EECS 122 Introduction to Computer Networks (Fall 2004) Network simulator 2 (ns-2)

1. EECS 122Introduction to Computer Networks(Fall 2004)Network simulator 2 (ns-2) Department of Electrical Engineering and Computer Sciences University of California Berkeley Slides: K. Fall, J. Heidemann, P. Huang, K. Lai, A. Parekh, I. Stoica, S. Shenker, J. Walrand

2. EECS 122: Introduction to Computer Networks Network Simulator ns2 Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

3. Outline • Goals • Discrete event simulation • Basic ns-2 • Examples • Project requirements

4. What is ns-2? • Discrete event simulator • Models network protocols • Wired, wireless, satellite • TCP, UDP, multicast, unicast • Web, telnet, ftp • Ad-hoc routing, sensor networks • Infrastructure: stats, tracing, error models, etc. • Multiple levels of detail in one simulator

5. Why simulate? • Can examine protocol in controlled environment • Repeatable experiments • Alternatives: • Experimentation: operational details, but: limited scale, limited flexibility • Analysis: can provide deeper understanding, but: ignores implementation details

6. ns-2 components • ns: the Network Simulation engine • executes tcl scripts containing simulation setup and events • nam: the Network AniMator • visualize ns output tcl script (specification of experiment) trace file (output) ns-2 nam

7. Discrete event simulation • model world as events • maintain queue of events, ordered by time • maintain [virtual time] • repeat: • extract event at head, set [virtual time]=event’s time • process it • if processing generates another event, add it to queue • each event takes predefined amount of virtual time, arbitrary amount of real time • having a slow CPU makes simulation run slower (in real time), but doesn’t change result

8. A. B. C. D. Discrete event example S D Event queue S D

9. programming language used for setting up simulation environment object oriented interpreted (slow) Used for Setting up topology Placing agents Injecting events Configuring tracing oTcl overview Examples: • variables • set x 10 • puts “x is $x” • expressions • set y [pow x 2] • set y [expr x+x*3] • control • if ($x>0) { return $x } else { return [expr -$x] } • while ($x >0) { puts $x • set x [eval x+1] }

10. Basic ns-2 • Create scheduler • set ns [new Simulator] • Create node • set <var> [$ns node] • example: set n0 [$ns node] • Create link • $ns <link-type> <node1> <node2> <bandwidth> <delay> <queuetype> • example: $ns duplex-link $n0 $n1 10Mb 100ms DropTail • Schedule event • $ns at <time> <event> • example: $ns at 10.0 “$ftp start” • Start scheduler • $ns run

11. #Create a simulator object set ns [new Simulator] #Create three nodes set n0 [$ns node] set n1 [$ns node] set n2 [$ns node] #Create link between the nodes $ns duplex-link $n0 $n1 4Mb 10ms DropTail $ns duplex-link $n2 $n1 1Mb 10ms DropTail $ns queue-limit $n1 $n2 10 #Create a TCP agent and attach it to node n0 set tcp0 [new Agent/TCP] $ns attach-agent $n0 $tcp0 #Create a TCP sink agent and attach it to node n2 set sink [new Agent/TCPSink] $ns attach-agent $n2 $sink #Connect both agents $ns connect $tcp0 $sink # create an FTP source set ftp [new Application/FTP] $ftp set maxpkts_ 1000 $ftp attach-agent $tcp0 #Inject starting events $ns at 0.0 "$ftp start" $ns at 10.0 "$ftp stop" $ns at 10.1 "finish" #Run the simulation $ns run Example ftp tcp tcp-sink n0 n1 n2

12. nam visualization demo • Now I show a an instance of nam running the example on the previous slide… We just saw this… Now I will show you this… tcl script (specification of experiment) trace file (output) ns-2 nam

13. Analysis

14. Project 2: ns2 -- overview • Goal: learn how to simulate networks with ns-2, understand how different TCP behaves • Task 1 • Familiarize with ns-2 • Write basic scripts to define network topologies, to specify transport agents, to log traces, to parse trace, and to mine the trace data to understand how the transport protocol behaves • Task 2 • Reverse engineering • Given a blackbox TCP implementation, you figure what the blackbox is. • Understand how different TCP behaves under packet losses. • Refer to the reference papers in the last slide.

15. Project 2: ns2 -- requirements • Project details: • Project 2 web page • You MUST: • use the version of ns2 supplied on the instructional machines • be familiar with the simulation results and the contents of your report, be prepared to answer questions • Project due 11:59PM, October 28.

16. Reference Papers • [1] K. Fall and S. Floyd. Simulation-based Comparisons of Tahoe, Reno, and SACK TCP. Computer Communication Review, 1996. (We strongly recommend to read this paper for project 2.) • [2] L. Brakmo, S. O'Malley, and L. Peterson. TCP Vegas: New Techniques for Congestion Detection and Avoidance. Proc. of SIGCOMM, 1994. (This paper explains TCP Vegas. If you feel the textbook does not cover enough about TCP vegas, please read this paper.) • [3] V. Jacobson and M. J. Karels. Congestion Avoidance and Control. Proc. of SIGCOMM, 1988. (This is the seminal paper in TCP congestion control.) • [4] D. Chiu and R. Jain. Analysis of the Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks. Computer Networks and ISDN Systems, 1989. (If you want to know how AIMD achieves efficiency and fairness more in detail, please read this one.)