This is a guest post by Steve Crocker of Shinkuro, Inc. and Bill Duvall of Consulair. Fifty years ago they were both present when the first packets flowed on the Arpanet.
On 29 October 2019, Professor Leonard (“Len”) Kleinrock is chairing a celebration at the University of California, Los Angeles (UCLA). The date is the fiftieth anniversary of the first full system test and remote host-to-host login over the Arpanet. Following a brief crash caused by a configuration problem, a user at UCLA was able to log in to the SRI SDS 940 time-sharing system. But let us paint the rest of the picture.
The Arpanet was a bold project to connect sites within the ARPA-funded computer science research community and to use packet-switching as the technology for doing so. Although there were parallel packet-switching research efforts around the globe, none were at the scale of the Arpanet project. Cooperation among researchers in different laboratories, applying multiple machines to a single problem and sharing of resources were all part of the vision. And over the fifty years since then, the vision has been fulfilled, albeit with some undesired outcomes mixed in with the enormous benefits. However, in this blog, we focus on just those early days.
In September 1969, Bolt, Beranek and Newman (BBN) in Cambridge, MA delivered the first Arpanet IMP (packet switch) to Len Kleinrock’s laboratory at UCLA. The Arpanet incorporated his theoretical work on packet switching and UCLA was chosen as the network measurement site for validation of his theories. The second IMP was installed a month later at Doug Engelbart’s laboratory at the Stanford Research Institute – now called SRI International – in Menlo Park, California. Engelbart had invented the mouse and his lab had developed a graphical interface for structured and hyperlinked text. Engelbart’s vision saw computer users sharing information over a wide-scale network, so the Arpanet was a natural candidate for his work. Today, we have seen that vision travel from SRI to Xerox to Apple to Microsoft, and it is now a part of everyone’s environment.
“IMP” stood for Interface Message Processor; we would now simply say “router.” Each IMP was connected to up to four host computers. At UCLA the first host was a Scientific Data Systems (SDS) Sigma 7. At SRI, the host was an SDS 940. Jon Postel, Vint Cerf and Steve Crocker were among the graduate students at UCLA involved in the design of the protocols between the hosts on the Arpanet, as were Bill Duvall, Jeff Rulifson, and others at SRI (see RFC 1 and RFC 2.)
SRI and UCLA quickly connected their hosts to the IMPs. Duvall at SRI modified the SDS 940 time-sharing system to allow host to host terminal connections over the net. Charley Kline wrote the complementary client program at UCLA. These efforts required building custom hardware for connecting the IMPs to the hosts, and programming for both the IMPs and the respective hosts. At the time, systems programming was done either in assembly language or special purpose hybrid languages blending simple higher-level language features with assembler. Notable examples were ESPOL for the Burroughs 5500 and PL/I for Multics. Much of Engelbart’s NLS system was written in such a language, but the time-sharing system was written in assembler for efficiency and size considerations.
Along with the delivery of the IMPs, a deadline of October 31 was set for connecting the first hosts. Testing was scheduled to begin on October 29 in order to allow a few days for necessary debugging and handling of unanticipated problems. In addition to the high-speed line that connected the SRI and UCLA IMPs, there was a parallel open, dedicated voice line. On the evening of October 29 Duvall at SRI donned his headset as did Charley Kline at UCLA, and both host-IMP pairs were started. Charley typed an L, the first letter of a LOGIN command. Duvall, tracking the activity at SRI, saw that the L was received, and that it launched a user login process within the 940. The 940 system was full duplex, so it echoed an “L” across the net to UCLA. At UCLA, the L appeared on the terminal. Success! Charley next typed O and received back O. Charley typed G, and there was silence. At SRI, Duvall quickly determined that an echo buffer had been sized too small, re-sized it, and restarted the system. Charley typed “LO” again, and received back the normal “LOGIN”. He typed a confirming RETURN, and the first host-to-host login on the Arpanet was completed.
Len Kleinrock noted that the first characters sent over the net were “LO.” Sensing the importance of the event, he expanded “LO" to “Lo and Behold”, and used that in the title of the movie called “Lo and Behold: Reveries of the Connected World.” See imdb.com/title/tt5275828.
