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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ARPANET | 4/7 | https://en.wikipedia.org/wiki/ARPANET | reference | science, encyclopedia | 2026-05-05T12:35:36.923745+00:00 | kb-cron |
=== IMP functionality === Because it was never a goal for the ARPANET to support IMPs from vendors other than BBN, the IMP-to-IMP protocol and message format were not standardized. However, the IMPs did nonetheless communicate amongst themselves to perform link-state routing, to do reliable forwarding of messages, and to provide remote monitoring and management functions to ARPANET's Network Control Center. Initially, each IMP had a 6-bit identifier and supported up to 4 hosts, which were identified with a 2-bit index. An ARPANET host address, therefore, consisted of both the port index on its IMP and the identifier of the IMP, which was written with either port/IMP notation or as a single byte; for example, the address of MIT-DMG (notable for hosting development of Zork) could be written as either 1/6 or 70. An upgrade in early 1976 extended the host and IMP numbering to 8-bit and 16-bit, respectively. In addition to primary routing and forwarding responsibilities, the IMP ran several background programs, titled TTY, DEBUG, PARAMETER-CHANGE, DISCARD, TRACE, and STATISTICS. These were given host numbers in order to be addressed directly and provided functions independently of any connected host. For example, "TTY" allowed an on-site operator to send ARPANET packets manually via the teletype connected directly to the IMP.
=== 1822 protocol === The starting point for host-to-host communication on the ARPANET in 1969 was the 1822 protocol, which defined the transmission of messages to an IMP. The message format was designed to work unambiguously with a broad range of computer architectures. An 1822 message essentially consisted of a message type, a numeric host address, and a data field. To send a data message to another host, the transmitting host formatted a data message containing the destination host's address and the data message being sent, and then transmitted the message through the 1822 hardware interface. The IMP then delivered the message to its destination address, either by delivering it to a locally connected host, or by delivering it to another IMP. When the message was ultimately delivered to the destination host, the receiving IMP would transmit a Ready for Next Message (RFNM) acknowledgment to the sending, host IMP.
=== Network Control Program === Unlike modern Internet datagrams, the ARPANET was designed to reliably transmit 1822 messages, and to inform the host computer when it loses a message; the contemporary IP is unreliable, whereas the TCP is reliable. Nonetheless, the 1822 protocol proved inadequate for handling multiple connections among different applications residing in a host computer. This problem was addressed with the Network Control Program (NCP), which provided a standard method to establish reliable, flow-controlled, bidirectional communications links among different processes in different host computers. The NCP interface allowed application software to connect across the ARPANET by implementing higher-level communication protocols, an early example of the protocol layering concept later incorporated in the OSI model. NCP was developed under the leadership of Steve Crocker, then a graduate student at UCLA. Crocker created and led the Network Working Group (NWG) which was made up of a collection of graduate students at universities and research laboratories, including Jon Postel at UCLA. They were sponsored by ARPA to carry out the development of the ARPANET and the software for the host computers that supported applications.
=== TCP/IP === Beginning in 1972, Louis Pouzin and Hubert Zimmermann at the IRIA in France pioneered a simplified end-to-end approach to networking that they called a catenet, which was discussed at the ICCC. Bob Kahn left BBN in 1972, briefly to be President of Telenet, and then moved to DARPA, first as program manager for the ARPANET, under Larry Roberts, then as director of the IPTO when Roberts left to be President of Telenet. Kahn worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across both. Steve Crocker, now at DARPA, and the leaders of British and French network projects founded the International Network Working Group (INWG) and, on Crocker's recommendation, Vint Cerf, now on the faculty at Stanford University, became its Chair. Bob Metcalfe developed the theory and practice behind Ethernet and the PARC Universal Packet. These groups considered how to interconnect packet switching networks with different specifications, that is, internetworking. Peter Kirstein put internetworking into practice at University College London in 1973. Research led by Kahn and Cerf resulted in the formulation of the Transmission Control Program in 1974. Its specification was written by Cerf with Yogen Dalal and Carl Sunshine at Stanford in December that year (RFC 675), and a group of researchers began publishing Internet Experiment Notes (IENs). The following year, testing began through concurrent implementations at Stanford, BBN and University College London. At first a monolithic design, the software was redesigned as a modular protocol stack in Version 4 in 1978, which reflected concepts from the French CYCLADES project directed by Pouzin and the ideas of Bob Metcalfe at Xerox Parc. This was installed in the ARPANET for production use in January 1983, replacing NCP. The development of the complete Internet protocol suite by 1989, as outlined in RFC 1122 and RFC 1123, and partnerships with the telecommunication and computer industry laid the foundation for the adoption of TCP/IP as a comprehensive protocol suite as the core component of the emerging Internet.
== Operation == ARPA was intended to fund advanced research. The ARPANET was a research project that was communications-oriented, rather than user-oriented in design. Nonetheless, in the summer of 1975, operational control of the ARPANET passed to the Defense Communications Agency. At about this time, the first ARPANET encryption devices were deployed to support classified traffic. The ARPANET Completion Report, written in 1978 and published in 1981 jointly by BBN and DARPA, concludes that:
... it is somewhat fitting to end on the note that the ARPANET program has had a strong and direct feedback into the support and strength of computer science, from which the network, itself, sprang.