Telnet Protocol specification(1)

　　IEN 148 J. Postel

　　RFC764ISI

　　June 1980

　　TELNETPROTOCOL SPECIFICATION

　　INTRODUCTION

　　The purpose of the TELNET Protocol is to provide a fairly general,　bi-directional, eight-bit byte oriented communications facility. Its　primary goal is to allow a standard method of interfacing terminal　devices and terminal-oriented processes to each other. It is　envisioned that the protocol may also be used for terminal-terminal　　communication ("linking") and process-process communication

　　(distributed computation).

　　GENERAL CONSIDERATIONS

　　A TELNET connection is a Transmission Control Protocol (TCP)　connection used to transmit data with interspersed TELNET control　information. TCP and the connection establishment procedure are　documentented in the ARPA Internet Protocol Handbook.

　　The TELNET Protocol is built upon three main ideas: first, the　concept of a "Network Virtual Terminal"; second, the principle of　negotiated options; and third, a symmetric view of terminals and　processes.

　　1. When a TELNET connection is first established, each end is　assumed to originate and terminate at a "Network Virtual Terminal",　or NVT. An NVT is an imaginary device which provides a standard,　network-wide, intermediate representation of a canonical terminal.

　　This eliminates the need for "server" and "user" Hosts* to keep　information about the characteristics of each other's terminals and　terminal handling conventions. All Hosts, both user and server, map　their local device characteristics and conventions so as to appear to　be dealing with an NVT over the network, and each can assume a　similar mapping by the other party. The NVT is intended to strike a　balance between being overly restricted (not providing Hosts a rich　enough vocabulary for mapping into their local character sets), and　being overly inclusive (penalizing users with modest terminals).

　　*NOTE: The "user" Host is the Host to which the physical terminal　is normally attached, and the "server" host is the Host which is　normally providing some service. As an alternate point of view,　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 sites 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 various "options" will be sanctioned which can 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, the　line width, the page length, 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.

　　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 effect. (It should be noted that　some time will elapse between the transmission of a request　and the receipt of an acknowledgment, which may be negative.

　　Thus, a site may wish to buffer data, after requesting an　[Page 2] Postel

　　RFC764, IEN 148 June 1980