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Lecture 2: Congestion Control and Avoidance

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Wei Tsang Ooi
Wei Tsang Ooi
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Congestion Avoidance! and Control 21 August 2009 CS5229 Semester 1 2009/10 1
Van Jacobson, “Congestion Avoidance and Control”, " SIGCOMM 1988 21 August 2009 CS5229 Semester 1 2009/10 2
Fixes to TCP in BSD" Handwaving arguments" Less rigorous math" Lots of “magical” hacks 21 August 2009 CS5229 Semester 1 2009/10 3
We assume " - the sender always has data to send" - each packet is of the same size" - TCP is message-oriented 21 August 2009 CS5229 Semester 1 2009/10 4
1986 21 August 2009 CS5229 Semester 1 2009/10 6
TCP throughput from LBL to UC Berkeley (two hops) dropped from 32K bps to 40 bps. 21 August 2009 CS5229 Semester 1 2009/10 7
RFC793 21 August 2009 CS5229 Semester 1 2009/10 8
Sending window = receiving window" No congestion control" Retransmit only when timeout 21 August 2009 CS5229 Semester 1 2009/10 9
Congestion Collapse: " sender sends too fast" routers delay/drop packets" sender retransmit" no useful data getting through 21 August 2009 CS5229 Semester 1 2009/10 10 10
21 August 2009 CS5229 Semester 1 2009/10 11 11
Observation: a TCP connection should obey" Conservation ! of ! Packets 21 August 2009 CS5229 Semester 1 2009/10 12 12
In equilibrium state, a new packet is not inserted until an old packet leaves. 21 August 2009 CS5229 Semester 1 2009/10 13 13
21 August 2009 CS5229 Semester 1 2009/10 14 14
How could this principle be violated? 21 August 2009 CS5229 Semester 1 2009/10 15 15
1. Never reaches equilibrium 21 August 2009 CS5229 Semester 1 2009/10 16 16
2. Inject a packet before the next packet leaves 21 August 2009 CS5229 Semester 1 2009/10 17 17
1. Getting to the equilibrium state 21 August 2009 CS5229 Semester 1 2009/10 18 18
Equilibrium state: " self-clocking 21 August 2009 CS5229 Semester 1 2009/10 19 19
21 August 2009 CS5229 Semester 1 2009/10 20 20
How to start the ‘clock’? 21 August 2009 CS5229 Semester 1 2009/10 21 21
Slow Start 21 August 2009 CS5229 Semester 1 2009/10 22 22
Add a new variable cwnd." Start/Restart: cwnd = 1." Upon receiving ACK, cwnd++." Send at most min(cwnd,rwin) 21 August 2009 CS5229 Semester 1 2009/10 23 23
21 August 2009 CS5229 Semester 1 2009/10 24 24
Never send more than 2x the max possible rate." (previously 200x is possible!) 21 August 2009 CS5229 Semester 1 2009/10 25 25
21 August 2009 CS5229 Semester 1 2009/10 26 26
2. Inject a packet before the next packet leaves 21 August 2009 CS5229 Semester 1 2009/10 27 27
2. Conservation at Equilibrium 21 August 2009 CS5229 Semester 1 2009/10 28 28
Something’s wrong with TCP timer 21 August 2009 CS5229 Semester 1 2009/10 29 29
21 August 2009 CS5229 Semester 1 2009/10 30 30
TCP (RFC793) Ri : smoothed RTT Mi : measured RTT RTO : timeout value 21 August 2009 CS5229 Semester 1 2009/10 31 31
Variation in RTT when network is loaded 21 August 2009 CS5229 Semester 1 2009/10 32 32
