by Darius Kazemi, May 1 2019
The technical content
This document is similar to RFCs 119 and 120 in that it documents an interface to the network that is “higher level” (more abstract and ideally easier to use) than the Network Control Program (NCP). In particular this document defines commands that a user of the UCSB On-Line System can enter in order to connect to remote hosts on the ARPANET.
It makes sense that RFCs 119, 120, and 121 were published together as they are all documentation for what seems like the exact same system, but implemented in three different programming environments.
This brief guide to the Culler-Fried On-Line System (eventually just called the “On-Line System”) is worth a read. It's important to note that “on-line” did not mean “on the internet” back then, because again, there was no internet! “On-line” simply meant that you were directly interfacing with a computer through an interactive terminal of some kind. A personal computer today, with no internet attached to it, would still be considered “on-line” by 1969 standards.