6.829 NS Tutorial

6.829 NS tutorial Network Simulator Network Simulator (ns-2) 6.829 tutorial Why Network simulation • • • protocol validation controlled experimental conditions low cost in $, time, collaboration, complexity Why NS? Provides: • • • • • protocols: TCP, UDP, HTTP, etc. Traffic Models: Web Traffic, CBR, Topology Generation tools Visualization tools large validation package (people believe it works) NS Structure • C++ event scheduler protocols (most) • TCL scripts protocols (mostly extensions to C++ core) • TCL objects expose an interface to C++ objects (shadow objects) system configuration (defaults, etc.) 1 minute TCL tutorial set a 1 set b [expr $a + 2] proc list_total {list} { set tmp 0 foreach l $list { incr tmp $l } return $tmp } set c [list_total {1 2 3 4 5}] puts "a: $a b: $b c: $c" 1 minute oTCL tutorial Class car car instproc init {args} { $self instvar wheels_ set wheels_ 4 } Class vw -superclass car vw instproc init {args} { $self instvar wheels_ vendor_ set vendor_ "vw" eval $self next $args } set a [new puts "A: a set b [new puts "B: a car] car with [$a set wheels_] wheels" vw] [$b set vendor_] with [$b set wheels_] wheels" ns 1: Creating Topology We first create a Simulator object which we use to generate nodes and links. This code will generate a topology (2 nodes, 1 link), but won't generate any traffic. Note: we can schedule any arbitrary tcl code to be run with a $ns at ... command. Here we schedule the end of the simulation at 5 seconds. # Create simulation object set ns [new Simulator] # Ask ns for nodes set n0 [$ns node] set n1 [$ns node] # Create a duplex link b/w n0 & n1 $ns duplex-link $n0 $n1 1Mb 10ms DropTail # Schedule End $ns at 5.0 "exit 0" # Run Simulation $ns run ns 2: Attaching Agents Agents implement differenct connections between nodes. These connections are of some protocol type: TCP, UDP, etc. Here we show a simple UDP example, since a UDP sink would do nothing we use the null sink. # Create a UDP agent set udp1 [new Agent/UDP] # Create a Null agent set sink1 [new Agent/Null] # Attach agent udp1 to node n0 $ns attach-agent $n0 $udp1 # Attach agent sink1 to node n1 $ns attach-agent $n1 $sink1 # Connect the agents $ns connect $udp1 $sink1 ns 2: TCP Agents Of course instead of using UDP we could use TCP, but then we'll have to use a specialized sink to generate acknowledgements. # Create a TCP agent set tcp1 [new Agent/TCP] # Create a Null agent set sink1 [new Agent/TCPSink] # Attach agent tcp1 to node n0 $ns attach-agent $n0 $tcp1 # Attach agent sink1 to node n1 $ns attach-agent $n1 $sink1 # Connect the agents $ns connect $tcp1 $sink1 ns 3: Attaching Sources Still, our network will not carry any data. We need a data source which will feed bytes to our agent. Here we generate a constant bit rate source (CBR). # Create Source set cbr1 [new Application/Traffic/CBR] # Configure Source $cbr1 set packetSize_ 500 $cbr1 set interval_ 0.005 # Attach source to agent $cbr1 attach-agent $udp1 # Schedule $ns at 0.5 # Schedule $ns at 4.5 cbr on "$cbr1 start" cbr off "$cbr1 stop" ns 4: Tracing While our current simulation will create sources and generate traffic which will travel over our link, it will not create any useful output for us. Here we tell our simulation to trace events for us. Note in the full code file that the trace-all and nam-trace-all commands must be run before nodes and agents are created, but the variable trace commands must be run afterwards: # all packet trace: $ns trace-all [open out.tr w] # animator trace: $ns namtrace-all [open out.nam w] # variable trace: set tf [open "cwnd.tr" w] set tracer [new Trace/Var] $tracer attach $tf $tcp trace cwnd_ $tracer nam Network Animator You can run the network animator using the nam command. %nam out.nam