There are two primary differences between replication between sites and within a site. Replication between sites is mail-based, while replication within a site occurs with direct Remote Procedure Calls (RPC) communication between directory servers. Whether replication is RPC or mail-based also determines who is responsible for initiating replication. Whether a particular naming context is RPC-based or mail-based, and when replication occurs is determined by the Reps-From, Reps-To, and Reps-To-Ext properties of the naming context.