When TCP Friendliness Becomes Harmful

Amit Mondal

Aleksandar Kuzmanovic

Northwestern University

http://networks.cs.northwestern.edu/

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Introduction

Internet was designed for throughput– TCP probes for available b/w even if it causes

packet losses– Interactive applications suffer!

Telnet, ssh, games, chat all use TCP

Telnet delay from typed character until echo– Includes transmission, propagation and

queuing delay– It loss, TCP retransmits

3 A. Mondal When TCP Friendliness Becomes Harmful

data packets

“dummy”packets

strict priority

TCP-fair rate

Upgrading mice to elephants

Padding misbehavior

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Source of the problem

How TCP detects packet loss?– Retransmission Timeout (RTO) (1 second)– Triple duplicate acknowledgment/Fast

retransmit (~ 10-100 ms)

How does it affect interactive applications?– Burst size (~ 1-2 packets)– Inter-burst time (~ seconds)

Packet loss is detected by timeout, which degrades the response time in orders of

magnitude

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A deeper look

RTO

improvement

A B C D

Incentive for misbehavior!

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Related Work

Packet marking & differential dropping[Guo and Matta ‘01]

Service differentiation[Noureddine and Tobagi ‘02]

Differential Congestion Notification[ Le et al. ’04]

Explicit congestion notification[Floyd ‘94]

TCP smart framing[Mellia et al. ‘05]

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Implication

Packet switched => Circuit switched

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Quantifying padding induced-gain

RTT

RTO

2RTO

RTO

2RTO

4RTO

1-p

p

p

p

1-Q

p

p

1-p

Q

1-p

1-p

1-p

TCP backlogged-fully for time response ExpectedTCP eInteractiv for time response Expected

Gain

Correlated packet loss (FIFO)

Random packet loss (RED)

P = packet loss ratioQ = prob. packet loss is detected

timeout

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Modeling

RED FIFO

TCP backlogged-fullyfor timeresponse Expected

TCP eInteractivfor timeresponse Expected Gain

Upgrading interactive TCP flows to fully

backlogged flows always pays-off

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Simulations

1) Queuing delay plays a role2) Consecutive packet losses can affect overall gain achieved

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Outline

Source of the problem

Related work

Implication

Padding-induced response time gain

Sustainable countermeasures

Conclusion

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Sustainable countermeasures (1)

Differentiated minRTO– Application-limited flows use reduced value for

minRTO parameter

Short-term padding with dummy packets – Enable that a packet loss is detected via fast

retransmit mechanism– Actual packet followed by three tiny dummy packets.

A diversity approach – TCP sends k (k>1, k is a small integer) copies of the

packet without violating congestion control mechanism

– In reality k=2 is sufficient

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Sustainable countermeasures (2)

Short-term padding and diversity approaches outperform fully-backlogged approach

Differentiated minRTO does not work

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Sustainable countermeasures (3)

RED FIFO

Short-term padding and diversity approaches outperform fully-backlogged approach

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Overhead and Sustainability

RED FIFO

Short-term padding and diversity approaches provide significantly friendlier environment

Issue: What happens when everyone adoptsour approach?

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Conclusion

TCP’s loss recovery mechanism degrades response time of interactive applications relative to long flows during congestion.

Incentive for padding misbehaviour and seriously degrades overall networks performance.

Explored simple, sustainable and easily deployable solutions.

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Questions?