Mastering Active Directory - Second Edition

Inter-site transport protocols

There are two transport protocols that can be used for replication via site links. The default protocol used in site link is IP, and it performs synchronous replication between available domain controllers. The SMTP method can be used when the link between sites is not reliable. It will use mail messages to replicate AD partition data, including the schema changes between domain controllers. If it uses the SMTP method, there should be a certification authority to encrypt the traffic.