Speaking of telnet, that protocol was actually much richer and had many more features than it would seem at a casual look.
I learned that when implementing a tiny part of the telnet protocol in a retro computer emulator's serial port emulation routines, mostly just to enable character mode and proper (non-)echoing. (I also implemented the RS232 control lines, so that telnet'ing into the port actually behaved like a physical terminal would with respect to carrier detect and all that, but I think there wasn't any overlap with the telnet protocol itself. Maybe flow control.)
Telnet also uses TCP's out of band data mechanism to send "urgent data" and a DATA MARK to interrupt and synchronize with runaway processes, and send important commands around flow control and before buffered data.
(Oh, Don Hopkins answering. I read you so often on TUHS!)
Didn’t know that, seems useful. Especially because another comment in this thread since mentioned that they often kill their ssh sessions because of runaway processes.
Overall, it seems to me that a lot was lost when giving up telnet.
I wrote a terminal emulator for the Apple ][ + Videx 80 column card in FORTH and reverse polish notation 6502 code, that supported SUPDUP including the line saving protocol that RMS hacked into ITS and EMACS (%TDSAV %TDRES), so Emacs could stash lines on the screen in a memory buffer (on the Apple's bank switched RAM card) and restore them later to paint the screen quickly when scrolling back and forth. (That was great at 300 baud!)
This is just the part of it that interprets SUPDUP codes -- layered on top of the rest of the terminal emulator that emulated H19 (VT52 + character/line insert/delete):
Marc Crispin (the author of the SUPDUP RFC #734, who was famous for his catchphrase "MM is not at fault!") also wrote another RFC #748 documenting the Telnet Randomly-Lose Option, which was useful for fuzzball testing.
IAC WILL RANDOMLY-LOSE
The sender of this command REQUESTS permission to, or confirms
that it will, randomly lose.
IAC WON'T RANDOMLY-LOSE
The sender of this command REFUSES to randomly lose.
IAC DO RANDOMLY-LOSE
The sender of this command REQUESTS that the receiver, or grants
the receiver permission to, randomly lose.
IAC DON'T RANDOMLY-LOSE
The command sender DEMANDS that the receiver not randomly lose.
I learned that when implementing a tiny part of the telnet protocol in a retro computer emulator's serial port emulation routines, mostly just to enable character mode and proper (non-)echoing. (I also implemented the RS232 control lines, so that telnet'ing into the port actually behaved like a physical terminal would with respect to carrier detect and all that, but I think there wasn't any overlap with the telnet protocol itself. Maybe flow control.)