β = 2 (recommended) tolerates only 30% load 21 August 2009 CS5229 Semester 1 2009/10 33 33
Idea: estimate the variation and use in calculating RTO 21 August 2009 CS5229 Semester 1 2009/10 34 34
Measuring Variation" variance: " costly (need to square)" mean error:" simpler 21 August 2009 CS5229 Semester 1 2009/10 35 35
21 August 2009 CS5229 Semester 1 2009/10 36 36
21 August 2009 CS5229 Semester 1 2009/10 38 38
21 August 2009 CS5229 Semester 1 2009/10 39 39
To prevent spurious timeout, 21 August 2009 CS5229 Semester 1 2009/10 40 40
To pick a value of k, consider bandwidth- dominated link. 21 August 2009 CS5229 Semester 1 2009/10 41 41
21 August 2009 CS5229 Semester 1 2009/10 42 42
R doubles each round during slow-start. 21 August 2009 CS5229 Semester 1 2009/10 43 43
21 August 2009 CS5229 Semester 1 2009/10 44 44
21 August 2009 CS5229 Semester 1 2009/10 45 45
21 August 2009 CS5229 Semester 1 2009/10 46 46
21 August 2009 CS5229 Semester 1 2009/10 47 47
3. Moving towards new equilibrium when path changes 21 August 2009 CS5229 Semester 1 2009/10 48 48
Idea: adjust cwnd when congestion happens 21 August 2009 CS5229 Semester 1 2009/10 49 49
Assume: congestion leads to packet loss, leads to timeout. 21 August 2009 CS5229 Semester 1 2009/10 50 50
On timeout, cwnd /= 2" On ACK, cwnd += 1/cwnd 21 August 2009 CS5229 Semester 1 2009/10 51 51
Why drop by half ?" 1. Slow-start: " we know R/2 works" 2. Steady state:" a new flow probably? 21 August 2009 CS5229 Semester 1 2009/10 52 52
Combining " slow-start " and " congestion avoidance 21 August 2009 CS5229 Semester 1 2009/10 53 53
TCP Tahoe 21 August 2009 CS5229 Semester 1 2009/10 54 54
21 August 2009 CS5229 Semester 1 2009/10 55 55
21 August 2009 CS5229 Semester 1 2009/10 56 56
cwnd: " “pipe size” probed" ssthresh: " “pipe size” during equilibrium 21 August 2009 CS5229 Semester 1 2009/10 57 57
new ack:" if (cwnd < ssthresh)" cwnd += 1" else" cwnd += 1/cwnd 21 August 2009 CS5229 Semester 1 2009/10 58 58
timeout/3rd dup ack:" retransmit all unacked" ssthresh = cwnd/2" cwnd = 1" 21 August 2009 CS5229 Semester 1 2009/10 59 59
Improving TCP Tahoe:" Packets still getting through in dup ack -- no need to reset the clock! 21 August 2009 CS5229 Semester 1 2009/10 60 60
TCP Reno 21 August 2009 CS5229 Semester 1 2009/10 61 61
timeout:" retransmit all unacked" ssthresh = cwnd/2" cwnd = 1" 21 August 2009 CS5229 Semester 1 2009/10 71 71
3rd duplicate ACK:! fast retransmission! (ie, retransmit 1st unack) " fast recovery" (details in Week 4)" ssthresh = cwnd = cwnd/2 21 August 2009 CS5229 Semester 1 2009/10 72 72
21 August 2009 CS5229 Semester 1 2009/10 73 73
AIMD! additive increase! multiplicative decrease 21 August 2009 CS5229 Semester 1 2009/10 74 74
Chiu and Jain, “Analysis of Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks”, Comp. Net. & ISDN Sys. 1989 21 August 2009 CS5229 Semester 1 2009/10 80 80

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Lecture 2: Congestion Control and Avoidance

  • 1. Congestion Avoidance! and Control 21 August 2009 CS5229 Semester 1 2009/10 1
