I've heard that the first time a message was sent through it was "lo" since the SDS 940 crashed before the full message of "login" was able to be sent through.

However, I have been unable to find how fast a message was transmitted through it. Does anyone know how long that message must have been taken to transmit, or an estimate at least?

  • 3
    Estimate: The speed was probably bounded by the speed of the leased line, and early modem rates were 50, 75 and 110. 300 baud was after 1971 to my knowledge. Also note that "login" was probably typed, so the actual speed of the characters was human typing speed. As for latency, you probably have two add 2x imp latency + 2x host latency + line latency. IMP specs should allow a guess.
    – dirkt
    Commented Jan 19 at 15:14
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    From an article: AT&T supplied the 50kbps connection between the two sites, with engineers set up an analogue phone call to confirm the receipt of messages. Ref: Who supplied the technology : ARPANET anniversary: The internet’s first transmission was sent 50 years ago today
    – Brian
    Commented Jan 19 at 15:18
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    The fact that only 'lo' of 'login' was sent is a reflection on typing speed rather than line speed. Presumably the protocol was character-interactive.
    – dave
    Commented Jan 19 at 15:44
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    And probably sent from a teletype, which requires a rest between keystrokes to regain the strength in your fingers :-)
    – dave
    Commented Jan 19 at 16:00
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    @jimc - not according to Kleinrock: 'At the UCLA end, Charley typed in the “l” and asked SRI “did you get the l?” “Got the l” came the voice reply. He typed in the “o,” “Did you get the o?”'. SRI was running an SDS 940 timesharing system, which if I recall correctly had command recognition: type el, oh, and it outputs gee, eye, en.
    – dave
    Commented Jan 21 at 19:22

3 Answers 3


From the IEEE Annals of the History of Computing 44(2) 8-19 (2022) titled Seeking High IMP Reliability in Maintenance of the 1970s ARPAnet on finds

The design called for a small computer called an Interface Message Processor (IMP) to be collocated with each of the computers (Hosts) that ARPA desired to have access to the network. Each IMP was to be connected to 2 or more other IMPs by 50 kbps telephone circuits to form a loose mesh network.

So at least the (leased dedicated) telephone circuits were 50 kbps.


Does anyone know how long that message must have been taken to transmit, or an estimate at least?

It's two characters, so even considering an inefficient protocol and slow lines (*1) will be considerably less than a second - except it maybe rather be limited by typing speed, wouldn't it?

*1 - Slow by today's speed, as it can be assumed that those were leased lines running at least at 19200 bps.


When using a packet-switched protocol such as TCP/IP with John Nagle's flow-control algorithm, a system that has sent out a packet of data will hold off on sending further data for the channel until it has received an acknowledgment. Once the acknowledgment arrives, the system will send a packet containing whatever data became available for transmission while the system was waiting for the acknowledgment, subject to packet-size limitations. This will yield "smooth" behavior in situations where turnaround latency is short, but avoid transmitting an excessive number of short packets when latency is longer.

Even on the faster Internet connections that were available in the 1980s and 1990s, it was common to see a noticeable hiccup between the first handful of characters that was sent on a link, and subsequent characters. If something went wrong between the first packet that was transmitted, and the successful receipt of an acknowledgment for it, it would hardly be unexpected that only "lo" would get transmitted no matter how fast or slow the link was.

  • 5
    I think you're on the right track - those characters typed in to telnet (or whatever the early equivalent was) were most likely sent in separate packets, due to the timing of the keystrokes, using the earliest TCP/IP protocol. But mentioning Nagle's algorithm and its behavior is a red herring as that wasn't invented at the time.
    – davidbak
    Commented Jan 19 at 19:17
  • @davidbak: Fair point about Nagel's algorithm not having been formalized yet, but I'm pretty sure there would have been some mechanism for aggregating bytes for transmission rather than sending a new packet for every byte.
    – supercat
    Commented Jan 19 at 19:41
  • Well in those early days I have no idea how sophisticated it was but there was probably a timing component - wait nnn ms and then send what you've got. Unless the early protocol they were using was line oriented...
    – davidbak
    Commented Jan 19 at 20:37
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    The host-to-host protocol described in RFC 11, which predates even NCP, appears likely to have been what was in use at the time. See 2.2.2 (b) for login.
    – dave
    Commented Jan 19 at 23:57
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    If you care to dig deeper, our user @LarsBrinkhoff has a github repo with the original BBN tech reports.
    – dave
    Commented Jan 20 at 0:04

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