Network Simulator ns-2: An Overview and Tutorial

1. 15-744: Computer Networking L-3 The network simulator ns-2 Slides loosely based on a tutorial by Polly Huang, ETH Zurich.

2. What is ns? • Network simulator • VINT Project -- Virtual Internet Testbed • Collaboration of many institutes • AT&T • UC Berkeley • XEROX PARC • ETH • Goals

3. ns --- what is it good for? Used to: • Evaluate performance of existing • network protocols. • Prototyping and evaluation of new protocols. • Large-scale simulations not possible • in real experiments.

4. ns • Event-driven simulator • Packet level • Link layer and up How does it work:

5. Otcl / Tcl tclcl C++ Development Status • 100K lines of C++ code • 70K lines of otcl code • 20K lines of documentation Current status:

6. Pure C++ objects Pure OTcl objects C++/OTcl split objects OTcl C++ ns otcl and C++: The Duality Your ns-script

7. Class mom mom instproc init {age} { $selfinstvar age_ set age_ $age } mom instproc greet {} { $selfinstvar age_ puts “$age_ years old mom: How are you doing?” } set a [new mom 45] $a greet Basic otcl • instead of single class declaration multiple definitions • instproc adds method • instvar adds instance variable • methods are always called through object ( $self) • init instproc instead of constructor

8. Class kid -superclass mom kid instproc greet {} { $selfinstvar age_ puts “$age_ years old kid: What’s up, dude?” } set b [new kid 15] $b greet Basic otcl - inheritance

9. Basic structure of ns-scripts • Creating the event scheduler • [Tracing] • Creating network topology • Creating Transport Layer - Agents • Creating Applications - Applications • Events!

10. 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

11. “Hello World” in ns simple.tcl set ns [new Simulator] $ns at 1 “puts \“Hello World!\”” $ns at 1.5 “exit” $ns run gs116% ns simple.tcl Hello World! gs116%

12. Creating Network • Nodes • set n0 [$ns node] • set n1 [$ns node] • Links & Queuing • $ns duplex-link $n0 $n1 <bandwidth> <delay> <queue_type>

13. Transport Layer Class Agent Agent/UDP Agent/TCP (=Tahoe) … Other TCP flavors

14. The transport layer: UDP • UDP • set udp [new Agent/UDP] • set null [new Agent/NULL] • $ns attach-agent $n0 $udp • $ns attach-agent $n1 $null • $ns connect $udp $null

15. The transport layer: TCP • TCP • set tcp [new Agent/TCP] • set tcpsink [new Agent/TCPSink] • $ns attach-agent $n0 $tcp • $ns attach-agent $n1 $tcpsink • $ns connect $tcp $tcpsink

16. Transport Layer Class Agent Agent/UDP Agent/TCP (=Tahoe) … Other TCP flavors Agent/TCP/FullTCP

17. Two-way TCP • FullTcp connection • set tcp1 [new Agent/TCP/FullTcp] • set tcp2 [new Agent/TCP/FullTcp] • $ns attach-agent $n1 $tcp1 • $ns attach-agent $n2 $tcp2 • $ns connect $tcp1 $tcp2 • $tcp2 listen

18. Application Layer Class Application {Traffic generators} (on top of UDP) {Simulated Applications} (on top of TCP)

19. Creating Traffic: On Top of TCP FTP • set ftp [new Application/FTP] • $ftp attach-agent $tcp • $ns at <time> “$ftp start” Telnet • set telnet [new Application/Telnet] • $telnet attach-agent $tcp

20. Creating Traffic: On Top of UDP • CBR • set src [new Application/Traffic/CBR] • Exponential or Pareto on-off • set src [new Application/Traffic/Exponential] • set src [new Application/Traffic/Pareto]

21. Attaching a traffic source • set cbr [new Application/Traffic/CBR] • $cbr attach-agent $udp • $ns at <time> “$cbr start”

22. Tracing Trace packets on all links: • set f[open out.tr w] • $ns trace-all $f • $ns flush-trace • close $f <event> <time> <from> <to> <pkt> <size>--<flowid> <src> <dst> <seqno> <aseqno> + 1 0 2 cbr 210 ------- 0 0.0 3.1 0 0 - 1 0 2 cbr 210 ------- 0 0.0 3.1 0 0 r 1.00234 0 2 cbr 210 ------- 0 0.0 3.1 0 0 Is tracing all links allways the best thing to do?

23. More Tracing • Tracing specific links • $ns trace-queue $n0 $n1 $f • Tracing variables • set cwnd_chan_ [open all.cwnd w] • $tcp trace cwnd_ • $tcp attach $cwnd_chan

24. Putting it all together set ns [new Simulator] set n0 [$ns node] set n1 [$ns node] $ns duplex-link $n0 $n1 1.5Mb 10ms DropTail Creating Topology set tcp [$ns create-connection TCP $n0 TCPSink $n1 0] Creating Transport layer set ftp [new Application/FTP] $ftp attach-agent $tcp Creating Applications $ns at 0.2 "$ftp start“ $ns at 1.2 ”exit“ Schedule Events $ns trace-queue $n0 $n1 $f $ns run

25. nam – the network animator n0 n1 • set nf [open out.nam w] • $ns namtrace-all $nf • exec nam out.nam & …

26. Computing routes • Unicast • $ns rtproto <type> • <type>: Static, Session, DV.

27. Network Dynamics: Link failures • $ns rtmodel-at <time> <up|down> $n0 $n1 • $ns rtmodel Trace <config_file> $n0 $n1 • $ns rtmodel <model> <params> $n0 $n1 <model>: Deterministic, Exponential

28. Agent/FTP Agent/FTP Agent/FTP Agent/FTP Issues in Simulations • Suppose you want to study the way TCP sources share a bottleneck link… Which topology? Background Traffic? Which traffic sources? When to start sources? What else affects results?

