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The Telnet Protocol

The Telnet protocol is often thought of as simply providing a facility for remote logins to computer via the Internet. This was its original purpose although it can be used for many other purposes.

It is best understood in the context of a user with a simple terminal using the local telnet program (known as the client program) to run a login session on a remote computer where his communications needs are handled by a telnet server program. It should be emphasised that the telnet server can pass on the data it has received from the client to many other types of process including a remote login server. It is described in RFC854 and was first published in 1983.

The Network Virtual Terminal

Communication is established using the TCP/IP protocols and communication is based on a set of facilities known as a Network Virtual Terminal (NVT). At the user or client end the telnet client program is responsible for mapping incoming NVT codes to the actual codes needed to operate the user's display device and is also responsible for mapping user generated keyboard sequences into NVT sequences.

The NVT uses 7 bit codes for characters, the display device, referred to as a printer in the RFC, is only required to display the "standard" printing ASCII characters represented by 7 bit codes and to recognise and process certain control codes. The 7 bit characters are transmitted as 8 bit bytes with most significant bit set to zero. An end-of-line is transmitted as the character sequence CR (carriage return) followed by LF (line feed). If it is desired to transmit an actual carriage return this is transmitted as a carriage return followed by a NUL (all bits zero) character.

NVT ASCII is used by many other Internet protocols.

The following control codes are required to be understood by the Network Virtual Terminal.

Name code Decimal Value Function
NULL NUL 0 No operation
Line Feed LF 10 Moves the printer to the next print line, keeping the same horizontal position.
Carriage Return CR 13 Moves the printer to the left margin of the current line.

The following further control codes are optional but should have the indicated defined effect on the display.

Name code Decimal Value Function
BELL BEL 7 Produces an audible or visible signal (which does NOT move the print head.
Back Space BS 8 Moves the print head one character position towards the left margin. [On a printing devices this mechanism was commonly used to form composite characters by printing two basic characters on top of each other.]
Horizontal Tab HT 9 Moves the printer to the next horizontal tab stop. It remains unspecified how either party determines or establishes where such tab stops are located.
Vertical Tab VT 11 Moves the printer to the next vertical tab stop. It remains unspecified how either party determines or establishes where such tab stops are located.
Form Feed FF 12 Moves the printer to the top of the next page, keeping the same horizontal position. [On visual displays this commonly clears the screen and moves the cursor to the top left corner.]

The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences.

Commands

The telnet protocol also specifies various commands that control the method and various details of the interaction between the client and server. These commands are incorporated within the data stream. The commands are distinguished by the use of various characters with the most significant bit set. Commands are always introduced by a character with the decimal code 255 known as an Interpret as command (IAC) character. The complete set of special characters is

Name Decimal Code Meaning
SE 240 End of subnegotiation parameters.
NOP 241 No operation
DM 242 Data mark. Indicates the position of a Synch event within the data stream. This should always be accompanied by a TCP urgent notification.
BRK 243 Break. Indicates that the "break" or "attention" key was hit.
IP 244 Suspend, interrupt or abort the process to which the NVT is connected.
AO 245 Abort output. Allows the current process to run to completion but do not send its output to the user.
AYT 246 Are you there. Send back to the NVT some visible evidence that the AYT was received.
EC 247 Erase character. The receiver should delete the last preceding undeleted character from the data stream.
EL 248 Erase line. Delete characters from the data stream back to but not including the previous CRLF.
GA 249 Go ahead. Used, under certain circumstances, to tell the other end that it can transmit.
SB 250 Subnegotiation of the indicated option follows.
WILL 251 Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option.
WONT 252 Indicates the refusal to perform, or continue performing, the indicated option.
DO 253 Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option.
DONT 254 Indicates the demand that the other party stop performing, or confirmation that you are no longer expecting the other party to perform, the indicated option.
IAC 255 Interpret as command

There are a variety of options that can be negotiated between a telnet client and server using commands at any stage during the connection. They are described in detail in separate RFCs. The following are the most important.

Decimal code Name RFC
1 echo 857
3 suppress go ahead 858
5 status 859
6 timing mark 860
24 terminal type 1091
31 window size 1073
32 terminal speed 1079
33 remote flow control 1372
34 linemode 1184
36 environment variables 1408

Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications.

Either end of a telnet dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form

	IAC,<type of operation>,<option>

The response is of the same form.

Operation is one of

Description Decimal Code Action
WILL 251 Sender wants to do something.
DO 252 Sender wants the other end to do something.
WONT 253 Sender doesn't want to do something.
DONT 254 Sender wants the other not to do something.

Associated with each of the these there are various possible responses

Sender Sent Receiver Responds Implication
WILL DO The sender would like to use a certain facility if the receiver can handle it. Option is now in effect
WILL DONT Receiver says it cannot support the option. Option is not in effect.
DO WILL The sender says it can handle traffic from the sender if the sender wishes to use a certain option. Option is now in effect.
DO WONT Receiver says it cannot support the option. Option is not in effect.
WONT DONT Option disabled. DONT is only valid response.
DONT WONT Option disabled. WONT is only valid response.

