1. How packets are routed in the Internet

2. The Problem A B How should packets travel from A to B?

3. Hop-by-hop forwarding IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3 DataB

4. How do the routers know what forwarding table to use?

5. Easy!! IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3

6. But how about this network?

7. In a future Unit you will learn how the forwarding tables are populated. Today, we’ll explore different approaches to get you thinking about it.

8. 8 A Game: Routing with Cards

9. You should have one card Keep card face down, do not look at it until I say so…. Your card tells you your node number, and who your neighbors are. 9

10. 5 12, 19 Your node ID Ignore these numbers for now!!

11. 2 2 3 3 4 4 5 5 6 6 7 7 8 8 1 1 15 14 13 12 11 10 9 9 16 18 19 20 21 22 23 24 17 31 30 29 28 27 26 25 32

12. 2 2 3 3 4 4 5 5 6 6 7 7 8 8 1 1 15 14 13 12 11 10 9 9 16 18 19 20 21 22 23 24 17 31 30 29 28 27 26 25 32

13. 5 12, 19 Your node ID Your neighbor nodes

14. 5 12, 19 12 19 3 5, 19 5, 12, 3

15. Rules You may not – Leave your seat (but you can stand) – Pass your card – Write anything You may – Ask nearby friends for advice – Shout to other participants (anything you want) – Curse your instructor You must: Participate 15

16. Task #1 Find a path from Node 1 to Node 40. When you are done, you must be able to repeat it correctly.

17. Go! 17

18. Task #2 Find the shortest path from Node 1 to Node 40. When you are done, you must be able to repeat it correctly.

19. An algorithm to find the shortest path

20. 1 1 24 14 27 25 15 9 9 6 6 10 32 26 30 12 4 4 23 22 3 3 29 31 40 7 7 28 17 5 5 19 8 8 20 13 11 21 18 16 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 55 5 6 5 6 6 7 8 88 7 7 9 10 11 9 12 10 12 10 11 12 4

21. 1 1 24 14 27 25 15 9 9 6 6 10 32 26 30 12 4 4 23 22 3 3 29 31 40 7 7 28 17 5 5 19 8 8 20 13 11 21 18 16 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 55 5 6 5 6 6 7 8 88 7 7 9 10 11 9 12 10 12 10 11 12 4

22. What if each link has a “cost”?

23. R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R3 1 4 4 2 9 4 1 1 5 1 5 2 “Expensive link”: It might be very long. e.g. a link from Europe to USA. Or it might be very busy. e.g. it connects to Google or CNN. Or it may be very slow. e.g. 1Mb/s instead of 100Mb/s. Cost = 4+9 = 13 Cost = 1+4+4+2 = 11

24. Find lowest cost path to R 8 R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R3 1 4 4 2 9 4 1 1 5 1 5 2 2 9 5 1 6 2 7 6 14 13

25. Find lowest cost path to R 8 R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R3 1 4 4 2 9 4 1 1 5 1 5 2 2 9 5 1 6 2 7 6 14 13 6 7 456

26. 1 1 24 14 27 25 15 9 9 6 6 10 32 26 30 12 4 4 23 22 3 3 29 31 40 7 7 28 17 5 5 19 8 8 20 13 11 21 18 Find the lowest cost path 16 1 6 1 1 1 1 1 7 1 1 1 2 7 1 1 1 12 3 1 4 3 1 1 10 1 1 8 1 4 11 1 5 6 2 1 1 1 1 1

27. 1 1 24 14 27 25 15 9 9 6 6 10 32 26 30 12 4 4 23 22 3 3 29 31 40 7 7 28 17 5 5 19 8 8 20 13 11 21 18 Solution 16 1 6 1 1 1 1 1 7 1 1 1 2 7 1 1 1 12 3 1 4 3 1 1 10 1 1 8 1 4 11 1 5 6 2 1 1 1 1 1 Cost = 22

28. How packets are routed in the Internet

29. Stanford University Istanbul Technical University The Public Internet Every organization is “autonomous” – they decide how packets are routed inside. They run our algorithm!

30. Stanford University Istanbul Technical University Every organization can run its own algorithm inside! AT&T Superonline

31. Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline A B C X Y Z Internet routing is a hierarchy

32. Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline He.net A B C X Y Z Internet routing is a hierarchy

33. Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline He.net IP AddressNext-hop AR2 B CR3 ELSER-Boundary-1 IP AddressNext-hop ITUR-Boundary-2 SUR8 A B C X Y Z R-Boundary-2 R-Boundary-3 R8 R2 R3 IP AddressNext-hop ITUR100 SUR-Boundary-2 R100

34. Summary

35. A B How should packets travel from A to B?

36. Hop-by-hop forwarding IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3 DataB

37. How do the routers know what forwarding table to use? Now you know!

38. Easy!