RFC 854 (rfc854) - Page 2 of 15

Telnet Protocol Specification

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