NCP makes interaction between clients and file servers possible by defining connection control and service request reply/encoding.
NCP provides its own session control and packet-level error checking instead of relying on other protocols for these functions. Consequently, the modularity of the protocol stack is reduced, but runs more efficiently.
Each NCP request packet submitted on a given connection must be assigned a sequence number by the client. The first request is assigned the number 1; that number is incremented by 1 for each subsequent request. When a file server finishes processing a request, it places the sequence number for that request in the response packet. The client can then make sure it is receiving the correct responses for the requests submitted.