is a value that the destination domain controller maintains for tracking the originating updates that are received from all source domain controllers.The source domain controller uses this value to reduce the set of attributes that it sends to the destination domain controller.The current USN is a 64-bit counter that is maintained by each Active Directory domain controller.At the start of each update transaction (originating or replicated) on a domain controller, the domain controller increments its current USN and associates this new value with the update request.It relies instead on the use of update sequence numbers (USNs) that are assigned by a counter that is local to each domain controller.Because these USN counters are local, it is easy to ensure that they are reliable and never "run backward" (that is, decrease in value).But if the attribute is updated twice on domain controller A and once on domain controller B, the update of domain controller A wins even if the clock of domain controller B is set forward to December 31, 9999.With Active Directory, clock skew can never prevent a value from being overwritten.
In these systems, it is important to keep the clocks on all directory servers synchronized.
Active Directory replication does not use timestamps as the primary mechanism to determine what updates "win" (are preserved) in a conflict resolution.
Instead, Active Directory uses volatility (number of changes) as the first element of the per-attribute "stamps" that are compared during conflict resolution. So if an attribute is updated once on domain controller A and once on domain controller B, the last writer's update wins.
You can use Ldp or ADSI Edit to read the current USN of a domain controller.
These tools use LDAP to read the is a value that the destination domain controller maintains to keep track of the most recent change that it has received from a specific source domain controller for an object in a specific directory partition.