Flexible Single Master Operations (FSMO, F is sometimes "floating"; pronounced Fiz-mo), or just single master operation or operations master, is a feature of Microsoft's Active Directory (AD). As of 2005, the term FSMO has been deprecated in favour of operations masters. FSMO is a specialized domain controller (DC) set of tasks, used where standard data transfer and update methods are inadequate. AD normally relies on multiple peer DCs, each with a copy of the AD database, being synchronized by multi-master replication. The tasks which are not suited to multi-master replication and are viable only with a single-master database are the FSMOs. FSMO roles Per-domain roles These roles are applicable at the domain level (i.e., there is one of each for every domain in a forest): The PDC Emulator (Primary Domain Controller) - This role is the most used of all FSMO roles and has the widest range of functions. The domain controller that holds the PDC Emulator role is crucial in a mixed environment where Windows NT 4.0 BDCs are still present. This is because the PDC Emulator role emulates the functions of a Windows NT 4.0 PDC. Even if all Windows NT 4.0 domain controllers have been migrated to Windows 2000 or later, the domain controller that holds the PDC Emulator role still does a lot. The PDC Emulator is the domain source for time synchronization for all other domain controllers; in a multi-domain forest, the PDC Emulator in each domain synchronizes to the forest root PDC Emulator. All other domain member computers synchronize to their respective domain controllers. It is critically important that computer clocks are synchronized across the forest because excessive clock skew causes Kerberos authentication to fail. In addition, all password changes occur on the PDC Emulator and receive priority replication. The RID Master - (Relative ID) This FSMO role owner is the single DC responsible for processing RID Pool requests from all DCs within a given domain. It is also responsible for moving an object from one domain to another during an interdomain object move. When a DC creates a security principal object such as a user or group, it attaches a unique SID to the object. This SID consists of a domain SID (the same for all SIDs created in a domain) and a relative ID (RID) that is unique for each security principal SID created in a domain. Each DC in a domain