telnet terminal network telnet is a general-purpose client-server application program telnet...
DESCRIPTION
Local loginTRANSCRIPT
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 mode2) character mode3) 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.