object classes and attributes. This list can be extended by other applications or by administrators. For example, your organization could create attributes and classes for storing badge numbers and social security numbers in Active Directory.

Active Directory and LDAP

When Microsoft decided to replace the Registry-based account management system in Windows NT with a real time directory service, rather than introducing 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)
DNS-Based Locator System

As I said in my previous post the TLAs were derived as it is in the new directory services like active directory. Microsoft continued the trend and decided to host the Active Directory service into a database, unlike DIT.
This database is made up of tables. For a SQL administrator it would really not very tough to understand what does a table in a database mean? So, active directory database is made up of tables, in which; rows represent objects and columns represent attributes associated with the objects. If you have started thinking that a SQL administrator can now manage active directory then stop imagining wild because, what makes this database different from others is the way the tables inside are managed. A component which manages all these tables is very often known as database engine.
LDAP standards do not define any particular table management technology. For Active Directory table management Microsoft chose to use a proven technology very well known as Extensible Storage Engine (ESE) which had proven its efficiency in earlier versions of Microsoft Exchange Server. Microsoft decided to use ESE over the SQL database engine because SQL engine does not work efficiently with object oriented databases which comply the LDAP structure. On the other hand, ESE engine was primarily developed to handle the object oriented databases.

Though, design and architecture of a LDAP database compliant with LDAP was ready; it was unusable for a normal until he could locate this information repository. They decided to build the entire directory around Domain Naming System (DNS). So when the LDAP client wants to discover the location of the directory services it reaches the location by using DNS queries. This was made possible by using the service locator (SRV). SRV is nothing but a pointer which redirects the clients to a specified service running on a server. If you decide to dig into the DNS architecture and functionality SRV is not really a very simple service component which can be understood real quick but Microsoft made it available by registering them automatically by using dynamic DNS.

I would have really loved to write down some more information in this post about the storage architecture but I am surrounded by some other attacks of my network these days and have to fix them up on priority basis. In the next post let’s have a closer understanding of a LDAP database that runs active directory. I will try to document how does Active Directory information is stored in the database which is run by ESE. Let me promise you that the next post will cover some basic information about the LDAP information model too. Till then keep searching the information around and fill it in your biological database. 😀

X.500 – An Overview and Components

The previous post was just to highlight the limitations of Windows NT 4.0 and the need why an a new version of directory services was required to be introduced in the market which could meet the rapidly growing network infrastructures.
Microsoft successfully rolled out its own product Windows NT4 by launching Active Directory Services.
Active Directory is not the only directory service which existing on this planet. Moreover of all Active Directory originated from the concept of Windows NT and Windows NT domain services were brought into market as an enhancement to the previously available directories.

In this post I am going to shed some light on evolution of directory services which will be backed up by explanation of directory service models and architectures. Following are key points;

Definition of Directory Service.
X.500 Overview and its components
How X.500 works.
Why LADAP and not X.500?
LDAP and its namespace model.

Definition of Directory Service.

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


The purpose behind implementation of X.500 was to create an information repository of people from different locations so that they can share their thoughts, locate each other, discover likes and dislikes, and head towards the brotherhood. The key features of X.500 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 X.500 architecture became very famous and was very much feasible too. The flexibility to store the information, sorting it out in proper manner, and making it available to end users made a drastic improvements in user experience, with a complex structure though. After all, flexibility can not be achieved without complexities.
To store all the information using this architecture the requirement of naming convention was born. This naming convention is as it is derived into Microsoft’s Active Directory Services too. This naming terminology is known as Three Letter Acronyms or TLA in short. They are as follows:

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).

DAP, DSP, DISP, and all other high-level communication protocols in X.500 use OSI networking.

How X.500 works.

I would like to explain it in the form of an example in stead of writing down anything else. The example below illustrates a working transaction on X.500 directory. It will be better enough to understand the basic functionality.

Let’s say that a group of industrialists which belong to the same class of market want form an organization so that they can speed up their business by sharing information among each other. To achieve this they will need a common resource which can handle the storage of information available at every member of the group.
This group is basically in dealing of old cars.
Now, before you try to understand what this example means I will suggest to have a look at the figure first.

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 LADAP and not X.500?

Several X.500 directory services are available, but few have achieved popularity. The problem with 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* a lot may go wrong in the jumble of Ds.

Few brains 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

Alright till yet. Now the actual adventure is to start about Active Directory, LDAP, LDAP model, LDAP namespace, Domains, Forests, OUs and much more. Before that I really need a refreshment . I would have been the university topper if I would have really worked so much hard in my college time. Well, I was never a bright student like the folks who developed todays well known Lightweight Directory Access Protocol.

The next post will cover up very interesting stuff like Domains and Forests and blah blah blah!!!

Active Directory Services

It was very busy time since past few days so it was very hard for me to take out some time post a new item here on the blog. Now, everything seems to be settled down and things are going very fine on my servers.
This post is going to cover the below mentioned key points today. Anything you feel is missing from the blog or can be made better please write your comments to me directly at

Ok now, lets understand the what active directory is and how it evolved.

International Telecommunication Union (ITU) is a United Nations agency that plays a role as a forum for governments that want to achieve consensus on global telecom issues. Several manufacturers and service providers among more than 130 countries across the globe are members of ITU.

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 known as Comité Consultatif International Téléphonique et Télégraphique (CCITT).