  • 2. Van Jacobson, “Congestion Avoidance and Control”, " SIGCOMM 1988 21 August 2009 CS5229 Semester 1 2009/10 2
  • 3. Fixes to TCP in BSD" Handwaving arguments" Less rigorous math" Lots of “magical” hacks 21 August 2009 CS5229 Semester 1 2009/10 3
  • 4. We assume " - the sender always has data to send" - each packet is of the same size" - TCP is message-oriented 21 August 2009 CS5229 Semester 1 2009/10 4
  • 5. 1986 21 August 2009 CS5229 Semester 1 2009/10 6
  • 6. TCP throughput from LBL to UC Berkeley (two hops) dropped from 32K bps to 40 bps. 21 August 2009 CS5229 Semester 1 2009/10 7
  • 7. RFC793 21 August 2009 CS5229 Semester 1 2009/10 8
  • 8. Sending window = receiving window" No congestion control" Retransmit only when timeout 21 August 2009 CS5229 Semester 1 2009/10 9
  • 9. Congestion Collapse: " sender sends too fast" routers delay/drop packets" sender retransmit" no useful data getting through 21 August 2009 CS5229 Semester 1 2009/10 10 10
  • 10. 21 August 2009 CS5229 Semester 1 2009/10 11 11
  • 11. Observation: a TCP connection should obey" Conservation ! of ! Packets 21 August 2009 CS5229 Semester 1 2009/10 12 12
  • 12. In equilibrium state, a new packet is not inserted until an old packet leaves. 21 August 2009 CS5229 Semester 1 2009/10 13 13
  • 13. 21 August 2009 CS5229 Semester 1 2009/10 14 14
  • 14. How could this principle be violated? 21 August 2009 CS5229 Semester 1 2009/10 15 15
  • 15. 1. Never reaches equilibrium 21 August 2009 CS5229 Semester 1 2009/10 16 16
  • 16. 2. Inject a packet before the next packet leaves 21 August 2009 CS5229 Semester 1 2009/10 17 17
  • 17. 1. Getting to the equilibrium state 21 August 2009 CS5229 Semester 1 2009/10 18 18
  • 18. Equilibrium state: " self-clocking 21 August 2009 CS5229 Semester 1 2009/10 19 19
  • 19. 21 August 2009 CS5229 Semester 1 2009/10 20 20
  • 20. How to start the ‘clock’? 21 August 2009 CS5229 Semester 1 2009/10 21 21
  • 21. Slow Start 21 August 2009 CS5229 Semester 1 2009/10 22 22
  • 22. Add a new variable cwnd." Start/Restart: cwnd = 1." Upon receiving ACK, cwnd++." Send at most min(cwnd,rwin) 21 August 2009 CS5229 Semester 1 2009/10 23 23
  • 23. 21 August 2009 CS5229 Semester 1 2009/10 24 24
  • 24. Never send more than 2x the max possible rate." (previously 200x is possible!) 21 August 2009 CS5229 Semester 1 2009/10 25 25
  • 25. 21 August 2009 CS5229 Semester 1 2009/10 26 26
  • 26. 2. Inject a packet before the next packet leaves 21 August 2009 CS5229 Semester 1 2009/10 27 27
  • 27. 2. Conservation at Equilibrium 21 August 2009 CS5229 Semester 1 2009/10 28 28
  • 28. Something’s wrong with TCP timer 21 August 2009 CS5229 Semester 1 2009/10 29 29
  • 29. 21 August 2009 CS5229 Semester 1 2009/10 30 30
  • 30. TCP (RFC793) Ri : smoothed RTT Mi : measured RTT RTO : timeout value 21 August 2009 CS5229 Semester 1 2009/10 31 31
  • 31. Variation in RTT when network is loaded 21 August 2009 CS5229 Semester 1 2009/10 32 32
  • 32. β = 2 (recommended) tolerates only 30% load 21 August 2009 CS5229 Semester 1 2009/10 33 33
