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       To write a ns2 program for implementing unicast routing protocol.


Step 1: start the program.
Step 2: declare the global variables ns for creating a new simulator.
Step 3: set the color for packets.
Step 4: open the network animator file in the name of file2 in the write mode.
Step 5: open the trace file in the name of file 1 in the write mode.
Step 6: set the unicast routing protocol to transfer the packets in network.
Step 7: create the required no of nodes.
Step 8: create the duplex-link between the nodes including the delay time,bandwidth and dropping                                               .            queue mechanism.
Step 9: give the position for the links between the nodes.
Step 10: set a tcp reno connection for source node.
Step 11: set the destination node using tcp sink.
Step 12: setup a ftp connection over the tcp connection.
Step 13: down the connection between any nodes at a particular time.
Step 14: reconnect the downed  connection at a particular time.
Step 15: define the finish procedure.
Step 16: in the definition of the finish procedure declare the global variables ns,file1,file2.
Step 17: close the trace file and namefile and execute the network animation file.
Step 18: at the particular time call the finish procedure.
Step 19: stop the program.

set ns [new Simulator]

#Define different colors for data flows (for NAM)
$ns color 1 Blue
$ns color 2 Red

#Open the Trace file
set file1 [open out.tr w]
$ns trace-all $file1

#Open the NAM trace file
set file2 [open out.nam w]
$ns namtrace-all $file2

#Define a 'finish' procedure
proc finish {} {
        global ns file1 file2
        $ns flush-trace
        close $file1
        close $file2
        exec nam out.nam &
        exit 3

# Next line should be commented out to have the static routing
$ns rtproto DV

#Create six nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n4 [$ns node]
set n4 [$ns node]
set n5 [$ns node]

#Create links between the nodes
$ns duplex-link $n0 $n1 0.3Mb 10ms DropTail
$ns duplex-link $n1 $n2 0.3Mb 10ms DropTail
$ns duplex-link $n2 $n3 0.3Mb 10ms DropTail
$ns duplex-link $n1 $n4 0.3Mb 10ms DropTail
$ns duplex-link $n3 $n5 0.5Mb 10ms DropTail
$ns duplex-link $n4 $n5 0.5Mb 10ms DropTail

#Give node position (for NAM)
$ns duplex-link-op  $n0 $n1 orient right
$ns duplex-link-op  $n1 $n2 orient right
$ns duplex-link-op $n2 $n3 orient up
$ns duplex-link-op $n1 $n4 orient up-left
$ns duplex-link-op  $n3 $n5 orient left-up
$ns duplex-link-op  $n4 $n5 orient right-up

#Setup a TCP connection
set tcp [new Agent/TCP/Newreno]
$ns attach-agent $n0 $tcp
set sink [new Agent/TCPSink/DelAck]
$ns attach-agent $n5 $sink
$ns connect $tcp $sink
$tcp set fid_ 1

#Setup a FTP over TCP connection
set ftp [new Application/FTP]
$ftp attach-agent $tcp
$ftp set type_ FTP

$ns rtmodel-at 1.0 down $n1 $n4
$ns rtmodel-at 4.5 up $n1 $n4

$ns at 0.1 "$ftp start"

$ns at 6.0 "finish"

$ns run