The ITU-T recommends 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. All these recommendations issued by ITU-T can be found at

Initially, Microsoft introduced directory services in its first ever and most stable operating system; very well known as Windows NT 4.0 This operating system lasted a very long time in the market and on the corporate networks. The domain services provided by Windows NT 4.0 started dominating Novell’s directory services. Slowly, it grabbed the market and Novell remained on the networks wherever it was implemented before. Off course no one would like to go for the black and white and non user friendly interfaces if he/she is provided a good looking and easy to use graphical user interface which made the tasks simpler for administrators and utilization of resources for the end users.

As the networks started growing rapidly the requirement of manageability, scalability and functionality of Windows NT 4.0 started decreasing. It resulted in the rise of new directory service named as Active Directory. Microsoft added some salient features in Active Directory services those offered more reliable structure of directory services. Firstly, it was introduced with the Windows 2000 Server version. Following are the features implemented in Microsoft Windows Server 2003.

Site scalability. The calculations for determining replication topology between sites have been streamlined. This corrects a problem where large organizations with hundreds of sites might experience replication failure because the topology calculations cannot be completed in the time allotted to them.
Backlink attribute replication. Group members are now replicated as discrete entities instead of replicating the entire group membership list as a single unit. This corrects a problem where membership changes made to the same group on different domain controllers in the same replication interval overwrite each other.
Federations. A new trust type called Forest was added to simplify transitive trust relationships between root domains in different forests. Using Forest trusts, it is possible to build a federation of independent Active Directory forests. This feature does not implement true “prune and graft” in Active Directory, but it goes a long way toward simplifying operations within affiliated organizations.
Simplified domain logon. Universal group membership can be cached at non-global catalog servers. This permits users to log on even if connectivity to a global catalog server is lost. This enhancement is coupled with a feature in XP where the domainname result of cracking a User Principal Name (UPN) is cached locally. This permits a user at an XP desktop to log on with the format even if a global catalog server is not available.
Application naming contexts. Windows Server 2003 introduces the capability to create new naming contexts to hold DNS record objects for Active Directory Integrated zones. One naming context holds domain zone records and one holds the _msdcs records used throughout a forest. These naming contexts make it possible to target replication of DNS zones only to domain controllers that are running DNS.
Eliminate piling onto new domain controllers. There is potential for a problem when an NT4 primary domain controller (PDC) is upgraded to Windows Server 2003. In this circumstance, all existing Windows 2000 and XP desktops will use the newly promoted PDC as a logon server. In Windows Server 2003, domain controllers can be configured to respond to modern Windows clients as if they were still classic NT domain controllers until sufficient domain controllers are available to handle local authentication. This feature is also available in Windows 2000 SP2 and later.
DNS diagnostics. Proper DNS configuration is critical for proper Active Directory operation. The Domain Controller promotion utility now performs a suite of DNS diagnostics to ensure that a suitable DNS server is available to register the service locator resource records associated with a Windows domain controller.
Fewer global catalog rebuilds. Adding or removing an attribute from the Global Catalog no longer requires a complete synchronization cycle. This minimizes the replication traffic caused by adding an attribute to the GC.
Management console enhancements. The Active Directory Users and Computers console now permits drag-and-drop move operations and modifying properties on multiple objects at the same time. There is also the capability of creating and storing custom LDAP queries to simplify managing large numbers of objects. The new MMC 2. 0 console includes scripting support that can eliminate the need to use the console entirely.
Real-time LDAP. Support was added for RFC 2589, “LDAPv3: Extensions for Dynamic Directory Services.” This permits putting time-sensitive information in Active Directory, such as a user’s current location. Dynamic entries automatically time out and are deleted if they are not refreshed.
Enhanced LDAP security. Support was added for digest authentication as described in RFC 2829, “Authentication Methods for LDAP.” This makes it easier to integrate Active Directory into non-Windows environments. Support was also added for RFC 2830, “LDAPv3: Extension for Transport Layer Security.” This permits using secure connections when sending LDAP (Lightweight Directory Access Protocol) queries to a domain controller.
Schema enhancements. The ability was added to associate an auxiliary schema class to individual objects rather than to an entire class of objects. This association can be dynamic, making it possible to temporarily assign new attributes to a specific object or objects. Attributes and object classes can also be declared defunct to simplify recovering from programming errors.
LDAP query enhancements. The LDAP search mechanism was expanded to permit searching for individual entries in a multivalued Distinguished Name (DN) attribute. This is called an Attribute Scoped Query, or ASQ. For example, an ASQ could be used to quickly list every group to which a specific user belongs. Support was also ad

ded for Virtual List Views, a new LDAP control that permits large data sets to be viewed in order instead of paging through a random set of information. This change permits Windows Server 2003 to show alphabetically sorted lists of users and groups in pick lists.
Interoperability. This enhances interoperability with Netscape and NetWare directory services, both of which use the inetOrgPerson object class to create User objects.
Speedier domain controller promotions. The capability was added for using a tape backup of the Active Directory database to populate the database on a new domain controller. This greatly simplifies domain controller deployments in situations where it is not practical to ship an entire server.
Scalability. The maximum number of objects that can be stored in Active Directory was increased to over one billion.

These were the enhancements made in the recent release of Windows, Microsoft Windows Server 2003 Active Directory Services. In the next posts I will be covering the remaining stuff related to Active Directory.

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