site level. Physically, the Active Directory information is held on one or more peer domain controllers, replacing the NT PDC/BDC model. Each DC has a copy of the Active Directory. Servers joined to Active Directory that is not domain controllers are called Member Servers. A subset of objects in the domain partition replicate to domain controllers that are configured as global catalogs. Global catalog (GC) servers provide a global listing of all objects in the Forest. Global Catalog servers replicate to themselves all objects from all domains and, hence, provide a global listing of objects in the forest. However, to minimize replication traffic and keep the GC's database small, only selected attributes of each object are replicated. This is called the partial attribute set (PAS). The PAS can be modified by modifying the schema and marking attributes for replication to the GC. Earlier versions of Windows used NetBIOS to communicate. Active Directory is fully integrated with DNS and requires TCP/IP—DNS. To be fully functional, the DNS server must support SRV resource records, also known as service records. Replication Active Directory synchronizes changes using multi-master replication. Replication by default is 'pull' rather than 'push', meaning that replicas pull changes from the server where the change was effected. The Knowledge Consistency Checker (KCC) creates a replication topology of site links using the defined sites to manage traffic. Intra-site replication is frequent and automatic as a result of change notification, which triggers peers to begin a pull replication cycle. Inter-site replication intervals are typically less frequent and do not use change notification by default, although this is configurable and can be made identical to intra-site replication. Each link can have a 'cost' (e.g., DS3, T1, ISDN, etc.) and the KCC alters the site link topology accordingly. Replication may occur transitively through several site links on same-protocol site link bridges, if the cost is low, although KCC automatically costs a direct site-to-site link lower than transitive connections. Site-to-site replication can be configured to occur between a bridgehead server in each site, which then replicates the changes to other DCs within the site. Replication for Active Directory zones is automatically configured when DNS is activated in the domain-based by the site. Replication of Active Directory uses Remote Procedure Calls (RPC) over IP (RPC/IP). Between Sites, SMTP can be used for replication, but only for changes in the Schema, Configuration, or Partial Attribute Set (Global Catalog)