The document discusses two different Network Control Programs (NCP): 1) The ARPANET NCP provided the transport layer for the ARPANET, handling connections and flow control between processes on different host computers. Application services like email and file transfer were built on top of the ARPANET NCP. 2) The IBM NCP ran on IBM equipment and allowed those machines to function as nodes in IBM's Systems Network Architecture (SNA). The IBM NCP provided link control and network layer functions to connect locations in a wide area network.

1. Introduction of NCP 49871009 李 盈

2. ARPANET NCP The ARPANET Network Control Program (NCP) provided the middle layers of the protocol stack running on an ARPANET host computer. (Sometimes the abbreviation NCP is mistakenly expanded to Network Control Protocol, but this term is not found in the contemporary documentation.)

3. NCP provided connections and flow control between processes running on different ARPANET host computers. Application services, like e-mail or file transfer, were built on top of NCP, using it to handle connections to other host computers.

4. On the ARPANET, the physical layer, the data link layer, and the network layer protocols used internally to the network were implemented on separate Interface Message Processors, called IMPs. The host usually connected to the IMP using another kind of interface, with different physical, data link and network layer specifications. The IMP's capabilities were specified by the Host/IMP Protocol in BBN Report 1822.

5. History Since lower protocol layers were provided by the IMP-host interface, NCP essentially provided a transport layer consisting of the ARPANET Host-to-Host Protocol (AHHP) and the Initial Connection Protocol (ICP).

6. 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.

7. On January 1 1983, known as flag day, 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.

8. IBM NCP The IBM Network Control Program (NCP) ran on an IBM 3704, 3705, IBM 2725 or 3745. It caused the machine to become a Systems Network Architecture (SNA) Physical Unit Type 4 (PU4).

9. A PU4 usually had SDLC links to remote cluster controllers (PU1/PU2) and optional SDLC links to other PU4s. Polling and addressing of the cluster controllers was performed by the NCP without mainframe intervention.

10. A local NCP connected to a System/370 channel via single address. A remote NCP had no direct connection to a mainframe. Instead a connection was made through a local NCP via SNA packet switching. NCP provided the link control and Network layer functions of a Wide area network.