Download - When TCP Friendliness Becomes Harmful Amit Mondal Aleksandar Kuzmanovic Northwestern University
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
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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?