NS-2

1. NS-2 Shahin Shayandeh December 2004 Session 1

2. Ns-2, the Network Simulator • A discrete event simulator • Simple model • Focused on modeling network protocols • Wired, wireless, satellite • Network protocols • TCP, UDP, multicast, unicast • Traffic source behavior • Web, telnet, ftp, CBR, VBR • Router queue management mechanism • Drop Tail, RED, CBQ • Ad hoc routing, sensor networks • Infrastructure: stats, tracing, error models, etc

3. Simplified User's View of NS

4. Discrete Event Simulation • 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 • Ns uses simple model: single thread of control  no locking or race conditions to worry about (very easy) • Packet-handling delay • Timers

5. A B Discrete Event Example t=1, A enqueues pkt on LAN t=1.01, LAN dequeues pkt and triggers B simple queuing model: Consider two nodes on an Ethernet:

6. Ns Architecture • Object-oriented (C++, OTcl) • Modular approach • Fine-grained object composition • Reusability • Maintenance • Performance (speed and memory) • Careful planning of modularity

7. C++ and OTcl Separation • “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)

8. Basic Tcl variables: set x 10 puts “x is $x” functions and expressions: set y [pow x 2] set y [expr $x*$x] control flow: if {$x > 0} { return $x } else { return [expr -$x] } while { $x > 0 } { puts $x incr x –1 } procedures: proc pow {x n} { if {$n == 1} { return $x } set part [pow x [expr $n-1]] return [expr $x*$part] } Also lists, associative arrays, etc. => can use a real programming language to build network topologies, traffic models, etc.

9. Basic otcl Class Person # constructor: Person instproc init {age} { $selfinstvar age_ set age_ $age } # method: Person instproc greet {} { $selfinstvar age_ puts “$age_ years old: How are you doing?” } # subclass: Class Kid -superclass Person Kid instproc greet {} { $selfinstvar age_ puts “$age_ years old kid: What’s up, dude?” } set a [new Person 45] set b [new Kid 15] $a greet $b greet

10. Using ns Problem Result analysis Simulation model Modify ns Setup/run simulation with ns

11. Hello World - Interactive Mode Interactive mode: $ ns % set ns [new Simulator] _o3 % $ns at 1 “puts \“Hello World!\”” 1 % $ns at 1.5 “exit” 2 % $ns run Hello World! Batch mode: simple.tcl set ns [new Simulator] $ns at 1 “puts \“Hello World!\”” $ns at 1.5 “exit” $ns run $ ns simple.tcl Hello World!

12. How to start? • Create a simulator object set ns [new Simulator] • NAM trace data set nf [open out.nam w] $ns namtrace-all$nf • 'finish' procedure proc finish {} { global ns nf $ns flush-trace close $nf exec nam out.nam & exit 0 } • Execute the 'finish' procedure after 5.0 seconds $ns at 5.0 "finish“ • starts the simulation $ns run

13. Creating Network • Nodes set n0 [$ns node] set n1 [$ns node] • Links and queuing $ns <link_type> $n0 $n1 <bandwidth> <delay> <queue_type> • <link_type>: duplex-link, simplex-link • <queue_type>: DropTail, RED, CBQ, FQ, SFQ, DRR queues

14. Two nodes, one link set n0 [$ns node] set n1 [$ns node] $ns duplex-link $n0 $n1 1Mb 10ms DropTail

15. Sending Data #Create a UDP agent and attach it to node n0 set udp0 [new Agent/UDP] $ns attach-agent $n0 $udp0 #Create a CBR traffic source and attach it to udp0 set cbr0 [new Application/Traffic/CBR] $cbr0 set packetSize_ 500 $cbr0 set interval_ 0.005 $cbr0 attach-agent $udp0 • Null agent which acts as traffic sink set null0 [new Agent/Null] $ns attach-agent $n1 $null0 • Two agents have to be connected with each other $ns connect $udp0 $null0 • CBR agent when to send data and when to stop sending $ns at 0.5 "$cbr0 start“ $ns at 4.5 "$cbr0 stop"

16. NAM