1. Administrivia From santaclaus@northpole.org Thu Nov 8 12:05:31 2001 Date: Thu, 8 Nov 2001 12:04:36 -0500 (EST) From: santaclaus@northpole.org no goodies for you just a lump of coal

2. Administrivia From santaclaus@northpole.org Thu Nov 8 12:05:31 2001 Return-Path: Received: from Princeton.EDU (postoffice.Princeton.EDU [128.112.129.120]) by upright.CS.Princeton.EDU (8.11.6/8.11.6) with ESMTP id fA8H5QQ29528 for ; Thu, 8 Nov 2001 12:05:31 -0500 (EST) Received: from yuma.Princeton.EDU (yuma.Princeton.EDU [128.112.128.89]) by Princeton.EDU (8.9.3/8.9.3) with SMTP id MAA29199 for dpd; Thu, 8 Nov 2001 12:04:36 -0500 (EST) Date: Thu, 8 Nov 2001 12:04:36 -0500 (EST) From: santaclaus@northpole.org Message-Id: Status: RO no goodies for you just a lump of coal

3. Why the internet works TCP/IP Other protocols –http –Smtp Lots of means of testing and seeing the network –Traceroute (on UNIX) (tracert on windows) –ping –www.arin.net (whois command)www.arin.net –www.caida.org (who sees who)www.caida.org http://www.caida.org/projects/internetatlas/gallery/ascore/dem o.xml (step by step)http://www.caida.org/projects/internetatlas/gallery/ascore/dem o.xml

4. When things go wrong Code-red virus –On July 19, 2001 more than 359,000 computers were infected with the Code-Red (CRv2) worm in less than 14 hours. At the peak of the infection frenzy, more than 2,000 new hosts were infected each minute. 43% of all infected hosts were in the United States, while 11% originated in Korea followed by 5% in China and 4% in Taiwan. The.NET Top Level Domain (TLD) accounted for 19% of all compromised machines, followed by.COM with 14% and.EDU with 2%. We also observed 136 (0.04%).MIL and 213 (0.05%).GOV hosts infected by the worm. –VisualizationVisualization

5. The Internet Where did it come from How has it grown Who owns the internet

6. Internet, as we know it 1960’sDoD built ARPANET –Experimental network –Would function even in event of nuclear war 1985 NSF created NSFNET –Based on ARPRANET –National backbone service –Free to educational and research institutions –Corporations (eg Sprint, MCI) built networks Networks linked to NSFNET

7. Internet, as we know it (cont.) 1993 InterNIC created by NSF –directory and database services (AT&T) –registration services (Network Solutions Inc.) –information services (General Atomics/CERFnet) 1989 http created by Tim Berners-Lee –Released to high energy physicists in 1991 –Mosaic in 1993 –Netscape in 1994

8. Internet – key events 1961 Kleinrock –packet-switching theory 1966 Roberts –Plan for ARPANET 1969 –First ARPANET With much help from Hobbes' Internet Timeline http://www.zakon.org/robert/internet/timeline Copyright (c)1993-2000 by Robert H Zakon.

9. First packets sent by Charley Kline at UCLA as he tried logging into SRI. The first attempt resulted in the system crashing as the letter G of LOGIN was entered. (October 29)

10. Internet – key events (cont.) 1973 –Ethernet invented –Email is 75% of ARPANET traffic 1974 –TCP designed 1978 –TCP split into TCP and IP 1980 –First virus

11. Internet – key events (cont.) 1983 –First nameserver –Cut over to TCP/IP 1984 –DNS (Domain Name System) introduced.com,.edu, … –Number of hosts above 1000 1988 –CERT (Computer Emergency Response Team) formed in response to Morris worm

12. Internet – key events (cont.) 1989 –Number of hosts above 100,000 1990 –ARPANET ceases to exist 1993 –Mosaic; WWW proliferates at a 341,634% annual growth rate of service traffic 1994 –25 th anniversary of ARPANET

13. Growth in number of hosts

14. Growth in number of domains

15. Growth in number of networks

16. WWW Growth

17. Who owns the internet? Central organizations –InterNIC and its successors (for names) –ISP’s for transportation The networking community –Setting and following standards IETF ( Internet Engineering Task Force) W3C (World Wide Web Consortium) –Network managers

18. Who owns the internet? (cont.) We all do –Information wants to be free –A lot of good social behavior is required

19. What happens next? Various scenarios… –Which one do you believe?

20. Networking (conclusion) At the bottom –Hardware generating packets –Protocols for packet transmission –Collision, backoff, … In the middle –TCP/IP is the key idea At the top –Some nice tools Ping, traceroute, … are useful –Ports on my machine for http, ftp, fingerd, ping, …

21. A few questions What does it mean to be connected to the internet? –How is the web different from the internet? What about security? –Child security in chatroom –Would/should you give a credit card number What’s next?

