Network Control Protocol
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'.
Details
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). (Technically, those streams are provided by the underlying ARPANET; NCP just uses those, and adds additional semantics, such as opening and closing connections.) It runs over:
- 1822/VDH at the bottom, which transfers 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 transfers messages between the local host and a distant host
The top layer consists of two semi-separate protocols (the former makes use of the latter):
- Initial Connection Protocol (ICP)
- ARPANET Host-to-Host Protocol (AHHP)
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 messages of a connection; instead, the messages belonging to a connection were identified by the links (provided by the 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 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. (The ICP was a higher-level protocol, part of NCP, 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 a 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 seemingly 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.
The reliability was at the lowest, HIP, layer, because that functionality was done first - when BBN did the original ARPANET. 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.
The protocol family was also updated slightly over its later 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 use of it was limited to the original number of nodes.
- The handling of the RFNM flow control was improved.
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)
- RFC-127, Comments on RFC 123 - gives a detailed example of an ICP transaction
- 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
- ARPANET Technical Information
- The ARPANET Protocol Handbook also available here: January 1978 edition, April 1976 edition.
- IMPCLK - a concise and clear description of the NCP protocol layers in a block comment near the top; from WAITS source