For example if the sender wants the other end to suppress go-ahead it would send the byte sequence

255(IAC),251(WILL),3

The final byte of the three byte sequence identifies the required action.

For some of the negotiable options values need to be communicated once support of the option has been agreed. This is done using sub-option negotiation. Values are communicated via an exchange of value query commands and responses in the following form.

 IAC,SB,<option code number>,1,IAC,SE

and

IAC,SB,<option code>,0,<value>,IAC,SE

For example if the client wishes to identify the terminal type to the server the following exchange might take place

Client   255(IAC),251(WILL),24
Server   255(IAC),253(DO),24
Server   255(IAC),250(SB),24,1,255(IAC),240(SE)
Client   255(IAC),250(SB),24,0,'V','T','2','2','0',255(IAC),240(SE)
The first exchange establishes that terminal type (option number 24) will be handled, the server then enquires of the client what value it wishes to associate with the terminal type. The sequence SB,24,1 implies sub-option negotiation for option type 24, value required (1). The IAC,SE sequence indicates the end of this request. The repsonse IAC,SB,24,0,'V'... implies sub-option negotiation for option type 24, value supplied (0), the IAC,SE sequence indicates the end of the response (and the supplied value).

The encoding of the value is specific to the option but a sequence of characters, as shown above, is common.

Telnet Negotiable Options

Many of those listed are self-evident, but some call for more comments.

  • Suppress Go Ahead

    The original telnet implementation defaulted to "half duplex" operation. This means that data traffic could only go in one direction at a time and specific action is required to indicate the end of traffic in one direction and that traffic may now start in the other direction. [This similar to the use of "roger" and "over" by amateur and CB radio operators.] The specific action is the inclusion of a GA character in the data stream.

    Modern links normally allow bi-directional operation and the "suppress go ahead" option is enabled.

  • echo

    The echo option is enabled, usually by the server, to indicate that the server will echo every character it receives. A combination of "suppress go ahead" and "echo" is called character at a time mode meaning that each character is separately transmitted and echoed.

    There is an understanding known as kludge line mode which means that if either "suppress go ahead" or "echo" is enabled but not both then telnet operates in line at a time mode meaning that complete lines are assembled at each end and transmitted in one "go".

  • linemode

    This option replaces and supersedes the line mode kludge.

  • remote flow control

    This option controls where the special flow control effects of Ctrl-S/Ctrl-Q are implemented.

Telnet control functions

The telnet protocol includes a number of control functions. These are initiated in response to conditions detected by the client (usually certain special keys or key combinations) or server. The detected condition causes a special character to be incorporated in the data stream.

  • Interrupt Process

    This is used by the client to cause the suspension or termination of the server process. Typically the user types Ctrl-C on the keyboard. An IP (244) character is included in the data stream.

  • Abort Output

    This is used to suppress the transmission of remote process output. An AO (238) character is included in the data stream.

  • Are You There

    This is used to trigger a visible response from the other end to confirm the operation of the link and the remote process. An AYT (246) character is incorporated in the data stream.

  • Erase character

    Sent to the display to tell it to delete the immediately preceding character from the display. An EC (247) character is incorporated in the data stream.

  • Erase line

    Causes the deletion of the current line of input. An EL (248) character is incorporated in the data stream.

  • Data Mark

    Some control functions such as AO and IP require immediate action and this may cause difficulties if data is held in buffers awaiting input requests from a (possibly misbehaving) remote process. To overcome this problem a DM (242) character is sent in a TCP Urgent segment, this tells the receiver to examine the data stream for "interesting" characters such as IP, AO and AYT. This is known as the telnet synch mechanism.

    A DM not in a TCP Urgent segment has no effect.

The telnet command

On most Unix systems a telnet session can be initiated using the telnet command. Most users simply type

telnet <remote host>

but if the user just types telnet then various options and subcommands are available which can be used to study the behaviour of the session.

Here's an exmaple of a telnet session from scitsc to ccub

bash$ telnet
telnet> toggle options
Will show option processing.
telnet> open ccub
Trying 134.220.1.20 ...
Connected to ccub.wlv.ac.uk.
Escape character is '^]'.
SENT do SUPPRESS GO AHEAD
SENT will TERMINAL TYPE (reply)
RCVD do TERMINAL TYPE (don't reply)
RCVD will SUPPRESS GO AHEAD (don't reply)
RCVD will ECHO (reply)
SENT do ECHO (reply)
RCVD do ECHO (reply)
SENT wont ECHO (reply)


UNIX(r) System V Release 4.0 (ccub)

RCVD dont ECHO (don't reply)
login: Login timed out after 60 seconds
Connection closed by foreign host.


These pages were produced to support a communication systems module that is no longer taught. Further communication systems notes are available on-line.
Peter Burden [removed]

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