A client DC sends an IDL_DRSGetNCChanges request (msgIn, of a type in the union DRS_MSG_GETCHGREQ) to a server to replicate directory objects in a given NC from the server NC replica to the client NC replica.
The response (msgOut, of a type in the union DRS_MSG_GETCHGREPLY) contains a set of updates that the client is to apply to its NC replica. Commonly, this set of updates is too large to send in a single response; in this case, multiple IDL_DRSGetNCChanges requests and responses must be sent before the server sends a response that indicates no additional updates are available.
This sequence of requests and responses is called a replication cycle, or "cycle". A client DC can request an action on a FSMO role (for example, a change in the FSMO role owner) by using a special replication cycle called an extended operation.