by Darius Kazemi, Jan 15 2019

In 2019 I'm reading one RFC a day in chronological order starting from the very first one. More on this project here. There is a table of contents for all my RFC posts.

Proposing TELNET

RFC-15 is authored by Steve Carr, a representative of University of Utah's computer science program (UTAH in ARPANET parlance). He was present at the first Network Working Group Meetings I mentioned in my RFC-1 post. I couldn't quickly find a lot of info on him but according to this article he was a grad student of early computer graphics expert David C. Evans when he authored this. (Incidentally, Carr would have been there at the same time as one of my personal computing heroes, Alan Kay.)

Titled “Network Subsystem for Time Sharing Hosts”, this RFC is dated Sep 25th, 1969, about one month before the first message would be sent over the ARPANET.

Technical content

This RFC is really important! It proposes a program called TELNET (now called Telnet) that solves the problem of how two computers running different hardware and different operating systems can somehow talk to one another over the network.

Carr begins by pointing to RFC-11 and summarizing Déloche's list of required “network primitives” that allow a computer to talk to ARPANET:

He says that these will operate at the level of system calls, “most likely a part of the resident monitor” for maximum speed/responsiveness. Both the SDS 940 and the PDP-10 offered a way for users to write their own system calls, and in the context of a time-shared computer, all users would need access to these calls.

A “resident monitor” is a tiny program that is always present in memory that is run over and over — it is resident in memory and it is generally used to monitor when critical things need to happen. Usually the available memory for this program is very small so it often is just a single instruction that says “go to this other place in memory and run the program there”. These aren't used in modern computers but similar concepts are used in the simple embedded systems you might find in a kitchen appliance.

The RFC describes how computers running different hardware and different operating systems could talk to each other in the context of a time-sharing system. The process goes:

Carr also states that Telnet needs to take up a really small amount of memory so that it's fast. Which is why it doesn't do much (there's explicitly no translating between different character sets).


Telnet has been in continuous use since 1969, and although it began to fall out of favor in the 1990s, it's had an incredible run as far as “ways to access the internet” go. Telnet was quickly adopted although there wasn't a formal attempt to standardize it beyond this proposal until RFC-97 in 1971.

In fact, telnet was standard on all modern consumer operating systems until very very recently. Apple only stopped shipping telnet on its Macs in 2017. On a personal note, Telnet was how I first accessed the internet, via bulletin board systems when I was ten years old in 1993.

In this RFC about logging in to a remote computer the word “password” is never used. In fact the only prerequisites for accessing a remote computer are: the remote computer is accepting remote logins, and the local computer has the local user cleared for connecting to remote computers. This is the modern equivalent of “if you don't want strangers logging in to your computer, deny all login attempts.” Of course, at this point all the potential users of ARPANET knew each other! So, not a big deal yet.

Carr mentions an “SPOP” on the SDS 940 system. I believe he is referring to “SYSPOP”, which functions much like a UUO does on a PDP-10. This would make sense as Carr would have been much more familiar with the PDP-10 since that's what they used at UTAH. (A PDP-10 at UTAH would eventually become the fourth node on the ARPANET.)

In a sense, Telnet is a simpler, text-only alternative proposal to the media-rich DEL system outlined in RFC-5 that was never implemented.

Further reading

A history of Telnet.

A set of notes on SDS-940 lectures given by Butler W. Lampson in June 1966. Lampson was one of the developers of the SDS 940, and would later go on to cofound Xerox PARC and help invent a million other things.

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About me

I'm Darius Kazemi. I'm a Mozilla Fellow and I do a lot of work on the decentralized web with both ActivityPub and the Dat Project.