Interface Message Processor

Interface Message Processor

The Interface Message Processor (IMP) was the packet-switching node used to connect computers to the original ARPANET in the late 1960s and 1970s. It was the first generation of what is known as a router today. [ [ IMP -- Interface Message Processor] , LivingInternet Accessed June 22 2007.] [ [ Looking back at the ARPANET effort, 34 years later] , Dave Walden, Accessed June 22 2007.] [ [ A Technical History of the ARPANET - A Technical Tour] , THINK Protocols team, Accessed June 22 2007.] An initial protocol was suggested in RFC 1. To connect to the ARPANET, host computers communicated with IMPs using a special high-speed bit-serial interface (defined in BBN Report 1822). The IMP itself was a ruggedized Honeywell DDP-516 minicomputer with special-purpose interfaces and software. In later years the IMPs were made from the non-ruggedized Honeywell 316.

The idea of the IMP being a separate computer was suggested by Wes Clark to Larry Roberts who led the ARPANET implementation for the Department of Defense's Advanced Research Projects Agency (ARPA). The IMP was implemented by Bolt Beranek and Newman (BBN).

The original IMP team at BBN consisted of:

* Team Leader: Frank Heart
* Software: Willy Crowther, Bob Kahn, Dave Walden, Bernie Cosell, Hawley Rising
* Hardware: Severo Ornstein, Ben Barker, Marty Thrope
* Unknown: Jim Geisman, Truett Thach, Bill Bertell (Honeywell) The first IMP delivered was to Leonard Kleinrock's group at UCLA on August 30, 1969. It was attached to a SDS Sigma-7. IMP number two was delivered to Douglas Engelbart's group at the Stanford Research Institute (SRI) on October 1, 1969. It was attached to an SDS-940. The first communication between these computers connected to IMPs was the first three letters of the word "login." The SRI machine crashed after the 'g' was transmitted. [cite book |last=Hambling|first=David|title=Weapons Grade|publisher=Carroll & Graf|year=2005|location=New York City|isbn=0-78671-769-6] A few minutes later the bug was fixed and they successfully logged in.

There was also a variant called the TIP which connected terminals instead of computers to the network, which was based on the 316. Initially, some Honeywell based IMPs were replaced with multiprocessing BBN Pluribus IMPs, but ultimately BBN developed a microprogrammed clone of the Honeywell processor.

IMPs were at the heart of the ARPANET until it was decommissioned 20 years later in 1989. The last IMP on the ARPANET was the one at the University of Maryland.


External links

* [ A Technical History of the ARPANET with photos of IMP]
* [ IMP history with photo of developers]
* [ Dave Walden's memories of the IMP and ARPANET]
*Internet STD 39, also known as BBN Report 1822, "Specification for the Interconnection of a Host and an IMP".

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