This is the default replication protocol and the only one that supports replication within a domain. It enables low-speed, synchronous replication of all AD DS partitions using remote procedure calls.
This is an asynchronous email-based protocol that can be used to replicate the schema and configuration partitions of the AD DS forest structure and the global catalog between domains. This protocol is useful for interdomain replication across unreliable links. To use this protocol, you must install an enterprise certification authority (CA) to sign the SMTP messages sent across the link. You also need to install SMTP on the domain controllers using this site link.