The server object that represents a domain controller in the Sites container of the configuration directory partition has a globally unique identifier (GUID) that identifies it to the replication system as a domain controller. This GUID, called the DSA (Directory System Agent) GUID, is used in USNs to track originating updates. It is also used by domain controllers to locate replication partners. The DSA GUID is the GUID of the NTDS Settings object (class nTDSDSA), which is a child object of the server object. Its value is stored in the objectGUID attribute of the NTDS Settings object.
The DSA GUID is created when Active Directory is initially installed on the domain controller and destroyed only if Active Directory is removed from the domain controller. The DSA GUID ensures that the DSA remains recognizable when a domain controller is renamed. The DSA GUID is not affected by the Active Directory restore process.
The Active Directory database has its own GUID, which the DSA uses to identify the database instance (version of the database). The database GUID is stored in the invocationId attribute on the NTDS Settings object. Unlike the DSA GUID, which never changes for the lifetime of the domain controller, the invocation ID is changed during an Active Directory restore process to ensure replication consistency. For more information about replication following a restore process, see “Active Directory Replication on a Restored Domain Controller” later in this section.
On domain controllers that are running Windows Server 2003, the invocation ID also changes when an application directory partition is removed from or added to the domain controller.
Determining Changes to Replicate: Update Sequence Numbers
A source domain controller uses USNs to determine what changes have already been received by a destination domain controller that is requesting changes. The destination domain controller uses USNs to determine what changes it needs to request.
The current USN is a 64-bit counter that is maintained by each Active Directory domain controller as the highestCommittedUsn attribute on the rootDSE object. At the start of each update transaction (originating or replicated), the domain controller increments its current USN and associates this new value with the update request.