1. Using Load-Balancing To Build High-Performance Routers Isaac Keslassy, Shang-Tse (Da) Chuang, Nick McKeown Stanford University

2. 2 R R R R R R Typical Router Architecture Input Switch Fabric Scheduler Output 1 1 2 2 1 1

3. 3  Traffic matrix:  Uniform traffic matrix: λ ij = λ Definitions: Traffic Matrix R R R R R R 1 N i 1 N j

4. 4  100% throughput: for any traffic matrix of row and column sum less than R, λ ij < μ ij Definitions: 100% Throughput R R R R R R 1 N i 1 N j

5. 5 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering

6. 6 Stanford 100Tb/s Router  “Optics in Routers” project  http://yuba.stanford.edu/or/  Some challenging numbers:  100Tb/s  160Gb/s linecards  640 linecards

7. 7 In Out R R R R R R Router capacity = NR Switch capacity = N 2 R 100% Throughput in a Mesh Fabric ? ? ? ? ? ? ? ? ? R R R R R R R R R R R R R

8. 8 R In Out R R R R R R/N If Traffic Is Uniform R R

9. 9 Real Traffic is Not Uniform R In Out R R R R R R/N R R R R R R R R R ?

10. 10 Out R R R R/N Load-Balanced Switch Load-balancing stageForwarding stage In Out R R R R/N R R R 100% throughput for weakly mixing traffic (Valiant, C.-S. Chang)

11. 11 Out R R R R/N In R R R R/N 1 1 2 2 3 3 Load-Balanced Switch

12. 12 Out R R R R/N In R R R R/N 3 3 2 2 1 1 Load-Balanced Switch

13. 13 Out R R R R/N In R R R R/N Intuition: Proof of 100% Throughput  Arrivals to second mesh:  Capacity of second mesh:  Second mesh: arrival rate < service rate

14. 14 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering ?

15. 15 Out R R R R/N In R R R R/N Packet Reordering 1 2

16. 16 Out R R R R/N In R R R R/N Bounding Delay Difference Between Middle Ports 1 2

17. 17 Out R R R R/N In R R R R/N 1 2 3 UFS (Uniform Frame Spreading) 1 2

18. 18 Out R R R R/N In R R R R/N FOFF (Full Ordered Frames First) 1 2

19. 19 FOFF (Full Ordered Frames First)  Input Algorithm  N FIFO queues corresponding to the N output flows  Spread each flow uniformly: if last packet was sent to middle port k, send next to k+1.  Every N time-slots, pick a flow: - If full frame exists, pick it and spread like UFS - Else if all frames are partial, pick one in round-robin order and send it 12 3 1 2 4 N

20. 20 Out R R R R/N In R R R R/N Bounding Reordering 1 2 3

21. 21 FOFF  Output properties  N FIFO queues corresponding to the N middle ports  If there are N 2 packets, one of the head-of-line packets is in order and can depart   Buffer size at most N 2 packets 11 1 2 2 3 3 3 Output 4 N

22. 22 FOFF Properties  Property 1: FOFF maintains packet order.  Property 2: FOFF has O(1) complexity.  Property 3: Congestion buffers operate independently.  Property 4: FOFF maintains an average packet delay within constant from ideal output-queued router.  Corollary: FOFF has 100% throughput for any adversarial traffic.

23. 23 In Out R R R R R R Output-Queued Router ? ? ? ? ? ? ? ? ? R R R R R R R R R R R R R

24. 24 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering

25. 25 Out R R R R/N In R R R R/N From Two Meshes to One Mesh One linecard In Out

26. 26 From Two Meshes to One Mesh First mesh In Out In Out In Out In Out One linecard Second mesh R R R R R

27. 27 From Two Meshes to One Mesh Combined mesh In Out In Out In Out In Out 2R R

28. 28 Many Fabric Options Options Space: Full uniform mesh Time: Round-robin crossbar Wavelength: Static WDM Any spreading device C 1, C 2, …, C N C1C1 C2C2 C3C3 CNCN In Out In Out In Out In Out N channels each at rate 2R/N One linecard

29. 29 AWGR (Arrayed Waveguide Grating Router) A Passive Optical Component  Wavelength i on input port j goes to output port (i+j-1) mod N  Can shuffle information from different inputs  1,  2 …  N NxN AWGR Linecard 1 Linecard 2 Linecard N  1  2  N Linecard 1 Linecard 2 Linecard N

30. 30 In Out In Out In Out In Out Static WDM Switching: Packaging AWGR Passive and Almost Zero Power A B C D A, B, C, D A, A, A, A B, B, B, B C, C, C, C D, D, D, D N WDM channels, each at rate 2R/N

31. 31 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering

32. 32 Scaling Problem  For N < 64, an AWGR is a good solution.  We want N = 640.  Need to decompose.

