Components of the X.500 Directory

Directory Information Database (DIB)
A collection of objects representing physical resources. Objects are made up of entries which consist of attributes and values and each entry has a unique distinguished name.

Directory Information Tree (DIT)
Logical tree structure representing hierarchical arrangements of objects. A set of rules are enforced to ensure the DIB remains stable and intact, known as the Directory schema.

Directory User Agent (DUA)
The DUA is an application process which acts as the client in a client/server approach to accessing the Directory. Users are people or programs that read or modify the Directory. The DUA requests information from the Directory which it then relays to the user.

Directory System Agent (DSA)
The DSA is not a single protocol or piece of software, but a collection of services and protocols managing a portion of a DIB. The DSA handles requests from the DUA for information which it relays back to the DUA. If the DSA cannot satisfy the request it passes the request on to another DSA.

Directory Access Protocol (DAP)
The DAP handles the exchange of requests and responses between a DUA and a DSA.

Directory System Protocol (DSP)
When a DSA cannot satisfy a request the request is passed onto another DSA using the DSP.

Directory Information Shadowing Protocol (DISP)
The DISPL performs the replication/shadowing of the DIB to another DSA.