Every Telnet implementation MUST include option negotiation and subnegotiation machinery [TELNET:2].
A host MUST carefully follow the rules of RFC-854 to avoid option-negotiation loops. A host MUST refuse (i.e, reply WONT/DONT to a DO/WILL) an unsupported option. Option negotiation SHOULD continue to function (even if all requests are refused) throughout the lifetime of a Telnet connection.
If all option negotiations fail, a Telnet implementation MUST default to, and support, an NVT.
Even though more sophisticated "terminals" and supporting option negotiations are becoming the norm, all implementations must be prepared to support an NVT for any user-server communication.