One spring day, a delivery truck from Honeywell turned down Moulton Street. Inside was the first 516 machine built to BBN's specifications. The refrigerator-sized computer was brought off the truck and onto a loading dock at the back of the systems division building and then was rolled into a large room, soon to be known as the IMP room, adjacent to the dock. The team had converted a storeroom into space for testing the IMPs by adding a raised computer floor, bright fluorescent lighting, and air-conditioning. The windowless room was where the youngest man on the team, twenty-two-year-old Ben Barker, would soon spend a lot of time.
Barker was an undergraduate student whose brilliance had caught Ornstein's attention in one of the classes he taught at Harvard. When BBN was awarded the ARPA contract, Heart had offered Barker a job, and Barker had taken a leave of absence to accept it. Barker, like Ornstein, was a hardware engineer and he showed signs of becoming an ace debuggerâsomeone who could rescue a project when the time came. He was placed in charge of setting up each IMP Honeywell delivered and debugging the hardware before it left BBN's shop.
This first machine was the prototype (IMP Number 0), a nonruggedized 516 containing Honeywell's initial implementation of BBN's interfaces. With the machine in the middle of the room, Barker ran power to the computer, plugged everything in, and turned it on.
Barker had built a tester and had written some debugging code. He was looking forward to working out whatever bugs the machine had. Undoubtedly there would be something that would need fixing, because there always was; bugs were part of the natural process of computer design. Heart and the whole team looked forward to finding out which parts of the IMP design worked and which needed more attention.
Barker tried loading the first IMP diagnostic program into the untested machine. He couldn't get it to work. So he loaded some other code, and that didn't work either. Barker tried a few other things and discovered that nothing worked. “The machine didn't come close to doing anything useful,” he said. So far, the first IMP was a fizzling dud.
Prior to the IMP project, people at BBN and Honeywell had interacted casually and relations were friendly. In the days leading up to the IMP project a sense of teamwork grew. Honeywell had devoted a special systems crew to work on the BBN contract from day one. At BBN's request, Honeywell had assigned one of its technicians exclusively to the task of shepherding Honeywell's part of the job to completion.
This was unusual. In general, minicomputer manufacturers like Honeywell didn't cater much to the special demands of their customers. “Most computer companies won't build specials at all,” said Heart. “Or if they do, it's under great duress.” Minicomputer salespeople went after a broad market, while mainframe computer makers were known to treat customers like royalty. Nobody in the minicomputer business did much hand-holding.
In the wake of IMP Number 0's gross failure, BBN's hardware chief Ornstein began to go over the design with the Honeywell team. He discovered that no one at Honeywell seemed to understand in much detail how the BBN-designed interfaces were supposed to work. He was surprised to learn that the technicians building the first interface didn't really understand the drawings. Honeywell hadn't attempted to develop any diagnostics to test the design; it had simply tried to produce a faithful implementation of the block diagrams that Ornstein had drawn and that BBN had included in its proposal to ARPA. The trouble was that in furnishing Honeywell with a set of fairly generic block diagrams, BBN assumed that Honeywell's familiarity and expertise with its own machines would enable the computer manufacturer to anticipate any peculiar problems with BBN's requested modifications to the model 516. Honeywell had its own logic modules, its own design system. But instead of working out the essential details in the blueprints, Honeywell had built BBN's machine without verifying that the BBNdesigned interfaces, as drawn, would work with the 516 base model.
Of course, neither BBN in drawing the block diagrams nor Honeywell in implementing the design actually had all of the necessary tools to create a perfectly working prototype IMP on the first pass. In building new computers, said Barker, the operative assumption is that you design something you think will work, get the prototype ready, start testing, then gradually fix the design errors until the machine passes the test. It would have been an engineering fluke if the machine ran perfectly straight away. But even as a first pass, the condition of this prototype machine was unacceptable.
