Lecture 2: Congestion Control and Avoidance
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The document summarizes Van Jacobson's 1988 paper on congestion avoidance and control in TCP. It describes how the original TCP specification led to congestion collapse in 1986 when network loads increased. Jacobson proposed modifications to TCP including slow start, congestion avoidance, and estimating retransmission timeouts (RTOs) based on measured variance in round-trip times. This introduced the concepts of additive increase, multiplicative decrease to gradually probe and respond to available bandwidth without overloading networks. The TCP Tahoe and Reno algorithms incorporated these ideas to provide early congestion control and avoidance in TCP.
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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