29. Another practical issue: Memory • Avoid trace-all • Use arrays for a sequence of variables • Instead of n$i, say n($i) ~ns/tcl/ex/cmcast-150.tcl: 150 nodes, 2200 links => 53MB 2420 nodes, 2465 linkd => 800MB

30. Basic ns-2: Not Covered • mobile IP • multicasting • satellite • emulation

31. Pure C++ objects Pure OTcl objects C++/OTcl split objects OTcl C++ ns Making changes to ns – where??? • Where would you implement • one-time configuration variables • complex procedures • per packet action

32. ns directory structure ns-allinone Tcl8.0 TK8.0 OTcl tclcl ns-2 nam-1 ... tcl Makefile.in C++ code mcast ... ex test lib examples validation tests OTcl code

33. New component purely in Otcl ns-allinone Tcl8.0 TK8.0 OTcl tclcl ns-2 nam-1 ... tcl Makefile.in C++ code mcast lib ... ex test mysrc msg.tcl validation tests examples OTcl code source msg.tcl

34. New component in C++ • Create C++ class, fill in methods • Define otcl linkage • Write otcl code (if any) • Build (and debug)

35. Pure OTcl objects Pure C++ objects C++/OTcl split objects C++ OTcl ns How does linkage work? • how to access Tcl variables from C++ • how is C++ object created from interpreter • ……

36. TclObject: Hierarchy and Shadowing C++ class hierarchy otcl class hierarchy TclObject TclObject Agent Agent TcpAgent Agent/TCP *tcp _o123 Agent/TCP otcl shadow object Agent/TCP C++ object

37. TclObject • Example set tcp [new Agent/TCP] => how is corresponding C++ object created? $tcp set window_ 500 => how is corresponding C++ variable set? $tcp advance 5000 => how is C++ procedure called?

38. TclObject: Hierarchy and Shadowing C++ class hierarchy otcl class hierarchy TclObject TclObject Agent Agent TcpAgent Agent/TCP *tcp _o123 Agent/TCP otcl shadow object Agent/TCP C++ object

39. TclObject::bind() • Link C++ member variables to otcl object variables • C++ TcpAgent::TcpAgent() { bind(“window_”, &wnd_); … … } • bind_time(), bind_bool(), bind_bw() • otcl $tcp set window_ 200

40. TclObject::command() • Implement otcl methods in C++ • Trap point: otcl method cmd{} • Send all arguments after cmd{} call to TclObject::command()

41. TclObject::command() OTcl space no such procedure $tcp advance TclObject::unknown{} $tcp cmd advance C++ space TcpAgent::command() match “advance”? Yes No Invoke parent: return Agent::command() process and return

42. TclObject::command() • otcl set tcp [new Agent/TCP] $tcp advance 10 • C++ int TcpAgent::command(int argc, const char*const* argv) { if (argc == 3) { if (strcmp(argv[1], “advance”) == 0) { int newseq = atoi(argv[2]); …… return(TCL_OK); } } return (Agent::command(argc, argv); }

43. TclObject • Example set tcp [new Agent/TCP] => how is corresponding C++ object created? $tcp set window_ 500 => how is corresponding C++ variable set? $tcp advance 5000 => how is C++ procedure called? ?

44. Agent/TCP constructor parent constructor TclObject constructor invoke parent constructor create C++ object which C++ object to create? – TclClass complete initialization complete initialization create OTcl shadow object parent (Agent) constructor AgentTCP constructor TclObject (C++) constructor invoke parent constructor invoke parent constructor do nothing, return bind variables and return bind variables and return TclObject: Creation and Deletion invoke parent constructor OTcl C++

45. TclClass • Static class TcpClass : public TclClass { • public: • TcpClass() : TclClass(“Agent/TCP”) {} • TclObject* create(int, const char*const*) { • return (new TcpAgent()); • } • } class_tcp;

46. Class Tcl • Singleton class with a handle to Tcl interpreter • Usage • Invoke otcl procedure • Obtain otcl evaluation results • Pass a result string to otcl • Return success/failure code to otcl

47. Class Tcl Tcl& tcl = Tcl::instance(); if (strcmp(argv[1], “now”) == 0) { tcl.resultf(“%g”, clock()); return TCL_OK; } if (strcmp(argv[1], “helloworld”) { tcl.evalc(“puts stdout Hello World”); return TCL_OK; } Passing results to the interpreter: Executing Otcl commands from C++

48. Class TclCommand • C++ implementation of global otcl commands class RandomCommand : public TclCommand { public: RandomCommand() : TclCommand("ns-random") {} virtual int command(int argc, const char*const* argv); }; int RandomCommand::command(int argc, const char*const* argv) { Tcl& tcl = Tcl::instance(); if (argc == 1) { sprintf(tcl.buffer(), "%u", Random::random()); tcl.result(tcl.buffer()); }

49. Summary