Telnet Protocols(1)

　　Network Working Group Jon Postel

　　Request for Comments: 318 UCLA-NMC

　　NIC: 9348 April 3, 1972

　　References: RFC139, 158, and NIC 7104

　　TelnetProtocol

　　At the October 1971 Network Working Group Meeting, I promised to　promptly produce a document which clearly and succinctly specified　and explained the Official Telnet Protocol. 　　The following pages present my understanding of the ad hoc Telnet　protocol. There are some who have serious questions about this　protocol. The proposed changes to the protocol are given in Section　IV.

　　Any comments should be promptly directed to me via the Network　Information Center (Ident = JBP) or by phone (213) 825-2368 or by　mail.

　　Jon Postel

　　SPADE Group

　　3804 Boelter Hall

　　UCLA

　　Los Angeles, California 90024

　　I. DEFINITION OF THE NETWORK VIRTUAL TERMINAL

　　The Network Virtual Terminal (NVT) is a bi-directional character　device. The characters are represented by 8 bit codes. The NVT has　no timing characteristics. The character codes 0 through 127 are the　USASCII codes. (Note all code values are given in decimal.) The　codes 128 through 255 are used for special control signals. The NVT　is described as having a printer and a keyboard. The printer　responds to incoming data and the keyboard produces outgoing data.

　　The Printer

　　The NVT printer has an unspecified carriage width (common values are　40, 72, 80, 120, 128, 132). The printer can produce representations　of all 95 USASCII graphics (codes 32 through 126). Of the 33 USASCII　control codes (0 through 31 and 127) the following 8 have specific　meaning to the NVT printer.

　　NAME CODE MEANING

　　NULL (NUL) 0 A no operation.

　　BELL (BEL) 7 Produces an audible or visible signal.

　　Back Space (BS) 8 Backspaces the printer one character　position.

　　Horizontal Tab (HT) 9 Moves the printer to next horizontal tab stop.

　　Line Feed (LF) 10 Moves the printer to next line (keeping　the same horizontal position).

　　Vertical Tab (VT) 11 Moves the printer to the next vertical　tab stop.

　　Form Feed (FF) 12 Moves the printer to the top of the　next page.

　　Carriage Return (CR) 13 Moves the printer to the left margin　of the current line.

　　The remaining USASCII codes (1 through 6, 14 through 31, and 127) do　not cause the NVT printer to take any action.

　　The Keyboard

　　The NVT Keyboard has keys or key combinations or key sequences for　generating all of the 128 USASCII codes. Note that although there　are codes which have no effect on the NVT printer, the NVT Keyboard　is capable of generating these codes.

　　The End of the Line Convention

   　The end of a line of text shall be indicated by the character　sequence Carriage Return Line Feed (CR, LF). This convention applies　to both the sending (Keyboard) and receiving (Printer) (virtual)　mechanisms.

　　Break and Reverse Break

　　The Telnet control signals provide a BREAK signal which can be used　to simulate the use of the break or attention or interrupt button　found on most terminals. This signal has no effect on the NVT. When　the BREAK Telnet control signal is used from server to user it is　sometimes called "reverse break". Such a reverse break has no effect　on the NVT.

　　II. DEFINITION OF TELNET PROTOCOL

　　The purpose of Telnet Protocol is to provide a standard method of　interfacing terminals devices at one site to processes at another　site.

　　The Telnet Protocol is built up from three major substructures, first　the Initial Connection Protocol (ICP), second the Network Virtual　Terminal (NVT), and third the Telnet control signals described　herein.

　　Telnet user and server processes follow the ICP to establish　connections. The term "Logger" has been associated with the set of　processes in the serving system which respond to the ICP and perform　the initial interactions e.g. obtain a name and password. The ICP is　defined and the initial socket number and byte size parameters are　defined in "Current Network Protocols" (NIC #7104).

　　The data transmitted between the user and server programs (and vice　versa) is treated as a character stream with embedded control　signals.

　　Note that all code values are given in decimal.