问题
What's the actual TELNET specification? I found RFC 854 which is very old.
回答1:
It looks like RFC 5198 is probably current. IF you look at the top of the RFC you referenced, there is a reference to RFC 5198 with a note that 5198 updates 854.
回答2:
854 is the last RFC for the protocol itself. NIC 15373 and IETF STD 8 are normative.
Several RFCs have proposed extensions eg RFC 1123, telnet over TCP. The most recent to specifically extend telnet was 4777, in 2006.
回答3:
The basic TELNET protocol has not changed significantly since RFC 854 was published (it was already a refinement of the TELNET protocol that was used in the Arpanet, prior to TCP/IP, so there were many years of experience with it). It was designed to allow for extensions using the OPTION mechanism, rather than changes to the protocol itself. There are many RFCs that describe options for the protocol, and RFC 1123 contains a number of clarifications and refinements to this (and other) protocols.
These days, TELNET is mostly an obsolete protocol. SSH has largely supplanted it, as it has built-in encryption and authentication (these are available as options in TELNET, but adoption is spotty).
来源：https://stackoverflow.com/questions/20790367/what-is-the-final-rfc-for-telnet-protocol