If Ornstein was concerned about Honeywell's performance, Barker was downright nervous. As the chief debugger, he was the one responsible for getting the machine to actually work. At this stage in the IMP project, the interfaces on the 516, he said, “wouldn't have come close to working even if Honeywell had implemented them properly.”
Barker was staring at weeks of concentrated work ahead. He felt the weight of the schedule suddenly grow heavier. If the BBN hardware team intended to hand off to the BBN programming team a working version of the modified Honeywell 516 any time soon, so that Crowther, Walden, and Cosell would have time for final debugging of their operating code, then the hardware specialists would have to hustle. With Ornstein's help, Barker would have to “take the stuff Honeywell had built,” he said, “and figure out how to make it actually do what it was intended to do.”
The arrival of the prototype IMP in its initial state marked a real setback; correcting the course would take time, and soon there would be precious little of that.
Armed with an oscilloscope, a wire-wrap gun, and an unwrap tool, Barker worked alone on the machine sixteen hours a day. The circuitry of the computer relied on pin blocks, or wire-wrapped boards, that served as the central connection points to which wires, hundreds upon hundreds of wires, were routed. There were numerous blocks of thirty-four pins into which logic boards were plugged and which carried components to form the correct circuits. After figuring out where the wires should actually go, Barker had to unwrap each tightly wound misconnected wire from its pin. The pins in each block were about an inch long, and were closely spaced (1/20th of an inch apart) in a square matrix; each block looked like a miniature bed of nails with wires streaming Medusa-like into and out of it. Once he determined where the correct wires should be reconnected, Barker used the wire-wrap gun to wrap each wire carefully on its correct pin. It was a long, laborious, and delicate process.
To complicate matters, Barker had a slight palsy in his hands. Working with a wire-wrap gun called for a steady hand and good concentration. The close spacing of the pins, the weight of the wire-wrap gun, and the size of the nozzle on the gun that had to be slipped down over a single pin amid a small forest of pins all conspired against him. The risk was in getting the wire on the wrong pin or bending or breaking a pin. You could destroy a lot of fine work if you weren't careful. So the shake in Barker's hands caused quite a stir among the other IMP Guys when Barker took his wire-wrap gun to the pin blocks inside the IMP.
Most of the rewiring was done with the power off. When something had to be done with the power on, it was done with little clip-leads that slipped onto the pins. Here, though, there was a very real danger of shorting things out and blowing circuits.
Barker spent months debugging the machine. Ornstein was overseeing the design corrections in the prototype and making sure they got relayed back to Honeywell's engineers. The next machine Honeywell was scheduled to deliver would be the first ruggedized 516 with all the design bugs worked out: IMP Number One destined for UCLA. “The sweat was to get working designs in time to ship,” said Barker. The summer was upon them.
Heart's wariness about the hordes of curious graduate students drove BBN to conceptualize even greater measures of protection for the IMPs. In time, among the most creative things Heart's team did was invent ways of obtaining critical operating data from the network IMPsâreading the machines'vital signsâunobtrusively from a distance. Heart wanted to be able to sit at a terminal in Cambridge and see what an IMP in Los Angeles, Salt Lake City or any place else was doing. BBN's full implementation of remote diagnostic tools and debugging capabilities would later become a huge asset. When the network matured, remote control would enable BBN to monitor and maintain the whole system from one operations center, collecting data and diagnosing problems as things changed. Periodically, each IMP would send back to Cambridge a “snapshot of its status,” a set of data about its operating conditions. Changes in the network, minor and major, could be detected. Heart's group envisioned someday being able to look across the network to know whether any machine was malfunctioning or any line was failing, or perhaps if anyone was meddling with the machines.
Nonetheless, BBN still hadn't even gotten the prototype IMP to a state in which it could run operational code. And the programming teamâCrowther, Walden, and Cosellâwas now moving into difficult territory: the design of a flexible, or “dynamic” routing system, allowing alternative routing, so that packets would automatically flow around troubled nodes and links. A fixed-routing scheme would have been straightforward: You would send a packet with clear instructions to travel via x, y, and z points on the map. But if point y was knocked out, all traffic would be held up. And that would thwart one of the advantages of a network with multiple links and nodes.
