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Lecture 4: TCP and TFRC

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Wei Tsang Ooi
Wei Tsang Ooi
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TCP" Reno, NewReno, SACK 4 September 2009 CS5229, Semester 1, 2009/10 1
TCP Reno 4 September 2009 CS5229, Semester 1, 2009/10 2
14 cwnd ssthresh 12 10 8 6 4 2 0 4 September 2009 CS5229, Semester 1, 2009/10 3
new ack:" if (cwnd < sstresh)" cwnd += 1" else" cwnd += 1/cwnd 4 September 2009 CS5229, Semester 1, 2009/10 5
timeout:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = 1" 4 September 2009 CS5229, Semester 1, 2009/10 6
3rd duplicate ACK:" fast retransmission" (ie, retransmit 1st unack) " fast recovery" (details today)" ssthresh = cwnd = cwnd/2 4 September 2009 CS5229, Semester 1, 2009/10 7
TCP’s rule" send more packets if " L + cwnd > H" [ L .. H-1 ] are outstanding packets 4 September 2009 CS5229, Semester 1, 2009/10 10
4 September 2009 CS5229, Semester 1, 2009/10 11
Data Packets ACK Packets (received up to this seq number) Congestion Window Sequence Number Time 4 September 2009 CS5229, Semester 1, 2009/10 12
3rd dup ack:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 4 September 2009 CS5229, Semester 1, 2009/10 13
Incorrectly clamping the congestion window too soon (without fast recovery) X 4 September 2009 CS5229, Semester 1, 2009/10 14
fast recovery:" keep the pipe occupied" 4 September 2009 CS5229, Semester 1, 2009/10 15
X 4 September 2009 CS5229, Semester 1, 2009/10 16
4 September 2009 CS5229, Semester 1, 2009/10 17
X X 4 September 2009 CS5229, Semester 1, 2009/10 18
TCP Reno timeout with multiple losses in a window 4 September 2009 CS5229, Semester 1, 2009/10 19
TCP NewReno 4 September 2009 CS5229, Semester 1, 2009/10 20
X X 4 September 2009 CS5229, Semester 1, 2009/10 21
4 September 2009 CS5229, Semester 1, 2009/10 22
3rd dup ack:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 + 3" remember highest 4 September 2009 CS5229, Semester 1, 2009/10 25
“complete” ack:" (all are acked) " cwnd = ssthresh 4 September 2009 CS5229, Semester 1, 2009/10 26
“partial” ack:" (acknowledge n packets)" retransmit" cwnd = cwnd - n + 1 4 September 2009 CS5229, Semester 1, 2009/10 27
Note: RFC2581/RFC2582 give the accurate/gory details. Simplified version is presented here (eg. cwnd vs FlightSize, update of cwnd upon partial ACK). 4 September 2009 CS5229, Semester 1, 2009/10 29
What does a dup ACK tell us? 4 September 2009 CS5229, Semester 1, 2009/10 30
“Coarse Feedback” 4 September 2009 CS5229, Semester 1, 2009/10 31
TCP SACK 4 September 2009 CS5229, Semester 1, 2009/10 32
Use TCP header options to report received segments. 4 September 2009 CS5229, Semester 1, 2009/10 33
SACK Blocks:" 1st block - report most recently received segments" subsequent blocks - repeat most recent previous blocks 4 September 2009 CS5229, Semester 1, 2009/10 34
pipe: num of outstanding packets in the pipe." send only if pipe < cwnd 4 September 2009 CS5229, Semester 1, 2009/10 35
scoreboard: which packets have been received? 4 September 2009 CS5229, Semester 1, 2009/10 36
X X 4 September 2009 CS5229, Semester 1, 2009/10 37
3rd dup ack:" pipe = cwnd - 3" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 + 3 4 September 2009 CS5229, Semester 1, 2009/10 38
subsequent dup ack:" cwnd++" pipe--" if pipe < cwnd" send packet, pipe++ 4 September 2009 CS5229, Semester 1, 2009/10 39
