e.g., FTP and SMTP.

      Telnet uses a single TCP connection, and its normal data stream
      ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with
      escape sequences to embed control functions.  Telnet also allows
      the negotiation of many optional modes and functions.

      The primary Telnet specification is to be found in RFC-854
      [TELNET:1], while the options are defined in many other RFCs; see
      Section 7 for references.

   3.2  PROTOCOL WALK-THROUGH

      3.2.1  Option Negotiation: RFC-854, pp. 2-3

         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.

         DISCUSSION:
              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.

      3.2.2  Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858

         On a host that never sends the Telnet command Go Ahead (GA),
         the Telnet Server MUST attempt to negotiate the Suppress Go
         Ahead option (i.e., send "WILL Suppress Go Ahead").  A User or
         Server Telnet MUST always accept negotiation of the Suppress Go




 



RFC1123                  REMOTE LOGIN -- TELNET             October 1989


         Ahead option.

         When it is driving a full-duplex terminal for which GA has no
         meaning, a User Telnet implementation MAY ignore GA commands.

         DISCUSSION:
              Half-duplex ("locked-keyboard") line-at-a-time terminals
              for which the Go-Ahead mechanism was designed have largely
              disappeared from the scene.  It turned out to be difficult
              to implement sending the Go-Ahead signal in many operating
              systems, even some systems that support native half-duplex
              terminals.  The difficulty is typically that the Telnet
              server code does not have access to information about
              whether the user process is blocked awaiting input from
              the Telnet connection, i.e., it cannot reliably determine
              when to send a GA command.  Therefore, most Telnet Server
              hosts do not send GA commands.

              The effect of the rules in this section is to allow either
              end of a Telnet connection to veto the use of GA commands.

              There is a class of half-duplex terminals that is still
              commercially important: "data entry terminals," which
              interact in a full-screen manner.  However, supporting
              data entry terminals using the Telnet protocol does not
              require the Go Ahead signal; see Section 3.3.2.

      3.2.3  Control Functions: RFC-854, pp. 7-8

         The list of Telnet commands has been extended to include EOR
         (End-of-Record), with code 239 [TELNET:9].

         Both User and Server Telnets MAY support the control functions
         EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP,
         SB, and SE.

         A host MUST be able to receive and ignore any Telnet control
         functions that it does not support.

         DISCUSSION:
              Note that a Server Telnet is required to support the
              Telnet IP (Interrupt Process) function, even if the server
              host has an equivalent in-stream function (e.g., Control-C
              in many systems).  The Telnet IP function may be stronger
              than an in-stream interrupt command, because of the out-
              of-band effect of TCP urgent data.

              The EOR control function may be used to delimit the