ppt.ppt

Telnet

TErminaL NETwork

TELNET is a general-purpose client-serverapplication program

TELNET enables the establishment of aconnection to a remote system in such a way thatthe local terminal appears to be a terminal at theremote system.TELNET enables the establishment of aconnection to a remote system in such a way thatthe local terminal appears to be a terminal at theremote system.

Local

Remote

Local login

Remote login

Network Virtual Terminal(NVT)

Universal interface character set

TELNET client translates characters (data orcommands) that come from the local terminal intoNVT form and delivers them to the network.TELNET client translates characters (data orcommands) that come from the local terminal intoNVT form and delivers them to the network.

TELNET server translates data and commandsfrom NVT form into the form acceptable by theremote computer.TELNET server translates data and commandsfrom NVT form into the form acceptable by theremote 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 charactersSome NVT control characters

Embedding

The same connection is used by TELNET forsending both data and control characters.TELNET accomplishes this by embedding thecontrol characters in the data stream.The same connection is used by TELNET forsending both data and control characters.TELNET accomplishes this by embedding thecontrol characters in the data stream.

Each sequence of control characters is precededby a special control character called Interpret ascontrol(IAC)Each sequence of control characters is precededby a special control character called Interpret ascontrol(IAC)

An example of embedding

Options

TELNET lets the client and server negotiateoptions before or during the use of the service.TELNET lets the client and server negotiateoptions before or during the use of the service.

Options are extra features available to a userwith a more sophisticated terminal.Options are extra features available to a userwith a more sophisticated terminal.

Binary : This option allows the receiver to interpret every 8bit character received as binary data except IAC

Echo : This options allows the server to echo data receivedfrom the client. This means every character sent by theclient to the sender will be echoed back to the screen ofthe 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 theserver site.

Timing mark: allows one party to issue a timing mark thatindicates 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 optionnegotiation between the client and the server.Four control characters are used for thispurposeTo use any of the options first requires optionnegotiation between the client and the server.Four control characters are used for thispurpose

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 remoteserver.Control characters can be used to control the remoteserver.

Escape character

When a user wants characters interpreted bythe client instead of the server, he can use anescape character, normally Ctrl+]When a user wants characters interpreted bythe client instead of the server, he can use anescape character, normally Ctrl+]

shown as ^]shown as ^]

Mode of Operation

Most TELNET implementations operate in oneof three modes:Most TELNET implementations operate in oneof three modes:

1)default mode1)default mode

2)character mode2)character mode

3)line mode3)line mode

Default Mode

In this mode echoing is done by the client

Will not sent to the server until a whole line iscompleted

After sending client wait for the go-aheadcommand from the server before accepting anymore 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 , lineerasing,)

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 isresponsible for translating the user-friendlycommands to the TELNET commands.The operating system (UNIX, for example)defines an interface to TELNET with user-friendly commands. The interface isresponsible for translating the user-friendlycommands to the TELNET commands.