Windows 2008 Active Directory Replication Protocols

                Replication is performed through replication transport protocols. Replications within a site perform by RPC-over-IP transport protocols. Replication between sites performs by two replication transport protocols IP – RPC over IP and SMTP over IP.  Two different sites can replicate only with SMTP over IP. RPC replication between sites can be scheduled and compressed, within a site is not compressed.

       SMTP, it enables low speed, asynchronous replication between the schema, configuration and global catalog partition, but not between domain directory partitions. SMTP traffic can be secured, monitor able and manageable across the environment

       RPC over IP, it enables low speed, point-to-point, and synchronous replication between all directory partitions. RPC replication is synchronous, where SMTP replication is asynchronous. Bandwidth is limited; it is a disadvantage to enforce an entire transaction to complete before another can begin. In SMTP transaction can be process simultaneously, so each transaction is receiving some attention all the time.

To adjust the default size of the replication packet that active directory replication data, by can modify the registry value in the following key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters.

RPC replication within a site:

Replicator intra site packet size (objects) 

Range: >=1

Replicator intra site packet size (bytes)
Range: >=10 KB

For RPC replication between sites:

Replicator inter site packet size (objects)
Range: >=1

Replicator inter site packet size (bytes)
Range: >=10 KB

For SMTP replication between sites:

Replicator async inter site packet size (objects)
Range: >=1

Replicator async inter site packet size (bytes)
Range: >=10 KB