Ns tutorial

NS2 Tutorial

Kameswari Chebrolu
Dept. of Computer Science and Engineering,
IIT Bombay

Motivation for Simulations
●
Cheap does not require costly equipment
●
Complex scenarios can be easily tested
●
Results can be quickly obtained – more ideas can
be tested in a smaller timeframe
●
The real thing isn't yet available
●
Controlled experimental conditions
– Repeatability helps aid debugging
●
Disadvantages: Real systems too complex to
model

Features of NS2
●
Protocols: TCP, UDP, HTTP, Routing algorithms,
MAC etc
●
Traffic Models: CBR, VBR, Web etc
●
Error Models: Uniform, bursty etc
●
Misc: Radio propagation, Mobility models , Energy
Models
●
Topology Generation tools
●
Visualization tools (NAM), Tracing

NS Structure
●
NS is an object oriented discreteevent simulator
– Simulator maintains list of events and executes one event after
another
– Single thread of control: no locking or race conditions
●
Back end is C++ event scheduler
– Protocols mostly
– Fast to run, more control
●
Front end is oTCL
– Creating scenarios, extensions to C++ protocols
– fast to write and change

TCL tutorial
set x 1
●
Variables: set y $x
●
Arrays: set a(0) 1

●
Printing: puts “$a(0) n”

●
Arithmetic Expression: set z = [expr $y + 5]

●
Control Structures: if {$z == 6} then { puts “Correct!”}
for {set i =0} {$i < 5} {incr i }{
puts “$i * $i equals [expr $i * $i]”
}

proc sum {a b} {
●
Procedures: return [expr $a + $b]
}

NS programming Structure

●
Create the event scheduler
●
Turn on tracing
●
Create network topology
●
Create transport connections
●
Generate traffic
●
Insert errors

Creating Event Scheduler
●
Create event scheduler: set ns [new simulator]
●
Schedule an event: $ns at <time> <event>
– event is any legitimate ns/tcl function
$ns at 5.0 “finish” proc finish {} {
global ns nf
●
Start Scheduler close $nf
exec nam out.nam &
$ns run
exit 0
}

Tracing
●
All packet trace
$ns traceall [open out.tr w]
<event> <time> <from> <to> <pkt> <size>

<flowid> <src> <dst> <seqno> <aseqno>

+ 0.51    0 1 cbr 500 0 0.0 1.0 0 2
  0.51    0 1 cbr 500 0 0.0 1.0 0 2
r  0.514  0 1 cbr 500 0 0.0 1.0 0 0
●
Variable trace set par [open output/param.tr w]
$tcp attach $par
$tcp trace cwnd_
$tcp trace maxseq_
$tcp trace rtt_

Tracing and Animation
●
Network Animator

set nf [open out.nam w]
$ns namtraceall $nf

proc finish {} {
global ns nf
close $nf
exec nam out.nam &
exit 0
}

Creating topology
●
Two nodes connected by a link
●
Creating nodes
set n0 [$ns node]
set n1 [$ns node]
●
Creating link between nodes
– $ns <link_type> $n0 $n1 <bandwidth> <delay>
<queuetype>

$ns duplexlink $n0 $n1 1Mb 10ms DropTail

Sending data
●
Create UDP agent
set udp0 [new Agent/UDP]
$ns attachagent $n0 $udp0
●
Create CBR traffic source for feeding into UDP agent
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attachagent $udp0
●
Create traffic sink
set null0 [new Agent/Null]
$ns attachagent $n1 $null0

Sending data
●
Connect two agents
$ns connect $udp0 $null0

●
Start and stop of data
$ns at 0.5 “$cbr0 start”
$ns at 4.5 “$cbr0 stop”

Creating TCP Connections
●
Create TCP agent and attach it to the node
set tcp0 [new Agent/TCP]
$ns attachagent $n0 $tcp0
●
Create a Null Agent and attach it to the node
set null0 [new Agent/TCPSink]
$ns attachagent $n1 $null0
●
Connect the agents
$ns connect $tcp0 $null0

Traffic on top of TCP
●
FTP
set ftp [new Application/FTP]
$ftp attachagent $tcp0

●
Telnet
set telnet [new Application/Telnet]
$telnet attachagent $tcp0

Introducing Errors
●
Creating Error Module
set err [new ErrorModel]
$err unit pkt_
$err set rate_ 0.01
$err ranvar [new RandomVariable/Uniform]
$err droptarget [new Agent/Null]

●
Inserting Error Module
$ns lossmodel $err $n0 $n1

Examples
●
UDP Script
●
Tracing (wired,wireless,tcp)
●
TCP without Loss
●
TCP with Loss

NS Internals

n0 n1

Port Application/FTP Port
Classifier dst_=1.0 Classifier dst_=0.0
Addr Agent/TCP Addr Agent/TCPSink
Classifier 0 Classifier 0
0 1
Link n0n1
entry_ 1 entry_ 0

Link n1n0

Summary
●
Simulators help in easy verification of protocols
in less time, money
●
NS offers support for simulating a variety of
protocol suites and scenarios
●
Front end is oTCL, back end is C++
●
NS is an ongoing effort of research and
development

Reference Material
●
http://www.isi.edu/nsnam/ns/
●
Marc Greis' tutorial
●
Jae Chung tutorial
●
Ns manual

Ns tutorial

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