View source for Network Control Protocol

The '''Network Control Program''', canonically abbreviated to '''NCP''' (which people often, but incorrectly, think stands for 'Network Control Protocol'), is the [[transport protocol]] run between [[host]]s connected to the [[ARPANET]], prior to the introduction of [[TCP/IP]].

Since then, many other networking software systems were also called NCP; the modern equivalent would be [[protocol stack|network stack]].

==Details==

NCP provides uni-directional [[reliable byte stream]]s, called 'connections', used by [[application]]s to talk to each other (usually a pair of connections, one in each direction). It runs over the [[Host-to-IMP Protocol]], and 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 [[Host-to-IMP Protocol#Host addresses|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 [[packet]]s; instead, [[Host-to-IMP Protocol#Links|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 circuit]]s in their properties, in that packets sent on one were received reliably (although there was an error message 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.

==Implementations==

{| class="wikitable"
! Operating System
! Status
! Site
! Machine
|-
| [[ITS]]
| '''Preserved'''
| [[MIT]]
| [[DEC]] [[PDP-10]]
|-
| [[WAITS]]
| '''Preserved'''
| [[SAIL]]
| DEC PDP-10
|-
| [[TENEX]]
| '''Preserved'''
| [[BBN]] and ''many'' more
| DEC PDP-10
|-
| [[Unix V6]]
| '''Preserved'''
| University of Illionis
| DEC [[PDP-11]]
|-
| 
| '''Preserved'''
| PARC?
| [[Xerox]] [[Alto]]
|-
| SEX
| Lost
| UCLA
| [[Scientific Data Systems|SDS]] Sigma 7
|- 
| [[OS/360]]
| ?
| UCLA
| [[IBM]] [[System/360]]
|-
| CP/CMS
| ?
| Lincoln Labs
| IBM System/360
|-
| TSS/360
| ?
| NASA Ames
| IBM System/360
|-
| [[Multics]]
| Maybe
| MIT
| GE 600, [[Honeywell 6000 series]]
|-
| [[TOPS-10]]
| Maybe
| ''Many''
| DEC PDP-10
|-
| [[TOPS-20]]
| Maybe
| ''Many''
| DEC PDP-10
|-
| ?
| 
| London
| DEC [[PDP-9]]
|-
| [[ELF]]
| 
| 
| DEC [[PDP-11]]
|-
| [[RSX-11M]]
| 
| 
| DEC PDP-11
|-
| [[4.1BSD]]
| 
| BBN 
| DEC [[VAX]]
|-
| [[VMS]]
| 
| 
| DEC VAX
|-
| 
| ?
| Lincoln Labs
| [[TX-2]]
|-
| SCOPE
| ?
| 
| [[CDC 6600]], 7600
|-
|
| ?
| Hawaii
| BCC 500
|-
| POSIX
| [https://github.com/larsbrinkhoff/linux-ncp In progress]
| Emulation
|
|}

{{semi-stub}}

==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 [http://www.rfc-editor.org/rfc/rfc165.pdf here])
* Alex McKenzie; Jon Postel "[http://www.cbi.umn.edu/hostedpublications/pdf/McKenzieNCP1972.pdf Host-to-Host Protocol for the ARPANET]", October 1977, NIC #8246, Network Information Center

==External links==

* [http://www.chiappa.net/~jnc/tech/arpanet.html ARPANET Technical Information]
** [http://www.chiappa.net/~jnc/tech/arpapkt.html Packet Formats]
** [http://www.chiappa.net/~jnc/tech/arpaprot.html ARPANET Protocol Handbook]
* [https://www.saildart.org/IMPCLK%5BS,SYS%5D IMPCLK] - a concise and clear description of the NCP protocol layers in a block comment near the top; from WAITS source

[[Category:Networking]]