Network Technologies essentials

Week 8: TCP congestion control

Compilation made by Tim Moors, UNSW AustraliaOriginal slides by David Wetherall, University of WashingtonISBN-10: 0133766624 © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5th ed, Prentice-Hall, 2011

Tim added green graffiti

Note that videos are numbered 7-*

Computer Networks 2

Fast Retransmit (2)Ack 10Ack 11Ack 12Ack 13

. . .

Ack 13

Ack 13Ack 13

Data 14. . . Ack 13

Ack 20. . . . . .

Data 20Third duplicate ACK, so send 14 Retransmission fills

in the hole at 14ACK jumps after loss is repaired

. . . . . .

Data 14 was lost earlier, but

got 15 to 20

7-7.6

Computer Networks 3

Effects of Congestion (2)• What happens to performance as we increase the load?

7-1.8

Computer Networks 4

Efficiency vs. Fairness (3)• If we care about efficiency:– Maximize total traffic in network– AB: 1 unit, BC: 1, and AC, 0 – Total traffic rises to 2 units!

A B C1 1

7-2.6

Computer Networks 5

Adapting over Time (2)

Flow 1 slows when Flow 2 starts

Flow 1 speeds up when Flow 2 stops

Time

Flow 3 limit is elsewhere

7-2.17

Computer Networks 6

AIMD Game (3)• AI and MD move the allocation

Host 1

Host 20 1

1

Fair, y=x

Efficient, x+y=1

OptimalAllocation

Congested

MultiplicativeDecrease

AdditiveIncrease

7-3.9

Computer Networks 7

AIMD Game (5)• Always converge to good allocation!

Host 1

Host 20 1

1

Fair

Efficient

Congested

A starting point

7-3.11

Computer Networks 8

Benefit of ACK Clocking (2)• Segments are buffered and spread out on slow link

Fast link Fast linkSlow (bottleneck) link

Segments “spread out”

7-5.5

Computer Networks 9

Additive Increase Timeline

Increment cwnd by 1 packet every cwnd ACKs (or 1 RTT)

7-6.9

Computer Networks 10

Slow-Start Solution• Start by doubling cwnd every RTT– Exponential growth (1, 2, 4, 8, 16, …)– Start slow, quickly reach large values

AI

Fixed

TimeWin

dow

(cw

nd)

Slow-start

7-6.5

Computer Networks 11

Slow-Start (Doubling) Timeline

Increment cwnd by 1 packet for each ACK

7-6.8

Computer Networks 12

Fast Recovery (2)

Ack 12Ack 13Ack 13

Ack 13Ack 13

Data 14Ack 13

Ack 20. . . . . .

Data 20Third 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 21Data 22

More ACKs advance window; may send

segments before jump

Ack 13

Exit Fast Recovery

7-7.10

Computer Networks 13

TCP Reno

MD of ½ , no slow-start

ACK clock running

TCP sawtooth

7-7.12

Computer Networks 14

ECN (Explicit Congestion Notification)• Router detects the onset of congestion via its queue– When congested, it marks affected packets (IP header)

7-8.5

Computer Networks 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