1. DMET 602: Networks and Media Lab
Amr El Mougy
Yasmeen Essam
Mohamed Karam

2. Exp 8: NS2

3. What is a Simulation?
“It is the imitation of the operation of a real-world process or system over time. The act of simulating something first requires that a model be developed; this model represents the key characteristics or behaviors/functions of the selected physical or abstract system or process. The model represents the system itself, whereas the simulation represents the operation of the system over time.” Wikipedia
Highly valuable tool for research and analysis

4. What is NS2 Object-oriented, discrete event-driven network simulator
Written in C++ and OTcl
Separates control path from data path implementations
By VINT: Virtual InterNet Testbed

5. Goals of NS2 Support networking research and education
Protocol design, traffic studies, etc
Protocol comparison
Provide a collaborative environment
Freely distributed, open source
Share code, protocols, models, etc
Allow easy comparison of similar protocols
Increase confidence in results
More people look at models in more situations
Experts develop models
Multiple levels of detail in one simulator

6. Functionalities of NS2 Wired world Wireless
Routing DV, LS, PIM-SM
Transportation: TCP and UDP
Traffic sources: web, ftp, telnet, cbr, stochastic
Queuing disciplines: drop-tail, RED, FQ, SFQ, DRR
QoS: IntServ and Diffserv
Emulation
Wireless
Ad hoc routing and mobile IP
Sensor networks, vehicular networks
Tracing, visualization, various utilities

7. NS Components NS, the simulator itself Nam, the network animator
Visualize ns (or other) output
Nam editor: GUI interface to generate ns scripts
Pre-processing:
Traffic and topology generators
Post-processing:
Simple trace analysis, often in Awk, Perl, or Tcl

8. NS Models Traffic models and applications: Transport protocols:
Web, FTP, telnet, constant-bit rate, real audio
Transport protocols:
unicast: TCP (Reno, Vegas, etc.), UDP
Multicast: SRM
Routing and queueing:
Wired routing, ad hoc rtg and directed diffusion
queueing protocols: RED, drop-tail, etc
Physical media:
Wired (point-to-point, LANs), wireless (multiple propagation models), satellite

9. Discrete Event Simulator
Model world as events
Simulator has list of events
Process: take next one, run it, until done
Each event happens in an instant of virtual (simulated) time, but takes an arbitrary amount of real time
simple
queuing
model:
Consider two nodes
on an Ethernet:
t=1, A enqueues pkt on LAN
t=1.01, LAN dequeues pkt
and triggers B A B

10. C++ and oTCL Sepatarion
“data” / control separation
C++ for “data”:
per packet processing, core of ns
fast to run, detailed, complete control
OTcl for control:
Simulation scenario configurations
Periodic or triggered action
Manipulating existing C++ objects
fast to write and change
running vs. writing speed
Learning and debugging (two languages)

11. NS Programming Create the event scheduler Turn on tracing
Create network
Setup routing
Insert errors
Create transport connection
Create traffic
Transmit application-level data

12. Creating an Event Scheduler
Create event scheduler
set ns [new Simulator]
Schedule events
$ns at <time> <event>
<event>: any legitimate ns/tcl commands
$ns at 5.0 “finish”
Start scheduler
$ns run
Example:
simple.tcl
set ns [new Simulator]
$ns at 1 “puts \“Hello World!\””
$ns at 1.5 “exit”
$ns run
bash-shell$ ns simple.tcl
Hello World!
bash-shell$
Transport Layer

13. Tracing set nf [open out.nam w] $ns trace-all $nf $ns namtrace-all $nf
Insert immediately after scheduler!
Trace packets on all links
set nf [open out.nam w]
$ns trace-all $nf
$ns namtrace-all $nf

14. Tracing

15. Creating a Simple Network
Nodes
set n0 [$ns node]
set n1 [$ns node]
Links and queuing
$ns duplex-link $n0 $n1 1Mb 10ms RED
$ns duplex-link $n0 $n1 <bandwidth> <delay> <queue_type>
<queue_type>: DropTail, RED, etc. n0 n1
Transport Layer

16. Queuing Methods
Drop Tail: When the buffer is full drop any newly arriving packets
Has global TCP congestion problem  all nodes stop and start at the same time
Random Early Detection (RED): Starts dropping before the buffer is full. The more data sent by a node, the higher the dropping probability
Stochastic Fairness Queuing: Supports QoS by supporting multiple queues. Packets are taken in a round robin fashion

17. Creating a Larger Topology
for {set i 0} {$i < 7} {incr i} {
set n($i) [$ns node] }
$ns duplex-link $n($i) $n([expr($i+1)%7]) 1Mb 10ms RED

18. Network Dynamics Link failures
Hooks in routing module to reflect routing changes
$ns rtmodel-at <time> up|down $n0 $n1
For example:
$ns rtmodel-at 1.0 down $n0 $n1
$ns rtmodel-at 2.0 up $n0 $n1

19. Creating a UDP Link set udp [new Agent/UDP] set null [new Agent/Null]
$ns attach-agent $n0 $udp
$ns attach-agent $n1 $null
$ns connect $udp $null

20. Creating Traffic on top of UDP
cbr
null
Constant Bit Rate
set cbr [new Application/Traffic/CBR]
$cbr set packetSize_ 500
$cbr set interval_ 0.005
$cbr attach-agent $udp

21. Creating a TCP Connection
set tcp [new Agent/TCP]
set tcpsink [new Agent/TCPSink]
$ns attach-agent $n0 $tcp
$ns attach-agent $n1 $tcpsink
$ns connect $tcp $tcpsink n1 n0
tcp
sink

22. Creating Traffic on top of TCP
ftp
sink
FTP
set ftp [new Application/FTP]
$ftp attach-agent $tcp
Telnet
set telnet [new Application/Telnet]
$telnet attach-agent $tcp

23. Inserting Packet Loss Creating Error Module Inserting Error Module
set loss_module [new ErrorModel]
$loss_module set rate_ 0.01
$loss_module unit pkt
$loss_module ranvar [new RandomVariable/Uniform]
$loss_module drop-target [new Agent/Null]
Inserting Error Module
$ns lossmodel $loss_module $n0 $n1

24. Post-Processing Procedure
Add a 'finish' procedure that closes the trace file and starts nam.
proc finish {} {
global ns nf
$ns flush-trace
close $nf
exec nam out.nam &
exit 0 }

25. Network Animator

26. Network Visualization: Nodes and Links
Color
$node color red
Shape (can’t be changed after sim starts)
$node shape box (circle, box, hexagon)
Label (single string)
$ns at 1.1 “$n0 label \”web cache 0\””
Links
$ns duplex-link-op $n0 $n1 color "green"
Label
$ns duplex-link-op $n0 $n1 label “backbone"

27. Topology Example Manual” layout: specify everything
$ns duplex-link-op $n(0) $n(1) orient right
$ns duplex-link-op $n(1) $n(2) orient right
$ns duplex-link-op $n(2) $n(3) orient right
$ns duplex-link-op $n(3) $n(4) orient 60deg
If anything missing  automatic layout

28. Simulation Example