WebTP: Protocol Design Issues

Jeng Lung & Yogesh Bhumralkar

Introduction

• Following key issues related to the design and testing of the protocol:– Congestion Window Control– Retransmission Timeout Scheme– Performance under Network Jitter

Congestion Control

• Background– WebTP uses TCP-style congestion control– 2 Phases:

• Slow Start

• Congestion Avoidance: Additive Increase/Multiplicative Decrease

– TCP biased against long connections; therefore, WebTP faces same dilemma.

Improving Fairness

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bFairness

• Maximize fairness by modifying Additive Increase

• Instead of increasing cwnd by 1/cwnd, increase it by K=c*rtt*rtt. This makes it more fair

• Problem: Find optimal K for fairness

• K is topology dependent but still want to ensure that the scheme works for WebTP.

Network Topology

2 Node Cwnd Fairness Study (TCP & WebTP Same RTT)

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4 Node Cwnd Fairness Study (WebTP has 10 times the RTT of TCP)

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Retransmission Scheme

• Current timeout set to:

• M is the receiver’s tolerance to jitter.

• Examine the effects of changing M on the number of dropped and duplicate packets.

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M vs. Drop/Dup Packets

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Jitter Study

• Any network has considerable random delays.

• The time between when the packet is transmitted to when it reaches the receiver varies a great deal - this phenomenon is called jitter.

• Simulated jitter by introducing random delay to each packet on the sender side.

WebTP with no jitter

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WebTP with [0, 3ms] jitter

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WebTP with no jitter

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WebTP with [0, 3ms] jitter

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Conclusion

• Congestion Window Scheme: Optimal K depends on network topology.

• Retransmission Scheme: Dropped packets increase with M whereas the number of duplicate packets goes down at higher M.

• Network Jitter: Higher jitter implies that a higher M is required to handle the delays that are introduced.