“partial” ack:" retransmit" cwnd = cwnd - n + 1" pipe -= 2" if pipe < cwnd" send packet, pipe++ 4 September 2009 CS5229, Semester 1, 2009/10 40
Idea of SACK:" Which packet has left the network?" Where is the gap?" Decouple when to send and what to send. 4 September 2009 CS5229, Semester 1, 2009/10 43
TCP SACK recovers faster than NewReno with multiple losses in a window. 4 September 2009 CS5229, Semester 1, 2009/10 44
Deployment 4 September 2009 CS5229, Semester 1, 2009/10 57
Feb 2004 NewReno Reno 8% Tahoe 15% 77% 4 September 2009 CS5229, Semester 1, 2009/10 58
70% " SACK capable 4 September 2009 CS5229, Semester 1, 2009/10 59
TCP BIC/CUBIC" Linux 2.6.x 4 September 2009 CS5229, Semester 1, 2009/10 63
Compound TCP " MS Windows Vista 4 September 2009 CS5229, Semester 1, 2009/10 64
TFRC" equation-based" congestion control 4 September 2009 CS5229, Semester 1, 2009/10 65
Not everyone uses TCP 4 September 2009 CS5229, Semester 1, 2009/10 66
UDP:" Media streaming" Gaming" VoIP 4 September 2009 CS5229, Semester 1, 2009/10 67
Why not congestion controlled?" 1. UDP has low delay, no need full reliability. UDP " is not congestion controlled. 4 September 2009 CS5229, Semester 1, 2009/10 68
Why not congestion controlled?" 2. No incentive. OTOH, there are incentives NOT to use congestion control. 4 September 2009 CS5229, Semester 1, 2009/10 69
Sally Floyd, h+p://www.icir.org/floyd/tcp_unfriendly.html  4 September 2009 CS5229, Semester 1, 2009/10 70
“Unresponsive Flows” 4 September 2009 CS5229, Semester 1, 2009/10 71
Bad: lead to unfairness and congestion collapse. 4 September 2009 CS5229, Semester 1, 2009/10 72
Unfairness: unresponsive flows consume more bandwidth than congestion controlled flows. 4 September 2009 CS5229, Semester 1, 2009/10 73
Congestion Collapse: wasting bandwidth by sending packets that will be dropped 4 September 2009 CS5229, Semester 1, 2009/10 77
Today: " a TCP-friendly" unreliable protocol 4 September 2009 CS5229, Semester 1, 2009/10 78
Idea: " send at a rate that a TCP flow would send 4 September 2009 CS5229, Semester 1, 2009/10 79
we can do the AIMD-thing" at the source, or 4 September 2009 CS5229, Semester 1, 2009/10 80
4 September 2009 CS5229, Semester 1, 2009/10 81
equation-based " congestion control 4 September 2009 CS5229, Semester 1, 2009/10 82
steady" fair" responsive 4 September 2009 CS5229, Semester 1, 2009/10 83
how to determine" RTT" tRTO" p 4 September 2009 CS5229, Semester 1, 2009/10 84
p is not packet loss rate" but " loss event rate 4 September 2009 CS5229, Semester 1, 2009/10 85
4 September 2009 CS5229, Semester 1, 2009/10 86
4 September 2009 CS5229, Semester 1, 2009/10 87
4 September 2009 CS5229, Semester 1, 2009/10 88
RTT can fluctuates 4 September 2009 CS5229, Semester 1, 2009/10 89
4 September 2009 CS5229, Semester 1, 2009/10 90
4 September 2009 CS5229, Semester 1, 2009/10 91
4 September 2009 CS5229, Semester 1, 2009/10 92
4 September 2009 CS5229, Semester 1, 2009/10 93
how to initialize? 4 September 2009 CS5229, Semester 1, 2009/10 94
slow start" (until loss occur)" Tnow = min(2Tprev, 2Trecv) 4 September 2009 CS5229, Semester 1, 2009/10 95
no loss history, how?" solve p given T, RTT 4 September 2009 CS5229, Semester 1, 2009/10 96
4 September 2009 CS5229, Semester 1, 2009/10 97
TFRC is now part of DCCP 4 September 2009 CS5229, Semester 1, 2009/10 98

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Lecture 4: TCP and TFRC

  • 1. TCP" Reno, NewReno, SACK 4 September 2009 CS5229, Semester 1, 2009/10 1
