1. Network Technologies essentials Week 8: TCP congestion control Compilation made by Tim Moors, UNSW Australia Original slides by David Wetherall, University of Washington ISBN-10: 0133766624 © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011 Tim added green graffiti Note that videos are numbered 7-*

2. Fast Retransmit (2) Computer Networks2 Ack 10 Ack 11 Ack 12 Ack 13... Ack 13 Data 14... Ack 13 Ack 20... Data 20 Third duplicate ACK, so send 14 Retransmission fills in the hole at 14 ACK jumps after loss is repaired... Data 14 was lost earlier, but got 15 to 20 7-7.6

3. Effects of Congestion (2) What happens to performance as we increase the load? Computer Networks3 7-1.8

4. Computer Networks4 Efficiency vs. Fairness (3) If we care about efficiency: – Maximize total traffic in network – A  B: 1 unit, B  C: 1, and A  C, 0 – Total traffic rises to 2 units! A B C 1 1 7-2.6

5. Adapting over Time (2) Computer Networks5 Flow 1 slows when Flow 2 starts Flow 1 speeds up when Flow 2 stops Time Flow 3 limit is elsewhere 7-2.17

6. Computer Networks6 AIMD Game (3) AI and MD move the allocation Host 1 Host 2 01 1 Fair, y=x Efficient, x+y=1 Optimal Allocation Congested Multiplicative Decrease Additive Increase 7-3.9

7. Computer Networks7 AIMD Game (5) Always converge to good allocation! Host 1 Host 2 01 1 Fair Efficient Congested A starting point 7-3.11

8. Benefit of ACK Clocking (2) Segments are buffered and spread out on slow link Computer Networks8 Fast link Slow (bottleneck) link Segments “spread out” 7-5.5

9. Additive Increase Timeline Computer Networks9 Increment cwnd by 1 packet every cwnd ACKs (or 1 RTT) 7-6.9

10. Computer Networks10 Slow-Start Solution Start by doubling cwnd every RTT – Exponential growth (1, 2, 4, 8, 16, …) – Start slow, quickly reach large values AI Fixed Time Window (cwnd) Slow-start 7-6.5

11. Slow-Start (Doubling) Timeline Computer Networks11 Increment cwnd by 1 packet for each ACK 7-6.8

12. Fast Recovery (2) Computer Networks12 Ack 12 Ack 13 Data 14 Ack 13 Ack 20... Data 20 Third duplicate ACK, so send 14 Data 14 was lost earlier, but got 15 to 20 Retransmission fills in the hole at 14 Set ssthresh, cwnd = cwnd/2 Data 21 Data 22 More ACKs advance window; may send segments before jump Ack 13 Exit Fast Recovery 7-7.10

13. TCP Reno Computer Networks13 MD of ½, no slow-start ACK clock running TCP sawtooth 7-7.12

14. ECN (Explicit Congestion Notification) Router detects the onset of congestion via its queue – When congested, it marks affected packets (IP header) Computer Networks14 7-8.5

15. END © 2013 D. Wetherall Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Computer Networks15