Program No 4

Simulating link errors  Consider the following network diagram: Here node # 2 act as a router. Any traffic to or from the LAN passes through it. Consider node # 1 running a FTP server, and node # 5 is downloading a file of size 4 MB. However, the link between node # 2 and # 3 is fault. It drops packets with a fixed probability of 0.2. Implement a link error model to reflect this. It may be noted here that the file download time will be more than we had in exercise # 2 of experiment # 1. Try different values of the simulation time to ensure that the file has been entirely transferred. Is the plot of bytes received a linear curve or non­linear? Why? Presence of link errors cause one or more packets to be retransmitted. Verify this from the "Packet Retransmissions" plot.

TCL Script:

set ns [new Simulator]
set namfile [open lab4.nam w]
$ns namtrace-all
$namfile set tracefile [open lab4.tr w]
$ns trace-all $tracefile

set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
set n5 [$ns node]
set n6 [$ns node]
$n1  color red
$n1 shape square

$ns duplex-link $n0 $n1 1Mb 10ms DropTail
$ns duplex-link-op $n0 $n1 orient right-down

$ns duplex-link $n0 $n2 1Mb 10ms DropTail
$ns duplex-link-op $n0 $n2 orient right

$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link-op $n1 $n2 orient right-up

$ns duplex-link $n2 $n3 1Mb 10ms DropTail
$ns duplex-link-op $n2 $n3 orient right

set lan [$ns newLan "$n3 $n4 $n5 $n6" 10Mb 10ms LL Queue/DropTail Mac/802_3 Channel]

set errmodel [new ErrorModel]
$errmodel set rate_ 0.2
$errmodel ranvar [new RandomVariable/Uniform]
$errmodel drop-target [new Agent/Null]
$ns lossmodel $errmodel $n2 $n3

set tcp [new Agent/TCP]
$ns attach-agent $n1 $tcp

set sink [new Agent/TCPSink]
$ns attach-agent $n5 $sink
$ns connect $tcp $sink

set ftp [new Application/FTP]
$ftp attach-agent $tcp
set filesize [expr 4*1024*1024]
$ns at 0.0 "$ftp send $filesize"

proc finish {}
{
global ns namfile tracefile
$ns flush-trace
close $namfile
close $tracefile

set awkCode
{
BEGIN{}
{
if($1 == "r" && $4 == 5 && $6 > 1000)
{
count_bytes = count_bytes + $6 - ($6 % 1000);
print $2,count_bytes >> "lab4.data";
}
else
if($1 == "d" && $5 == "tcp" && $6 > 1000)
{
count_packets++; print $2,count_packets >> "lab4_packets.data";
}
}
END{}
}
exec awk $awkCode lab4.tr
exec nam lab4.nam &
exec xgraph -bb -tk -x Time -x Bytes lab4.data -bg white &
exec xgraph -bb -tk -x Time -y packets lab4_packets.data -bg white &
exit 0

}
$ns at 100.0 "finish"
$ns run