RFC 854 (rfc854) - Page 2 of 15
Telnet Protocol Specification
Alternative Format: Original Text Document
RFC 854 May 1983 applicable even in terminal-to-terminal or process-to-process communications, the "user" host is the host which initiated the communication. 2. The principle of negotiated options takes cognizance of the fact that many hosts will wish to provide additional services over and above those available within an NVT, and many users will have sophisticated terminals and would like to have elegant, rather than minimal, services. Independent of, but structured within the TELNET Protocol are various "options" that will be sanctioned and may be used with the "DO, DON'T, WILL, WON'T" structure (discussed below) to allow a user and server to agree to use a more elaborate (or perhaps just different) set of conventions for their TELNET connection. Such options could include changing the character set, the echo mode, etc. The basic strategy for setting up the use of options is to have either party (or both) initiate a request that some option take effect. The other party may then either accept or reject the request. If the request is accepted the option immediately takes effect; if it is rejected the associated aspect of the connection remains as specified for an NVT. Clearly, a party may always refuse a request to enable, and must never refuse a request to disable some option since all parties must be prepared to support the NVT. The syntax of option negotiation has been set up so that if both parties request an option simultaneously, each will see the other's request as the positive acknowledgment of its own. 3. The symmetry of the negotiation syntax can potentially lead to nonterminating acknowledgment loops -- each party seeing the incoming commands not as acknowledgments but as new requests which must be acknowledged. To prevent such loops, the following rules prevail: a. Parties may only request a change in option status; i.e., a party may not send out a "request" merely to announce what mode it is in. b. If a party receives what appears to be a request to enter some mode it is already in, the request should not be acknowledged. This non-response is essential to prevent endless loops in the negotiation. It is required that a response be sent to requests for a change of mode -- even if the mode is not changed. c. Whenever one party sends an option command to a second party, whether as a request or an acknowledgment, and use of the option will have any effect on the processing of the data being sent from the first party to the second, then the command must be inserted in the data stream at the point where it is desired that it take Postel & Reynolds
