Telnet  TErminaL NETwork  TELNET is a general-purpose client-server application program  TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system.

Login  Local  Remote

Local login

Remote login

Network Virtual Terminal(NVT)  Universal interface character set  TELNET client translates characters (data or commands) that come from the local terminal into NVT form and delivers them to the network.  TELNET server translates data and commands from NVT form into the form acceptable by the remote computer.

Concept of NVT

NVT Character Set  Data Characters  Control Characters  Both are 8 bit bytes

Data Characters  Use NVT ASCII  7 lower order are same as US ASCII  Higher order one is 0

Control Characters  8 bit character set with highest orders bit set

Some NVT control characters

Embedding  The same connection is used by TELNET for sending both data and control characters. TELNET accomplishes this by embedding the control characters in the data stream.  Each sequence of control characters is preceded by a special control character called Interpret as control(IAC)

An example of embedding

Options  TELNET lets the client and server negotiate options before or during the use of the service.  Options are extra features available to a user with a more sophisticated terminal.

 Binary : This option allows the receiver to interpret every 8 bit character received as binary data except IAC  Echo : This options allows the server to echo data received from the client. This means every character sent by the client to the sender will be echoed back to the screen of the client terminal. Client terminal will not echo characters  Suppress go- ahead : suppresses go-ahead(GA) character

 Status: to get status of the options being enabled at the server site.  Timing mark: allows one party to issue a timing mark that indicates all previously received data has been processed  Terminal Type: client to send its terminal type  Terminal Speed: client to send its terminal speed  Line mode: enable to switch to line mode

Option Negotiation  To use any of the options first requires option negotiation between the client and the server. Four control characters are used for this purpose

Offer to enable an option

Request to enable an option

Offer to disable an option

Request to disable an option

Example 1: Echo option

Controlling the server  Control characters can be used to control the remote server.

Escape character  When a user wants characters interpreted by the client instead of the server, he can use an escape character, normally Ctrl+]  shown as ^]

Mode of Operation  Most TELNET implementations operate in one of three modes: 1) default mode 2) character mode 3) line mode

Default Mode  In this mode echoing is done by the client  Will not sent to the server until a whole line is completed  After sending client wait for the go-ahead command from the server before accepting any more from clients  Operation is half duplex

Character mode  Each character typed is sent by the client to server  Server echo the character  TCP segments must send for each character

Line Mode  Client send one line  Line editing is possible( echoing, character erasing, line erasing,)  Full duplex mode  Will not wait for go-ahead

User interface  The operating system (UNIX, for example) defines an interface to TELNET with user- friendly commands. The interface is responsible for translating the user-friendly commands to the TELNET commands.