The original request from ARPA had specified dynamic routing, without offering a clue as to how to make it work. Crowther had found a way to do it. He was building a system of dynamic-routing tables that would be updated every split second or so. These tables would tell the IMPs in which direction to forward each packet that hadn't yet reached its destination. The routing tables would reflect such network conditions as line failures and congestion, and they would route packets the shortest way possible. Making this design actually work seemed a mind-bending problemâuntil Crowther came up with a simple, perfect set of code. Crowther “always had his head right down in the bits,” as Ornstein described Crowther's uncanny, intuitive talent.
Crowther's dynamic-routing algorithm was a piece of programming poetry. “It was incredibly minimalistic and worked astoundingly well,” Walden observed. Crowther was regarded by his colleagues as being within the top fraction of 1 percent of programmers in the world. On occasion the graceful minimalism of Crowther's code wasn't enough to handle the complexity of real-world systems. Other programmers would have to fine-tune what Crowther had created. But his core ideas were more often than not brilliant. “Most of the rest of us made our livings handling the details resulting from Will's use of his brain,” Walden observed.
Flow control was another programming challenge. But when Kahn looked at Crowther's code and saw how he had implemented control of the flow of packets from one side of the network to the other, he was worried. Messages between hosts were to be transmitted by the subnet over logical “links.” The subnet would accept one message at a time on a given link. Messages waiting to enter the subnet were stored in a memory buffer (a waiting area inside the machine), in a queue. The next message wasn't sent until the sending IMP received an acknowledgment (similar to a return receipt) saying that the previous message had arrived error-free at the destination host. When one link was cleared and a new message could be sent, the subnet would notify the sending IMP by means of a special control signal that the BBN engineers called Ready for Next Message, or RFNM (pronounced RUFF-num). Messages in the sending host's buffers, waiting for links to clear, were like patrons in a restaurant waiting for tables; RFNMs were the equivalent of the maître d's announcing “Your table is ready.”
This meant it was impossible to send a continuous stream of messages over any single link through the system from one host to another. RFNM was a congestion-control strategy designed to protect the IMPs from overload, but at the expense of reducing service to the host. Kahn had studied the problem of congestion control even before the ARPA network project began. His reaction to Crowther's solution was that the links and RFNMs would still allow fatal congestion of the network's arteries. The IMP's buffers would fill up, he said. You'd have incomplete messages sitting in the receiving IMPs waiting for their final packets to arrive so the entire message could be reassembled, and there would be no room for the packets to arrive.
Kahn's reassembly lockup scenario has an analog in the shipping business. Say that a Toyota dealership in Sacramento orders sets of replacement engine blocks and pistons from a warehouse in Yokohama. Both items together are essential for the jobs the dealer has at hand. In the Yokohama harbor, freighters are loading large containers, all the same size, filled with products of many kinds. The engine blocks and pistons wind up in separate ships. When the container of engine blocks arrives in San Francisco, it is unloaded to a warehouse of containers whose contents are also partial shipments awaiting the arrival of other parts before being sent onward: components for television sets, pin blocks for pianos, and so on. When the freighter with the pistons arrives, it finds the warehouse full. Every later ship has the same problem: Nobody can unload; nothing can leave the warehouse. Deadlock. Solution? The Yokohama shipper agrees to call ahead next time to reserve space for all containers that go together. If space is unavailable, he waits until it becomes available before shipping.
Kahn also predicted another type of deadlock that might lead to loss of packets. He said that it would occur in heavy traffic within the subnet when the buffers of one IMP were filled with packets routed to another, and vice versa. A kind of standoff would result, in which neither one would be able to accept the other's packets. The way the routing software had been written the IMPs would discard the packets.