  • 2. TCP Reno 4 September 2009 CS5229, Semester 1, 2009/10 2
  • 3. 14 cwnd ssthresh 12 10 8 6 4 2 0 4 September 2009 CS5229, Semester 1, 2009/10 3
  • 4. new ack:" if (cwnd < sstresh)" cwnd += 1" else" cwnd += 1/cwnd 4 September 2009 CS5229, Semester 1, 2009/10 5
  • 5. timeout:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = 1" 4 September 2009 CS5229, Semester 1, 2009/10 6
  • 6. 3rd duplicate ACK:" fast retransmission" (ie, retransmit 1st unack) " fast recovery" (details today)" ssthresh = cwnd = cwnd/2 4 September 2009 CS5229, Semester 1, 2009/10 7
  • 7. TCP’s rule" send more packets if " L + cwnd > H" [ L .. H-1 ] are outstanding packets 4 September 2009 CS5229, Semester 1, 2009/10 10
  • 8. 4 September 2009 CS5229, Semester 1, 2009/10 11
  • 9. Data Packets ACK Packets (received up to this seq number) Congestion Window Sequence Number Time 4 September 2009 CS5229, Semester 1, 2009/10 12
  • 10. 3rd dup ack:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 4 September 2009 CS5229, Semester 1, 2009/10 13
  • 11. Incorrectly clamping the congestion window too soon (without fast recovery) X 4 September 2009 CS5229, Semester 1, 2009/10 14
  • 12. fast recovery:" keep the pipe occupied" 4 September 2009 CS5229, Semester 1, 2009/10 15
  • 13. X 4 September 2009 CS5229, Semester 1, 2009/10 16
  • 14. 4 September 2009 CS5229, Semester 1, 2009/10 17
  • 15. X X 4 September 2009 CS5229, Semester 1, 2009/10 18
  • 16. TCP Reno timeout with multiple losses in a window 4 September 2009 CS5229, Semester 1, 2009/10 19
  • 17. TCP NewReno 4 September 2009 CS5229, Semester 1, 2009/10 20
  • 18. X X 4 September 2009 CS5229, Semester 1, 2009/10 21
  • 19. 4 September 2009 CS5229, Semester 1, 2009/10 22
  • 20. 3rd dup ack:" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 + 3" remember highest 4 September 2009 CS5229, Semester 1, 2009/10 25
  • 21. “complete” ack:" (all are acked) " cwnd = ssthresh 4 September 2009 CS5229, Semester 1, 2009/10 26
  • 22. “partial” ack:" (acknowledge n packets)" retransmit" cwnd = cwnd - n + 1 4 September 2009 CS5229, Semester 1, 2009/10 27
  • 23. Note: RFC2581/RFC2582 give the accurate/gory details. Simplified version is presented here (eg. cwnd vs FlightSize, update of cwnd upon partial ACK). 4 September 2009 CS5229, Semester 1, 2009/10 29
  • 24. What does a dup ACK tell us? 4 September 2009 CS5229, Semester 1, 2009/10 30
  • 25. “Coarse Feedback” 4 September 2009 CS5229, Semester 1, 2009/10 31
  • 26. TCP SACK 4 September 2009 CS5229, Semester 1, 2009/10 32
  • 27. Use TCP header options to report received segments. 4 September 2009 CS5229, Semester 1, 2009/10 33
  • 28. SACK Blocks:" 1st block - report most recently received segments" subsequent blocks - repeat most recent previous blocks 4 September 2009 CS5229, Semester 1, 2009/10 34
  • 29. pipe: num of outstanding packets in the pipe." send only if pipe < cwnd 4 September 2009 CS5229, Semester 1, 2009/10 35
  • 30. scoreboard: which packets have been received? 4 September 2009 CS5229, Semester 1, 2009/10 36
  • 31. X X 4 September 2009 CS5229, Semester 1, 2009/10 37
  • 32. 3rd dup ack:" pipe = cwnd - 3" retransmit 1st unacked" ssthresh = cwnd/2" cwnd = cwnd/2 + 3 4 September 2009 CS5229, Semester 1, 2009/10 38
  • 33. subsequent dup ack:" cwnd++" pipe--" if pipe < cwnd" send packet, pipe++ 4 September 2009 CS5229, Semester 1, 2009/10 39
