Network Control Protocol

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The Network Control Protocol, a later backronym of the old acronym NCP (which changed its meaning over time; it originally stood for Network Control Program, a somewhat different meaning), is the protocol suite originally created for use in the then-new ARPANET, prior to the later creation and introduction of TCP/IP.

NCP was disabled in the ARPANET on January 1, 1983 (by blocking use of link 0), to force conversion to TCP/IP. (Technically, a few mission-critical sites which did not have their TCP/IP software working yet could get temporary 'stays of execution', but basically NCP was turned off on January 1.)

Since then, other networking software systems were also called 'Network Control Protocol'.


NCP provides uni-directional reliable byte streams, called 'connections', used by applications to talk to each other (usually a pair of connections, one in each direction). It runs over:

  • 1822/VDH at the bottom, to transfer bits/'messages' (ARPANET jargon for packets) between the host and its local IMP
  • on top of that, the Host-to-IMP Protocol, to transfer messages between the local host and a distant host

The top layer consists of two semi-separate protocols:

  • Initial Connection Protocol (ICP)
  • ARPANET Host-to-Host Protocol (AHHP)

The connection was identified by a 'socket' at each end, along with the addresses of the two hosts; socket numbers were 32 bits long. Only one connection was permitted to each socket at any host.

However, sockets did not appear in messages; instead, links were used. When a connection was set up, between one host/socket to another, it used a particular link, specified by the receiver, and no other connection could use that link until the connection was closed.

Links were like virtual circuits in their properties, in that packets sent on one were received reliably (although there was an error message to the host from its IMP when that didn't happen) and in order at the other end of the link, but they had no open/close - a host just started using a link.

One link, 0, was special - it was the 'control link'. All messages involved in opening and closing a connection were sent over the control link - only data messages belonging to a connection went over a connection's link.


The protocol family was updated slightly over its lifetime.

  • In late 1975 the 1822 leader was expanded from 32 bits to 96. Among other things, this allowed addressing more network nodes. The old format was still supported, but was limited to the original number of nodes.

See also

Further reading

  • Jon Postel, "Official Initial Connection Protocol", June 1971, NIC #7101, UCLA-NMC (this does not seem to be online, but an early version, which is almost identical to the final version, can be found here)
  • Alex McKenzie; Jon Postel "Host-to-Host Protocol for the ARPANET", October 1977, NIC #8246, Network Information Center

External links