Directory System Agent

A Directory System Agent (DSA) is the element of an X.500 directory service that provides User Agents with access to a portion of the directory (usually the portion associated with a single Organizational Unit).^[1][2][3] X.500 is an international standard developed by the International Organization for Standardization (ISO), and the International Telecommunication Union (ITU-T). The model and function of a directory system agent are specified in ITU-T Recommendation X.501.

Active Directory

In Microsoft's Active Directory the DSA is a collection of servers and daemon processes that run on Windows Server systems that provide various means for clients to access the Active Directory data store.^[4][5]

Clients connect to an Active Directory DSA using various communications protocols:

• LDAP version 3.0—used by Windows 2000 and Windows XP clients^[4][5]
• LDAP version 2.0^[4][5]
• Security Account Manager (SAM) interface—used by Windows NT clients^[4][5]
• MAPI RPC interface—used by Microsoft Exchange Server and other MAPI clients^[4][5]
• A proprietary RPC interface—used by Active Directory DSAs to communicate with one another and replicate data amongst themselves^[4][5]