PDC Emulator Allows Windows Server 2003 to act as the primary domain controller. Provides replication support for the backup domain controller. Assists with time and group policy synchronization. Infrastructure Master - (One per domain) Responsible for updating the group to user references whenever there are changes. Also receives new names. Relative ID Master - RID - (One per domain) Ensures that every object created has a unique ID number. Schema Master - (One per forest) Responsible for maintaining and modifying the AD schema. Domain Naming Master - (One per forest) Responsible for adding and deleting domains in a forest.