Lab 2: Admin details

1. Lab 2: Admin details • Lab 2 is out! • Due: • November 4th, 12 noon • November 6th, 12 noon (90% cap) • Assignment and starter code online • Start early! On average, took students 14 hours to complete Lab 2 last year

2. Functionality

3. Functionality Write an application that: • Takes in data from one host

4. Functionality Write an application that: • Takes in data from one host • Turns those data into packets

5. Functionality Write an application that: • Takes in data from one host • Turns those data into packets • Transmits those packets reliably and in-order to the other host

6. Example

7. Beware the appearance of simplicity

8. Why the Internet isn't just "tubes" • A packet can be corrupted • A packet can be delayed • A packet can be dropped • A packet can be duplicated

9. corrupted delayed dropped duplicated

10. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's

11. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's

12. corrupted delayed dropped duplicated

13. corrupted delayed dropped duplicated Please deposit $100 into my account Please transfer $100 from my bank account to Phil's

14. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's Please deposit $100 into my account Please transfer $100 from my bank account to Phil's Please deposit $100 into my account

15. corrupted delayed dropped duplicated

16. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's

17. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's

18. corrupted delayed dropped duplicated

19. corrupted delayed dropped duplicated Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's

20. Tools for ordered, reliable streams • Checksums • Sequence numbers • Acknowledgments • Timeouts

21. cksums seq nos acks timeouts

22. cksums seq nos acks timeouts Hash: cksum(data) integer

23. cksums seq nos acks timeouts Hash: cksum(data) integer cksum("a") 1

24. cksums seq nos acks timeouts Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's

25. cksums seq nos acks timeouts Tag packet with checksum before sending Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's

26. cksums seq nos acks timeouts Tag packet with checksum before sending Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's cksum("Please ...") cksum("Please ...") Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's

27. cksums seq nos acks timeouts Note: cksum("Please transfer ... $100 ...") cksum("Please transfer ... $14837135 ...")

28. cksums seq nos acks timeouts Note: cksum("Please transfer ... $100 ...") cksum("Please transfer ... $14837135 ...") cksum("Please ...") cksum("Please ...") Please transfer $100 from my bank account to Phil's Please transfer $14837135 from my bank account to Phil's

29. cksums seq nos acks timeouts Monotonically increasing number appended to packet

30. cksums seq nos acks timeouts Monotonically increasing number appended to packet Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's

31. cksums seq nos acks timeouts Monotonically increasing number appended to packet Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152

32. cksums seq nos acks timeouts Monotonically increasing number appended to packet Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152

33. cksums seq nos acks timeouts Monotonically increasing number appended to packet Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152

34. cksums seq nos acks timeouts Monotonically increasing number appended to packet Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152 Sender: Last sequence number sent (and last sequence number ack-d) Receiver: Last sequence number received

35. cksums seq nos acks timeouts Receiver acknowledges received packets If sender does not receive ack, retransmits

36. cksums seq nos acks timeouts Receiver acknowledges received packets If sender does not receive ack, retransmits Please transfer $100 from my bank account to Phil's Seq no: 152

37. cksums seq nos acks timeouts Receiver acknowledges received packets If sender does not receive ack, retransmits Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152

38. cksums seq nos acks timeouts Receiver acknowledges received packets If sender does not receive ack, retransmits Please transfer $100 from my bank account to Phil's Please transfer $100 from my bank account to Phil's Seq no: 152 Seq no: 152 Please transfer $100 from my bank account to Phil's Seq no: 152 Ack no: 153

39. Checksums Sequence numbers Acknowledgments Timeouts Duplicate Corrupt Delay Drop

40. Stop and wait If you have a Sender and a Receiver: • Sender sends a data message. • Sender waits until receiver acknowledges reception • If Sender receives ack, sends its next available packet • If Sender does not receive ack, the Sender re-transmits the data that had not been acknowledged

41. Beware the appearance of simplicity

42. Difficulties • Conceptual • The "little" things

43. Conceptual: Event-based code

44. Conceptual: Event-based code • What are the events? • What parts of code map to events? • How to respond to events? • How track state in? When transition between states?

45. Events • Receive data packet • Receive ack packet • Input goes from empty to having data • Output goes from full to having room • Timer expires • Create a new connection ------ • Receive shutdown from opposite end • Receive shutdown from input

46. Conceptual: Event-based code • What are the events? • What parts of code map to events? • How to respond to events? • How track state in? When transition between states?

47. Events • Create a new connection rel_create • Receive data packet • Receive ack packet rel_recvpkt • Input goes from empty to having data rel_read • Output goes from full to having room rel_output • Timer expires rel_timer

48. Conceptual: Event-based code • What are the events? • What parts of code map to events? • How to respond to events? • How track state in? When transition between states?

49. Events • Create a new connection rel_create • Receive data packet • Receive ack packet rel_recvpkt • Input goes from empty to having data rel_read • Output goes from full to having room rel_output • Timer expires rel_timer

50. Timer expires