final telnet chapt-8

7/25/2019 Final Telnet Chapt-8

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Telnet


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REMOTE LOGIN Users may want to run application programs at a

remote site and create results that can be

transferred to their local site.

For example: Students may connect to their

university computer Labs from their home to access

application programs for doing homework

assignments or proects.

!est solution is a general purpose client server

program that allows a user to access an application

program on a remote computer.

"fter logging on# a user can use services available

on the remote computer and transfer the results

back to the local computer.

$ypically $elnet uses $%& port 23.


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TELNET $'L('$ is an abbreviation for TErminaLNETwork.

)t is standard TCP/IPprotocol for virtual terminal serviceas proposed by ISO.

$'L('$ enables the establishment of connection to a

remote system in such a way that# the local terminal

appears to be a terminal at the remote system.

$'L('$ is a general purpose Client-Serverapplication

program.

Time Sharing Environment )t was designed at a time

when most of the *Ss# such as !NI"# were operating in

timesharing environment. $he interaction between the

user and the computer occurs through a terminal# which

is usually a combination of keyboard# monitor and

mouse.


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TELNET LOGGING

Lo#al Log-in +hen a user logs

into a local timesharing system#it is called as a local log,in.

" user types at a terminal#

running a terminal 'mulator# the

keystrokes are accepted by theterminal driver.

$he terminal driver passes the

characters to the *S.

$hen the *S# in turn interpretsthe combination of characters

and invokes the desired

application program or utility.


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TELNET LOGGINGRemote Log-in " user wants to access an application program

located on a remote machine.

!oth# the $'L('$ %lient and Server programs are used.

$he user sends a keystroke to a terminal driver# where the

local *S accepts the characters but does not interpret them.

$he characters are sent to $'L('$ %lient# which converts

the characters to a universal character set called Net$or%

&irt'al Terminal (N&T)characters and delivers them to the

local TCP/IPstack.

$he text in N&Tform# travels through the internet and arrives

at the $%&-)& stack at the remote machine. ere the characters are delivered to the *S and passed to

the $'L('$ server# which changes characters to the

corresponding characters understandable by the remote

computer.


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TELNET LOGGING

owever# remote *S is designed to receive characters

only from a terminal driver and not from a $'L('$ server.

ence# the *S uses a pseudo-terminal driver to receive

the characters# which in turn emulates the characters

coming from a terminal.

$he *S then passes the received characters to the

appropriate application program.


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NET*OR+ &IRT!,L TERMIN,L (N&T) +e are dealing with heterogeneous systems and want to

access any remote computer in the world.

$'L('$ solves this problem by defining universal interfacecalled as N&Tcharacter set.

/ia this interface# the client $'L('$ translates characters

01ata-%ommands2 that come from local terminal into N&T

form and delivers them to the network. $he $'L('$ server translates 1ata and %ommands from

N&Tform into the form acceptable by the remote computer.

N&T uses 3 character sets one for ata and other for

Control. !oth are 4,bit.

or ata(/$ uses 4,bit character set 05 out of which are

same as "S%))2 and highest order bit is 6.

or Control (/$ uses 4,bit character set where the

highest order bit is set to 7.


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NET*OR+ &IRT!,L TERMIN,L (N&T)

ata Chara#ter Control Chara#ter


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N&T Control Chara#terChara#ter e#imal 0inar1 Meaning

EO 23 444 En5 o6 6ile

EOR 237 4 En5 o6 re#or5

SE 284 4444 S'9option en5

NOP 28 444 No operation

M 282 444 ata mar%

0R+ 283 44 0rea%

IP 288 444 Interr'pt pro#e

,O 28: 44 ,9ort o'tp't

,;T 28 44 ,re 1o' there 444 Erae line

G, 287 44 Go ahea5

S0 2:4 44 S'9option 9egin

*ILL 2: 4 ,greement to ena9le option

*ONT 2:2 44 Re6'al to ena9le option

O 2:3 4 ,pproval to option re?'et

ON@T 2:8 4 enial o6 option re?'et

I,C 2:: Interpret (the neAt #hara#ter) a #ontrol


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Em9e55ing $'L('$ uses only one $%&-)& connection# the server uses well known port

38 and client uses an ephemeral port 0short lived2.

$he same connection is used for sending both data and control characters.

$'L('$ embeds control characters in data stream and distinguishes data

from control characters# by a special control character called Interpret ,

Control (I,C).

For example: User wants Server to display a file 9file7 on remote server.

#at file7

Suppose the filename has been mistyped as 9filea instead of 9file7# thenthe user uses the 9!ackSpace 02 key to correct this situation.

#atfilea;backspace


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Option $'L('$ lets the client,server negotiate option before or

during the use of service.

*ptions are extra features available to a user with moresophisticated terminal.

Users with simpler terminals can use simpler features.

Following table shows some common options.

Co5e Option Meaning4 0inar1 Interpret a >-9it 9inar1 tranmiion.

E#ho E#ho the 5ata re#eive5 on one i5e to other.

3 S'ppre go ahea5 S'ppre go ahea5 ignal a6ter 5ata

: Stat' Re?'et the tat' o6 TELNET.

Timing mar% e6ine the timing mar%.

28 Terminal t1pe Set the terminal t1pe.

32 Terminal pee5 Set the terminal pee5.

38 Line mo5e Change to line mo5e.


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Option Negotiation $o use any of the options mentioned in the previous

table# first re=uires option negotiation between client and

server. Four control characters are needed for this purpose.

Chara#ter e#imal 0inar1 Meaning

+)LL 3>7 77777677 7. *ffering to enable.

3. "ccepting a re=uest to enable.

+*($ 3>3 77777766 7. ?eecting a re=uest to enable.

3. *ffering to disable.

8. "ccepting a re=uest to disable.

1* 3>8 77777767 7. "pproving an offer to enable.3. ?e=uesting to enable.

1*(@$ 3>A 77777776 7. 1isapproving an offer to enable.

3. "pproving an offer to disable.

8. ?e=uesting to disable.


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Option Negotiation EAample $he client wants the server to '%* each character sent to the

server.

$he re=uest consists of 8 characters: I,CB Oand ECO. $he server informs the client by sending 8 character approval:

I,C# *ILLand ECO.


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S'9option Negotiation EAample "n example of Suboption

negotiation: $he client

wants to negotiate the

type of terminal.

S'9option Negotiation Some options re=uire additional information for example: $o define the

type or speed of a terminal# the negotiation includes a tring or a

n'm9er to define the t1peor pee5.

)n either case# the 3 Suboption %haracters indicated in following table#are needed for Suboption (egotiation.

Chara#ter e#imal 0inar1 Meaning

S' 3A6 77776666 Suboption end

S! 3>6 77777676 Suboption begin


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Mo5e o6 Operation Bost $'L('$ implementations operate in 7 of 8 modes:

$he e6a'lt mo5e# Chara#ter Mo5eor Line mo5e.

e6a'lt Mo5e used if no other modes are invoked throughoption negotiation.

, )n this# the echoing is done by the client# user types a

character and client echoes the character on the screen but

does not send it until a whole line is completed.

Chara#ter Mo5e )n this# each character typed is sent by clientto the server.

, $he server normally echoes the character back to be displayed

on client screen.

, )t also creates overhead for the network because 8 $%&segments must be sent for each character of data.

Line Mo5e " new mode called Line mode# Line editing

0echoing# character erasing# line erasing and so on2 is done by

the client. $he client then sends the whole line to the server.


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Thank

YOU

final telnet chapt-8

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