Origins Of Computer Network Routing

As we mentioned, a viable commercial computer timesharing requires a reliable and ubiquitous means to connect many remote users to that computer. Which implies a network. Which implies routing. But what do you do when you’re developing one of the world’s first computer networks? You invent the predecessor of MPLS. Norm Hardy continues his story.

Development of Tymnet II
By Norm Hardy

As Tymshare acquired more and different host machines, it became even clearer that upgrades were needed to the design of Tymnet.  The PDP-10 arrived in 1970 and was adapted to Tymnet by Bill Weiher so as to avoid the expensive “per-user” communications equipment from DEC.  Any programmer will tell you that one program knowing addresses in another is a very bad idea, but was the case in Tymnet 1. It worked pretty well but did limit architectural progress.

Laroy broached the idea that the supervisor would merely construct a needle which would pass thru the net guiding the creation of a new circuit. The needle would carry a 4 bit nibble, for each node, to steer the needle as it passed that node during the circuit construction. The supervisor would still need to know the network topology and how each node numbered each link. It would also know the node and link loads. It would no longer need to model the routing tables in each node. Routing table entries would be allocated locally. Twenty-five years later this pattern became the main notion behind MPLS routing (Multiprotocol Label Switching). The cost of core memory for nodes had come down and the new node code was larger but more modular and efficient. At this time nodes typically had 8K or 12K of 16 bit words. A few had 16K.

Once a particularly nasty bug struck the New Orleans node. Usually node bugs would have the civility to strike first, or eventually, near Cupertino, but this one took drastic measures. We loaded an extra core bank with the code for New Orleans and mailed it there in a shoe box. Upon the next crash they swapped core and mailed the core module with the crash state back to Cupertino. We found the bug.

When Tymshare opened the Paris office with its own 940 computer, there was consternation over the idea that the European network would be managed from the states. This consternation was largely overcome by rewriting a few manuals to describe the operation differently. The Paris 940 ran a supervisor so that when the trans-Atlantic link was out they would still be in service. Once when all of the American supervisors had gone down, (a very rare occurrence) the Paris node tried to take over the American network via a 2400 bit/sec link. It failed for by that time 2400 bit/sec did not suffice to control even a night load in the states.

Tymnet Service

Our timesharing computers attracted business now by those who found it convenient that their program and its data were accessible from a large number of geographical sites. Some customers, however, found our timesharing computers inadequate to their tasks, even with special pricing. There was increasing call, inside Tymshare and out, to make Tymnet serve hosts other than our own. We gradually did this. This required a considerable programming staff to work with owners of diverse hosts to connect those hosts to Tymnet.  Many techniques were used including DMA (Direct Memory Access).

This new business was easy to start because the simplest host interface mechanism was the early cable per connection configuration that required no changes to the host; just power, floor space and termination of a leased line to our site. The prospective customer had very little up-front cost to impede him. After the first day his computer would be accessible by a variety of terminal types from a variety of US cities and even several foreign countries. Here is a memento from the NLM, and some perspective from our first customer. Also see a contemporary directory of institutions with computers connected to Tymnet.

All through this development new varieties of terminals were coming to market. They came with higher baud rates and peculiar timing requirements. We would teach Tymnet about these new terminals and the node near the terminal would take care of the peculiarities. Other users of these terminals would have to adapt their host software to accommodate these strange timing requirements. It gradually dawned on us that the adaptation of various hosts to various terminals was a strategic marketing advantage.

With the advent of Tymnet service the Division became a company in its own right in 1979. After McDonnell Douglas bought Tymshare, and then sold it to EDS, Tymnet was sold to British Telecom (who was simultaneously merging with MCI).  BT renamed Tymnet as “Global Network Services.”   At its peak there were 6600 nodes in Tymnet, at nearly as many locations.

Don Johnson, who has been connected with Tymnet for many years, said on Monday, March 24, 2003, “We shut down Tymnet at 11:48 AM PST this morning.”

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