  • 33. Idea: estimate the variation and use in calculating RTO 21 August 2009 CS5229 Semester 1 2009/10 34 34
  • 34. Measuring Variation" variance: " costly (need to square)" mean error:" simpler 21 August 2009 CS5229 Semester 1 2009/10 35 35
  • 35. 21 August 2009 CS5229 Semester 1 2009/10 36 36
  • 36. 21 August 2009 CS5229 Semester 1 2009/10 38 38
  • 37. 21 August 2009 CS5229 Semester 1 2009/10 39 39
  • 38. To prevent spurious timeout, 21 August 2009 CS5229 Semester 1 2009/10 40 40
  • 39. To pick a value of k, consider bandwidth- dominated link. 21 August 2009 CS5229 Semester 1 2009/10 41 41
  • 40. 21 August 2009 CS5229 Semester 1 2009/10 42 42
  • 41. R doubles each round during slow-start. 21 August 2009 CS5229 Semester 1 2009/10 43 43
  • 42. 21 August 2009 CS5229 Semester 1 2009/10 44 44
  • 43. 21 August 2009 CS5229 Semester 1 2009/10 45 45
  • 44. 21 August 2009 CS5229 Semester 1 2009/10 46 46
  • 45. 21 August 2009 CS5229 Semester 1 2009/10 47 47
  • 46. 3. Moving towards new equilibrium when path changes 21 August 2009 CS5229 Semester 1 2009/10 48 48
  • 47. Idea: adjust cwnd when congestion happens 21 August 2009 CS5229 Semester 1 2009/10 49 49
  • 48. Assume: congestion leads to packet loss, leads to timeout. 21 August 2009 CS5229 Semester 1 2009/10 50 50
  • 49. On timeout, cwnd /= 2" On ACK, cwnd += 1/cwnd 21 August 2009 CS5229 Semester 1 2009/10 51 51
  • 50. Why drop by half ?" 1. Slow-start: " we know R/2 works" 2. Steady state:" a new flow probably? 21 August 2009 CS5229 Semester 1 2009/10 52 52
  • 51. Combining " slow-start " and " congestion avoidance 21 August 2009 CS5229 Semester 1 2009/10 53 53
  • 52. TCP Tahoe 21 August 2009 CS5229 Semester 1 2009/10 54 54
  • 53. 21 August 2009 CS5229 Semester 1 2009/10 55 55
  • 54. 21 August 2009 CS5229 Semester 1 2009/10 56 56
  • 55. cwnd: " “pipe size” probed" ssthresh: " “pipe size” during equilibrium 21 August 2009 CS5229 Semester 1 2009/10 57 57
  • 56. new ack:" if (cwnd < ssthresh)" cwnd += 1" else" cwnd += 1/cwnd 21 August 2009 CS5229 Semester 1 2009/10 58 58
  • 57. timeout/3rd dup ack:" retransmit all unacked" ssthresh = cwnd/2" cwnd = 1" 21 August 2009 CS5229 Semester 1 2009/10 59 59
  • 58. Improving TCP Tahoe:" Packets still getting through in dup ack -- no need to reset the clock! 21 August 2009 CS5229 Semester 1 2009/10 60 60
  • 59. TCP Reno 21 August 2009 CS5229 Semester 1 2009/10 61 61
  • 60. timeout:" retransmit all unacked" ssthresh = cwnd/2" cwnd = 1" 21 August 2009 CS5229 Semester 1 2009/10 71 71
  • 61. 3rd duplicate ACK:! fast retransmission! (ie, retransmit 1st unack) " fast recovery" (details in Week 4)" ssthresh = cwnd = cwnd/2 21 August 2009 CS5229 Semester 1 2009/10 72 72
  • 62. 21 August 2009 CS5229 Semester 1 2009/10 73 73
  • 63. AIMD! additive increase! multiplicative decrease 21 August 2009 CS5229 Semester 1 2009/10 74 74
  • 64. Chiu and Jain, “Analysis of Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks”, Comp. Net. & ISDN Sys. 1989 21 August 2009 CS5229 Semester 1 2009/10 80 80