33. 33 A Different Representation of the Mesh In Out In Out In Out In Out R 2R Mesh 2R In Out In Out In Out In Out R 2R R

34. 34 A Different Representation of the Mesh In Out In Out In Out In Out R In Out In Out In Out In Out R 2R/N

35. 35 1 2 3 4 Example: N=8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 2R/8

36. 36 When N is Too Large Decompose into groups (or racks) 4R/4 2R2R2R2R 1 2 3 4 5 6 7 8 2R2R 2R2R 1 2 3 4 5 6 7 8 4R

37. 37 When N is Too Large Decompose into groups (or racks) 12L 2R 12L Group/Rack 1 Group/Rack G 12L 2R Group/Rack 1 12L 2R Group/Rack G 2RL 2RL/G

38. 38 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering

39. 39 When Linecards Fail 12L 2R 12L Group/Rack 1 Group/Rack G 12L 2R Group/Rack 1 12L 2R Group/Rack G 2RL 2RL/G 2RL Solution: replace mesh with sum of permutations = + + 2RL/G ≤ 2RL 2RL/G G *

40. 40 Hybrid Electro-Optical Architecture Using MEMS Switches 12L 2R 12L Group/Rack 1 Group/Rack G 12L 2R Group/Rack 1 12L 2R Group/Rack G MEMS Switch MEMS Switch

41. 41 When Linecards Fail 12L 2R 12L Group/Rack 1 Group/Rack G 12L 2R Group/Rack 1 12L 2R Group/Rack G MEMS Switch MEMS Switch

42. 42 Fiber Link Capacity 12L 2R 12L Group/Rack 1 Group/Rack G 12L 2R Group/Rack 1 12L 2R Group/Rack G MEMS Switch MEMS Switch MEMS Switch Link Capacity ≈ 64 λ’s * 5 Gb/s/λ = 320 Gb/s = 2R      Laser/ Modulator    MUX

43. 43 Group/Rack 1 1 2 2R 4R Group/Rack 2 12 2R 4R Number of MEMS Switches Example: 2 Groups of 2 Linecards 12 2R Group/Rack 1 12 2R Group/Rack 2 4R 2R

44. 44  Theorem: M≡L+G-1 MEMS switches are sufficient for bandwidth. Number of MEMS Switches  Examples:  G groups, L i linecards in group i,

45. 45 Implementation of a 100Tb/s Load-Balanced Router Linecard Rack 1 L = 16 160Gb/s linecards 5556 12 40 x 40 static MEMS Switch Rack < 100W L = 16 160Gb/s linecards Linecard Rack G = 40 L = 16 160Gb/s linecards

46. 46 Group A 1 2 2R 4R Group B 12 2R 4R Packet Schedule 12 2R Group A 12 2R Group B 4R 2R

47. 47 At each time-slot:  Each transmitting linecard sends one packet  Each receiving linecard receives one packet  (MEMS constraint) Each transmitting group i sends at most one packet to each receiving group j through each MEMS connecting them In a schedule of N time-slots:  Each transmitting linecard sends exactly one packet to each receiving linecard Rules for Packet Schedule

48. 48 Packet Schedule T+1T+2T+3T+4 Tx LC A1???? Tx LC A2???? Tx LC B1???? Tx LC B2???? Tx Group A Tx Group B

49. 49 Packet Schedule T+1T+2T+3T+4 Tx LC A1A1A2B1B2 Tx LC A2B2A1A2B1 Tx LC B1B1B2A1A2 Tx LC B2A2B1B2A1 Tx Group A Tx Group B

50. 50 Bad Packet Schedule T+1T+2T+3T+4 Tx LC A1A1A2B1B2 Tx LC A2B2A1A2B1 Tx LC B1B1B2A1A2 Tx LC B2A2B1B2A1 Tx Group A Tx Group B

51. 51 Group Schedule T+1T+2T+3T+4 Tx Group AAB Tx Group BAB

52. 52 Good Packet Schedule T+1T+2T+3T+4 Tx LC A1A1A2B1B2 Tx LC A2B2B1A2A1 Tx LC B1B1B2A1A2 Tx LC B2A2A1B2B1  Theorem: There exists a polynomial-time algorithm that finds the correct packet schedule.  Verilog implementation < 50ms. Tx Group A Tx Group B

53. 53 Router Wish List Scale to High Linecard Speeds  No Centralized Scheduler  Optical Switch Fabric  Low Packet-Processing Complexity Scale to High Number of Linecards  High Number of Linecards  Arbitrary Arrangement of Linecards Provide Performance Guarantees  100% Throughput Guarantee  Delay Guarantee  No Packet Reordering

54. 54 Summary  The load-balanced switch  Does not need any centralized scheduling  Can use a mesh  Using FOFF  It keeps packets in order  It guarantees 100% throughput  Using the hybrid electro-optical architecture  It scales to high port numbers  It tolerates linecard failure

55. 55 References  Initial Work  C.-S. Chang, D.-S. Lee and Y.-S. Jou, "Load Balanced Birkhoff- von Neumann Switches, part I: One-Stage Buffering," Computer Communications, Vol. 25, pp. 611-622, 2002.  Extensions  I. Keslassy, S.-T. Chuang, K. Yu, D. Miller, M. Horowitz, O. Solgaard and N. McKeown, "Scaling Internet Routers Using Optics," ACM SIGCOMM '03, Karlsruhe, Germany, August 2003.  I. Keslassy, S.-T. Chuang and N. McKeown, “ A Load-Balanced Switch with an Arbitrary Number of Linecards, ” IEEE Infocom ’ 04, Hong Kong, March 2004.

56. Thank you.