Active Directory Replication Transports

Windows 2000 used three transports for replication of Active Directory ( AD ).

• High speed synchronous RPC over IP.
  Active Directory always uses high
  speed RPC over IP for intrasite replication. Required. Not configurable. The
  replicated data is not compressed.
  
• Point-to-Point low speed synchronous RPC over IP.
  Active Directory
  always uses low speed RPC over IP to replicate between domain controllers in the
  same domain but in separate sites. Required. Not configurable.
  
• Low speed, asynchronous SMTP.
  SMTP is limited to replication of schema
  and global catalog. Low speed RPC over IP can also be used for this replication
  type.

Replicated data transmitted between sites is compressed to
reduce the transport load on low capacity links. In synchronous replication, the
destination domain controller sends the requested replication data and waits for
the source domain controller to reply that replication is successful before
it honors requests for updates from other source domain controllers. In
asynchronous replication, the destination domain controller sends the requested
replication data, and does not wait for a successful replication message from
the source domain controller before initiating replication with another site.