Network Control Protocol

From Computer History Wiki
Revision as of 17:58, 31 December 2024 by Jnc (talk | contribs) (→‎Details: make into link)
Jump to: navigation, search

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), was 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'.

Details

NCP provided uni-directional reliable byte streams, called 'connections', used by applications to talk to each other (usually a pair of connections, one in each direction). (Technically, those streams are provided by the underlying ARPANET; NCP just used those, and added additional semantics, such as opening and closing connections.) It ran over:

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

The top layer consisted of two semi-separate protocols (the former made use of the latter):

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

However, socket numbers did not appear in the messages of a connection; instead, the messages belonging to a connection were identified by the link (provided by HIP). The link number appeared in every message, as links were used to carry them all. When a connection was set up, between one host/socket and 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 messages 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. (The ICP, a higher-level protocol, part of NCP, was performed to open a connection, though.)

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

History

Understanding the details of the operation of NCP may be difficult for those who know TCP, because NCP is quite different, and seemingly unnecessarily complicated. Like the vertebrate eye (which seems extraordinarily badly arranged, with necessarily transparent components placed in front of the optical receptors), this complexity and seeming bad design, is an accident of its evolution (after all, the designers of NCP were groping in the then-dark when it was all built). So, the first step in understanding it is to forget everything you know about today's networks - and accept that there will be major differences and infelicities.

The reliability was at the lowest, HIP, layer, because that functionality was done first - when BBN did the original ARPANET design. The ICP is built on top of the AHHP because it only became clear that the functionality of a generic ICP was needed fairly late in the process - after the AHHP had already been designed and implemented.

The protocol family was also updated slightly over its later lifetime:

  • In late 1975 the 1822 'leader' (the header of a 'message') was expanded from 32 bits to 96. Among other things, this allowed addressing more network nodes. The old format was still supported, but use of it limited the message 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" (archived), October 1977, NIC #8246, Network Information Center. Also reproduced in RFC 6529.

External links

Retrieved from "https://gunkies.org/w/index.php?title=Network_Control_Protocol&oldid=35761"