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The term Network Control Program (sometimes abbreviated as NCP) has two different meanings within the context of packet-switching: ARPANET context and IBM context.
ARPANET context
The ARPANET Network Control Program (NCP) provided the shared elements of the protocol stack running on an ARPANET host computer. NCP provided connections and flow control between processes running on different ARPANET host computers. Application services, like e-mail or file transfer, used an interface to the NCP to handle connections to other host computers.
On the ARPANET, the physical layer, the data link layer, and the network layer protocols were implemented on separate Interface Message Processors, called IMPs. The bottom of a host's protocol stack connected to the IMP's reliable packet delivery system with numeric host addresses. The IMP's capabilities were specified by the Host/IMP Protocol in BBN Report 1822.
The NCP represented the shared layers of the protocol stack resident on the host computer. Since lower protocol layers were provided by the IMP, NCP essentially provided a transport layer consisting of the ARPANET Host-to-Host Protocol (AHHP) and the Initial Connection Protocol (ICP). The AHHP defined procedures to transmit a unidirectional, flow controlled data stream between two hosts. The ICP defined the procedure for establishing a bidirectional pair of such streams between a pair of host processes. Application protocols (e.g. FTP, SMTP, etc.) accessed network services through an interface to the top layer of the NCP, a forerunner to the Berkeley sockets interface.
On January 1, 1983, NCP was rendered obsolete when the ARPANET changed its core networking protocols from NCP to the more flexible and powerful TCP/IP protocol suite, marking the start of the Internet as we know it today.
IBM context
The IBM Network Control Program ran on an IBM 3705 or 3745. It caused the machine to become a Systems Network Architecture Physical Unit Type 4 (PU4). One or more SDLC links communicated with other physical units (usually PU2 or PU4). A single System 360 channel address (the 3705 native subchannel) carried traffic to and from VTAM running on the mainframe which acted as an SNA PU5.
Further reading
NCP | NCP | NCP | Network Control Program
"Network Control Program"