22. Programming November 13, 2001

23. Where we are Built a machine Built an operating system to control the machine Now, want to run programs under the operating system

24. What is programming We did machine language –Single instructions that tell the hardware what to do –Primitive Arithmetic, simple branching, communication with memory We built state machines –States using memory –Transitions modeling tasks

25. Problem solving  We’ve seen Truth tables Logic gates States and transitions in a state machine Machine language  Now, higher level programming language

26. To build a computer program Figure out what you want to do –Understand the rules that guide the process you are automating Make sure that your rules are complete Translate the rules into the computer language –Build structures to hold your data –Build tools to manipulate them Make sure that the program does what the rules say

27. Figuring out the rules For traffic lights, –We stored data that told us the current color of lights –We read input from sensors –We had rules that told us whether to change state –We had rules that told us how to change state

28. Light A Traffic Light Behavior IF A=1 AND B=0 Always IF A=0 AND B=1 Otherwise Light B Otherwise Always

29. Turn Memory Into Variables Store data to tell current color of lights –Dim LightA, LightB as Integer ‘ 0 for red, 1 for yellow, 2 for green Read input from sensors –Dim SensorA, SensorB as Integer ‘ tell if cars are waiting

30. Turn Rules Into Statements Decide whether to change state –If LightA = 2 And SensorA = 0 And SensorB = 1 Then ‘ here we want to specify that the colors change –If LightB = 2 And SensorA = 1 And SensorB = 0 Then ‘ again, we want to specify that the colors change

31. Build shell of program Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeGreenToYellow(LightB) ChangeYellowToRed(LightB) ChangeRedToGreen(LightA)

32. Some Rules Statements have to be in blocks How does the computer know that it is If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) And Not If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB)

33. Some Rules Statements have to be in blocks If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) End If

34. More Rules We have to tell the program to loop Do While condition action Loop

35. More Rules We have to tell the program to loop Do While StillWantToControlTraffic RunMyTrafficControlProgram Loop

36. Procedures Must fill in functions to change lights Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub Private Sub ChangeYellowToRed(Light As Integer) Light = 2 End Sub Private Sub ChangeRedToGreen(Light As Integer) Light = 0 End Sub

37. Could build Procedure of Procedures ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) Could become the command ChangeLights(LightA,LightB) Private Sub ChangeLights(Light1 As Integer, Light2 As Integer) ChangeGreenToYellow(Light1) ChangeYellowToRed(Light1) ChangeRedToGreen(Light2) End Sub

38. Using the procedure ChangeLights(LightB,LightA) then does ChangeGreenToYellow(LightB) ChangeYellowToRed(LightB) ChangeRedToGreen(LightA)

39. The program Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub Private Sub ChangeYellowToRed(Light As Integer) Light = 2 End Sub Private Sub ChangeRedToGreen(Light As Integer) Light = 0 End Sub Private Sub ChangeLights(Light1 As Integer, Light2 As Integer) ChangeGreenToYellow(Light1) ChangeYellowToRed(Light1) ChangeRedToGreen(Light2) End Sub

40. The program (cont.) Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeLights(LightA,LightB) End If If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeLights(LightB,LightA)

41. Make it happen forever Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer Dim StillWantToControlTraffic as Integer StillWantToControlTraffic = 1 Do While StillWantToControlTraffic If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeLights(LightA,LightB) End If If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeLights(LightB,LightA) End If Loop

42. What could go wrong? Program could get confused –Check for consistency Replace Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub With Private Sub ChangeGreenToYellow(Light As Integer) If (Light = 0) Then Light = 1 Else ReportInconsistency() End If End Sub

43. Building a bigger program Could write this as a subroutine –Private sub ControlTrafficLight(light1,light2,sensor1,sensor2) Could reuse the subroutine to do a whole string of lights. But how would we keep track of hundreds of lights?

44. Arrays Build arrays –LightNS[1], LightNS[2], LightNS[3], … –LightEW[1]. LightEW[2]. LightEW[3], … –SensorNS[1], SensorNS[2], SensorNS[3], … –SensorEW[1], SensorEW[2], SensorEW[3], …

45. Arrays (con’t). Keep track of things in array ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i]) Control all the traffic lights in Manhattan For i = 1 To 100 ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i]) Next i But, –lights may want to communicate –Light at an intersection carries more information

46. Object oriented programming Figure out characteristics of your data –objects Figure out operations you will want to perform –Methods Modern idea in programming.

47. Objects Traffic light at intersection involves –Lights in each direction Call them red, yellow and green and not 0,1,2 –Sensors in each direction –Timing (rate of change in each direction) Timings needn’t be the same –Neighboring Lights May affect change as much as sensors

48. Methods Method of querying color of light Method of changing color of light Method of scheduling a color change later …

49. What happens to the program? Compiled or interpreted –Eventually it gets translated into machine language If compiled –Can store executable and run again If interpreted –Interpret each time it is executed

50. What does the compiler do? Identifies variables (need space in RAM) –Uses stores and loads to get values to registers Parses commands –Turns each command into a string of machine language commands Sets things up for execution

51. History of Programming Languages Fortran (1954) for scientific Cobol (1959) for business Algol (1958) more universal Fortran Lisp (1958) string/concept oriented APL (1960) formula oriented

52. History of Programming Languages PL/1 (1964) from Algol + Fortran Basic (1964) for everyone to use Simula (1967) combines with Algol to yield Smalltalk (1969) – object oriented BCPL  B  C (1971) Algol  Pascal (1971)  Modula 1,2,3,

53. History of Programming Languages C++ (1983) C with object oriented features –Often C is still used Awk (1978)  Perl (1987) report generators –Web programming language Java (1991) object oriented and portable –Web applets, devices Visual Basic(1991) macros and programs –Core of Microsoft systems

54. What makes a good language Does the task you want Keeps you from making mistakes Supports debugging when you need it Has a strong tool kit

55. Next class Look at a bigger system Some internals of how the compiler works Some notable bugs How do we know if we’ve gotten it right?