IMPs continued to be installed on the Arpanet at the rate of roughly one per month over the next two years. Soon we had a spectacularly large network with more than twenty hosts, and the connections between the IMPs were permanent telephone lines operating at the lightning speed of 50,000 bits per second.
Today, all computers come with hardware and software to communicate with other computers. Not so back then. Each computer was the center of its own world, and expected to be connected only to subordinate “peripheral” devices – printers, tape drives, etc. Many even used different character sets. There was no standard method for connecting two computers together, not even ones from the same manufacturer. Part of what made the Arpanet project bold was the diversity of the hardware and software at the research centers. Almost all of the hosts at these sites were time-shared computers. Typically, several people shared the same computer, and the computer processed each user’s computation a little bit at a time. These computers were large and expensive. Personal computers were fifteen years in the future, and smart phones were science fiction. Even Dick Tracy’s fantasy two-way wrist radio envisioned only voice interaction, not instant access to databases and sharing of pictures and videos.
Each site had to create a hardware connection from the host(s) to the IMP. Further, each site had to add drivers or more to the operating system in its host(s) so that programs on the host could communicate with the IMP. The protocols for host to host communication were in their infancy and unproven.
During those first two years when IMPs were being installed monthly, we met with students and researchers at the other sites to develop the first suite of protocols. The bottom layer was forgettably named the Host-Host protocol. Telnet, for emulating terminal dial-up, and the File Transfer Protocol (FTP) were on the next layer above the Host-Host protocol. Email started as a special case of FTP and later evolved into its own protocol. Other networks sprang up and the Arpanet became the seedling for the Internet, with TCP providing a reliable, two-way host to host connection, and IP below it stitching together the multiple networks of the Internet. But the Telnet and FTP protocols continued for many years and are only recently being phased out in favor of more robust and more secure alternatives.
The hardware interfaces, the protocols and the software that implemented the protocols were the tangible engineering products of that early work. Equally important was the social fabric and culture that we created. We knew the system would evolve, so we envisioned an open and evolving architecture. Many more protocols would be created, and the process is now embodied in the Internet Engineering Task Force (IETF). There was also a strong spirit of cooperation and openness. The Request for Comments (RFCs) series of notes were open for anyone to write and everyone to read. Anyone was welcome to participate in the design of the protocol, and hence we now have important protocols that have originated from all corners of the world.
In October 1971, two years after the first IMP was installed, we held a meeting at MIT to test the software on all of the hosts. Researchers at each host attempted to login, via Telnet, to each of the other hosts. In the spirit of Samuel Johnson’s famous quote, the deadline and visibility within the research community stimulated frenetic activity all across the network to get everything working. Almost all of the hosts were able to login to all of the other hosts. The Arpanet was finally up and running. And the bakeoff at MIT that October set the tone for the future: test your software by connecting to others. No need for formal standards certification or special compliance organizations; the pressure to demonstrate your stuff actually works with others gets the job done.
 The SDS 940 had a maximum memory size of 65K 24-bit words. The time-sharing system along with all of its associated drivers and active data had to share this limited memory, so space was precious and all data structures and buffers were kept to the minimum possible size. The original host-to-host protocol called for terminal emulation and single character messages, and buffers were sized accordingly. What had not been anticipated was that in a full duplex system such as the 940, multiple characters might be echoed for a single received character. Such was the case when the G of LOG was echoed back as “GIN” due to the command completion feature of the SDS 940 operating system.
 “50,000” is not a misprint. The telephone lines in those days were analog, not digital. To achieve a data rate of 50,000 bits per second, AT&T used twelve voice grade lines bonded together and a Western Electric series 303A modem that spread the data across the twelve lines. Several years later, an ordinary “voice grade” line was implemented with digital technology and could transmit data at 56,000 bits per second, but in the early days of the Arpanet 50Kbs was considered very fast. These lines were also quite expensive.
 In the papers that described the Host-Host protocol, the term Network Control Program (NCP) designated the software addition to the operating system that implemented the Host-Host protocol. Over time, the term Host-Host protocol fell into disuse in favor of Network Control Protocol, and the initials “NCP” were repurposed.
 Samuel Johnson - ‘Depend upon it, sir, when a man knows he is to be hanged in a fortnight, it concentrates his mind wonderfully.’