  • 34. “partial” ack:" retransmit" cwnd = cwnd - n + 1" pipe -= 2" if pipe < cwnd" send packet, pipe++ 4 September 2009 CS5229, Semester 1, 2009/10 40
  • 35. Idea of SACK:" Which packet has left the network?" Where is the gap?" Decouple when to send and what to send. 4 September 2009 CS5229, Semester 1, 2009/10 43
  • 36. TCP SACK recovers faster than NewReno with multiple losses in a window. 4 September 2009 CS5229, Semester 1, 2009/10 44
  • 37. Deployment 4 September 2009 CS5229, Semester 1, 2009/10 57
  • 38. Feb 2004 NewReno Reno 8% Tahoe 15% 77% 4 September 2009 CS5229, Semester 1, 2009/10 58
  • 39. 70% " SACK capable 4 September 2009 CS5229, Semester 1, 2009/10 59
  • 40. TCP BIC/CUBIC" Linux 2.6.x 4 September 2009 CS5229, Semester 1, 2009/10 63
  • 41. Compound TCP " MS Windows Vista 4 September 2009 CS5229, Semester 1, 2009/10 64
  • 42. TFRC" equation-based" congestion control 4 September 2009 CS5229, Semester 1, 2009/10 65
  • 43. Not everyone uses TCP 4 September 2009 CS5229, Semester 1, 2009/10 66
  • 44. UDP:" Media streaming" Gaming" VoIP 4 September 2009 CS5229, Semester 1, 2009/10 67
  • 45. Why not congestion controlled?" 1. UDP has low delay, no need full reliability. UDP " is not congestion controlled. 4 September 2009 CS5229, Semester 1, 2009/10 68
  • 46. Why not congestion controlled?" 2. No incentive. OTOH, there are incentives NOT to use congestion control. 4 September 2009 CS5229, Semester 1, 2009/10 69
  • 48. “Unresponsive Flows” 4 September 2009 CS5229, Semester 1, 2009/10 71
  • 49. Bad: lead to unfairness and congestion collapse. 4 September 2009 CS5229, Semester 1, 2009/10 72
  • 50. Unfairness: unresponsive flows consume more bandwidth than congestion controlled flows. 4 September 2009 CS5229, Semester 1, 2009/10 73
  • 51. Congestion Collapse: wasting bandwidth by sending packets that will be dropped 4 September 2009 CS5229, Semester 1, 2009/10 77
  • 52. Today: " a TCP-friendly" unreliable protocol 4 September 2009 CS5229, Semester 1, 2009/10 78
  • 53. Idea: " send at a rate that a TCP flow would send 4 September 2009 CS5229, Semester 1, 2009/10 79
  • 54. we can do the AIMD-thing" at the source, or 4 September 2009 CS5229, Semester 1, 2009/10 80
  • 55. 4 September 2009 CS5229, Semester 1, 2009/10 81
  • 56. equation-based " congestion control 4 September 2009 CS5229, Semester 1, 2009/10 82
  • 57. steady" fair" responsive 4 September 2009 CS5229, Semester 1, 2009/10 83
  • 58. how to determine" RTT" tRTO" p 4 September 2009 CS5229, Semester 1, 2009/10 84
  • 59. p is not packet loss rate" but " loss event rate 4 September 2009 CS5229, Semester 1, 2009/10 85
  • 60. 4 September 2009 CS5229, Semester 1, 2009/10 86
  • 61. 4 September 2009 CS5229, Semester 1, 2009/10 87
  • 62. 4 September 2009 CS5229, Semester 1, 2009/10 88
  • 63. RTT can fluctuates 4 September 2009 CS5229, Semester 1, 2009/10 89
  • 64. 4 September 2009 CS5229, Semester 1, 2009/10 90
  • 65. 4 September 2009 CS5229, Semester 1, 2009/10 91
  • 66. 4 September 2009 CS5229, Semester 1, 2009/10 92
  • 67. 4 September 2009 CS5229, Semester 1, 2009/10 93
  • 68. how to initialize? 4 September 2009 CS5229, Semester 1, 2009/10 94
  • 69. slow start" (until loss occur)" Tnow = min(2Tprev, 2Trecv) 4 September 2009 CS5229, Semester 1, 2009/10 95
  • 70. no loss history, how?" solve p given T, RTT 4 September 2009 CS5229, Semester 1, 2009/10 96
  • 71. 4 September 2009 CS5229, Semester 1, 2009/10 97
  • 72. TFRC is now part of DCCP 4 September 2009 CS5229, Semester 1, 2009/10 98