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2004.5.18.

Last Class

This ClassChapter 6.3. ~ 6.4.TCP congestion control

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When congestion happens …

ssthresh = ½ of cwnd In case of TCP Tahoe, cwnd = 1

When timeout happened/triple ACKs arrivedStart from minimum sending rate (slow start)

In case of TCP Reno, cwnd = ssthreshWhen triple duplicate ACKs arriveSending rate decreased only by half (fast recovery)

When timeout happened, same as in Tahoe.

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Problems with TCP Reno

Triple ACKs often fails to be triggered due to either Losses in burst Small sending window

Timeout needs unnecessarily long delay. Congestion control in Reno

Need to create packet losses to find the available bandwidth of the connection

Continually congesting the network Creating losses for other connections sharing the link. Oscillations

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TCP Vegas

IdeasDetect congestion before losses occur

•Compare RTT between two ACKs•If delay has increased, retransmit before a third ACK

Lower the rate when this imminent packet loss is detected•Estimate the thruput by cwnd and RTT•Increase/decrease cwnd by 1

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How would you compare Tahoe, Reno, and Vegas?

Which town offers best skiing? Highest probability to win a jackpot? All employ AIMD

Distributed – no coordination between connections needed?

Efficient – desired load level close to total?Fairness – connections split the share equally?

Responsive to and smooth in equilibrium?

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Congestion Avoidance

Instead of reacting to congestion, try to predict when congestion is about to happen and reduce sending rates

DECbit router sets congestion bit to notify users of impending congestion

Random Early Detection Source-based Congestion Avoidance

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Random Early Drop (RED)

Main idea Instead of waiting for the queue to get full to start dropping, it drops arriving packets with some drop probability whenver queue length exceeds some threshold.

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MaxT MinT

MinT MaxT

Drop Probability

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MaxP

AvgQLen

Drop allDrop with

some p

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AvgQLen = (1-α) AvgQLen + α SampleLen

Queue Length

Time

Average

Instantaneous

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Drop Probability of RED

TempP = MaxP ·(AvgQLen – MinT)/(MaxT-MinT)

P = TempP/(1-count · TempP)

count = # of pkts not dropped while MinT < AvgQLen < MaxT

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Source-Based Congestion Avoidance

Key ideaFigure out that some router’s queue is building up

Monitor RTT for increase

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Source-Based Congestion Avoidance Mechanisms

1. If CurrRTT > (minRTT+maxRTT)/2 Then decrease cwnd by 1/8

2. If (CurrW – OldW) x (CurrRTT – OldRTT) > 0 Then decrease cwnd by 1/8 Else increase cwnd by 1/8

3. Per RTT, increase/decrease cwnd by 1 pkt Compare thruput with previous one

4. TCP Vegas

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When congestion happens …

ssthresh = ½ of cwnd In case of TCP Tahoe, cwnd = 1

When timeout happened/triple ACKs arrivedStart from minimum sending rate (slow start)

In case of TCP Reno, cwnd = ssthreshWhen triple duplicate ACKs arriveSending rate decreased only by half (fast recovery)

When timeout happened, same as in Tahoe.

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Problems with TCP Reno

Triple ACKs often fails to be triggered due to either Losses in burst Small sending window

Timeout needs unnecessarily long delay. Congestion control in Reno

Need to create packet losses to find the available bandwidth of the connection

Continually congesting the network Creating losses for other connections sharing the link. Oscillations

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TCP Vegas

IdeasDetect congestion before losses occur

•Compare RTT between two ACKs•If delay has increased, retransmit before a third ACK

Lower the rate when this imminent packet loss is detected•Estimate the thruput by cwnd and RTT•Increase/decrease cwnd by 1

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Quality of Service

Real-time applicationsneed more than best-effort require some form of QoS guarantee intolerant of loss/delay

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VoIP Timing Charts

Sue speaks

encoded and packetized

at receiver

played out

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QoS Support Approaches

IntServ’s RSVP DiffServ

EF/AF ATM

CBRVBR-rt, VBR-nrtABRUBR

Equation-based congestion control

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Next Class

HW #8 due End-to-End data (Chap. 7)