What is the actual TELNET specification? I found RFC 854 , which is very old.
It looks like RFC 5198 is probably relevant. If you look at the top of the RFC that you are referencing, there is a link to RFC 5198 with a note that 5198 updates 854.
854 is the latest RFC for the protocol itself. NIC 15373 and IETF STD 8 are normative.
Several RFCs have proposed extensions, such as RFC 1123, telnet over TCP. Recently, there were 4777 telnet extensions in 2006.
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