1. Introduction to ns-2: “The” Network Simulator
Goals:
tutorial overview of ns
flavor of ns: simple example, model specification, execution and trace analysis
Daniel Figueiredo
November, 2004

2. Ns-2, the network simulator
Academic project over 10 years old
freely distributed, open source
Currently maintained by ISI
DARPA + NSF projects
~ 200K LoC, ~400 page manual
Large user base
mostly academics
“de facto” standard in networking research
Main website:

3. Ns Functionality Discrete event simulator
Traffic models and applications
Web, FTP, telnet, audio, sensor nets
Transport protocols
TCP (Reno, SACK, etc), UDP, multicast
Routing and queuing
static routing, DV routing, multicast, ad-hoc routing
queuing disciplines: drop-tail, RED, FQ
Link layer
wired, wireless, satellite
Infrastructure
tracing, visualization, error models, etc
modify or create your own modules

4. Ns components
Ns, the simulator itself (this is all we’ll have time for)
nam, the Network AniMator
visualize Ns (or other) output
GUI input simple Ns scenarios
pre-processing:
traffic and topology generators
post-processing:
simple trace analysis, often in Awk, Perl, or Tcl
Tutorial:
Ns by example:

5. Ns Software Structure: C++ and OTCL
Uses two languages
C++ for packet-processing
per packet processing
fast to run, detailed, complete control
OTCL for control [our focus]
simulation setup, configuration, occasional actions
fast to write and change

6. Steps when using Ns Create OTCL script with network model
nodes, links, traffic sources, sinks, etc.
Parameterize simulation objects
queue sizes, link speeds, TCP flavor and parameters (more than 30)
Collect statistics
dump everything to trace, post process it
gather stats during simulation within OTCL script
modify Ns source code
Run Ns multiple times
confidence intervals

7. Creating a Basic Ns Model
Create the event scheduler
Create nodes and links
Create connection
Create traffic sources/sinks
Enable tracing

8. Creating Event Scheduler
Create scheduler
set ns [new Simulator]
Schedule event
$ns at <time> <event>
<event>: any legitimate Ns/TCL commands
Start scheduler
$ns run

9. Creating Network (Nodes + Links)
set n0 [$ns node]
set n1 [$ns node]
Links: connect together two nodes
$ns duplex-link $n0 $n1 <bandwidth> <delay> <queue_type>
<delay> determines propagation delay
<queue_type> determines queueing policy
DropTail, RED, CBQ, FQ, SFQ, DRR

10. Transport and Traffic Models
Two layer approach
Transports:
TCP, UDP, multicast, etc.
transport protocol instances attach to nodes
Traffic (applications): (known as agents)
Web, ftp, telnet, audio, etc.
application instances attach to transport protocol instances
generates traffic into transport protocol

11. Creating Transport Channels: TCP
source and sink
set t_src [new Agent/TCP/Newreno]
set t_dst [new Agent/TCPSink]
“Newreno” flavor of TCP
attach to nodes and each other
$ns attach-agent $n0 $t_src
$ns attach-agent $n1 $t_dst
$ns connect $t_src $t_dst

12. Creating Traffic over TCP Channels
FTP
create traffic model
set ftp [new Application/FTP]
default is “infinite” file size
attach to TCP channel
$ftp attach-agent $t_src
schedule start time
$ns at <time> “$ftp start”

13. Creating Transport Channels: UDP
source and sink
set u_src [new Agent/UDP]
set u_dst [new Agent/NULL]
attach them to nodes, then connect to each other
$ns attach-agent $n0 $u_src
$ns attach-agent $n1 $u_dst
$ns connect $u_src $u_dst

14. Creating Traffic over UDP Channels
CBR
set cbr [new Application/Traffic/CBR]
$cbr set packetSize_ 512
$cbr set interval_ 0.250
$cbr attach-agent $u_src
$ns at <time> “$cbr start”

15. Tracing Trace packets on individual links Tracefile format: packet
<event> <time> <from> <to> <pkt> <size>--<flowid> <src> <dst> <seqno> <aseqno>
tcp
tcp
r tcp
packet
type
packet
flags
time
seq
number
packet ID
+ enqueue
dequeue
r receive
d drop
packet
length
flow ID
nodes involved
in this event
source
dest
addresses

16. Tracing via Monitors Queue monitor Flow monitor
queue to access the link
Flow monitor
particular flow within queue
set fmon [$ns makeflowmon Fid]
$ns attach-fmon $link $fmon
$ns at <time> “puts $fmon set pdrops_”
to specify a link, we need:
set link [$ns link $n0 $n1]

17. Walk-through example FTP/TCP 10 FTP/TCP dest 2 FTP/TCP sources sources
TCP SACK
W_max = 32
MSS = 1400
TCP NewReno
W_max = 32
MSS = 1400
1 CBR/UDP
source
FTP/TCP
dest
10 FTP/TCP
sources
2 FTP/TCP
sources 1 2
acks
acks
1 On-Off (voice)
UDP source
OnOff/UDP
CBR/UDP
dest
2 Mbps full duplex
5 msec prop delay
100 packet buffer
Drop-tail policy
1 Mbps full duplex
10 msec prop delay
700 packet buffer
Drop-tail policy

18. Ns Trace file : NS-trace.txt
...
tcp
r tcp
exp
cbr
r tcp
tcp
r exp
cbr
tcp
r cbr
exp
tcp
tcp
r tcp
r exp
exp

19. Flow Monitor Trace : packet-trace.txt
...

20. Results - Queue Statistics
Red - instantaneous
Green - running average
Blue - average w/o transient
Light blue - packet drops

21. Results - Flow Statistics
Red - TCP flow 1
Green - TCP flow 2
Number of packets in queue mimics TCP Window behavior

22. Performance Queue Statistics
Multiple runs to obtain confidence intervals