A TCP With Guaranteed Performance in Networks with Dynamic Congestion and Random

Wireless Losses

Stefan Schmid, ETH Zurich

Roger Wattenhofer, ETH Zurich

2nd Annual International Wireless Internet Conference (WICON)

Boston, MA, USA, August 2006

DistributedComputing

Group

Stefan Schmid, ETH Zurich @ WICON 2006 2

Large Data Transfers (1)

CERN, Geneva

ETH, Zurich

Stefan Schmid, ETH Zurich @ WICON 2006 3

Large Data Transfers (2)

CERN, GenevaETH, Zurich

TCP Connection

Lecture

E = mc2

Stefan Schmid, ETH Zurich @ WICON 2006 4

Large Data Transfers (3)

• Characteristics of transfer:- Internet can be congested- Available bandwidth changes over time- Packets may be lost, especially on wireless link

CongestionLosses

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TCP Congestion Control (1)

• TCP avoids congestion- Congestion control lies at the heart of TCP- Prevents congestion collapses of Internet (e.g., 1980)

• How to prevent congestion?

• Senders reduce sending rate when Internet is congested- senders maintain congestion window

- strategy: „additive-increase, multiplicative-decrease“ (AIMD)

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TCP Congestion Control (2)

• How does a sender know about congestion?

• When packets are lost, TCP sender assumes that routers are overloaded!

For packets lost for other reasons than congestion,

throughput is reduced unnecessarily!

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TCP Congestion Control (3)Lecture

E = mc2

Wasted throughput: student may seek to increase her download bandwidth (selfishly)!

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In this paper…

• A model is presented which comprises both dynamic changes of the available bandwidth (dynamic congestion) and random packet losses (e.g., wireless links)

• Model allows for formal analysis of transfer protocol‘s performance!

• Thereby, a selfish perspective is assumed- We look at protocols which aim at maximizing their throughput, regardless of consequences for other participants (no fairness).

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Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 10

Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 11

Basic Model (1)

• Time is divided into synchronous rounds

• Framework of online algorithms:

- Adversary chooses available bandwidth ut

- Protocol chooses sending rate xt

• In addition, all packets are lost in a given round with probability p

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Basic Model (2)

• Gain of transfer protocol ALG at time t:

• Gain of optimal (offline) transfer strategy OPT:

• We are interested in minimizing the (strict) competitive ratio, i.e., the gain of OPT divided by the gain of ALG.

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Basic Model (3)

• Takes into account an opportunity cost

• Assumption: No packets are transmitted at all if rate too large

- Pessimistic, but losses engender overhead (e.g., time-outs)

Goal of ALG is to always send at (or slightly lower) rate

of currently available bandwidth. Thereby, ALG does

not know whether losses are due to

congestion or wireless links!

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• In practice, it can be assumed that congestion does not change too abruptly over time.

• Therefore, we bound the adversary ADV which chooses the available bandwidth as follows (multiplicative changes):

ut has to be chosen from [ut-1/μ, ut-1μ]

Basic Model (4)

• A similar model but without random losses has been studied by Karp, Koutsoupias, Papadimitriou and Shenker (FOCS 2000)!

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• Transfer protocol achieving a provable performance:

Provable Performance in Basic Model

• In order to compensate wireless losses, TCPW increases the bandwidth by a factor larger than μ after successful rounds (aggressive MIMD strategy).

• Strict competitive ratio: at most 4(μ2+ μ)

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Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 17

Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 18

• Network traffic is often bursty.

• Network calculus has introduced the notion of leaky-bucket arrival curves to study queuing theory from a worst-case perspective.

• Also a reasonable model for dynamics on transport layer!

Extending the Model with Bursts

leaky bucket arrival curve

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• Dynamics of ADV has to correspond to leaky-bucket constraints

New Dynamic Adversary

• where

Adversary can accumulate power in some rounds to change available bandwidth more abruptly later!

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Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 21

Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

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The TCP Wichita Transfer Protocol for Bursty Environment

• After successful transmissions, rate is increased by a factor of 2μ2.

• xt>ut: Transmission rate compared to OPT can be described by Markov chain.

- Only in case of random errors, TCPW loses ground / reduces too much!

• Analysis: Look at cases x>u (fail) and x<u (pot. success) individually.

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Analysis

• In rounds where xt>ut, TCPW has gain = 0!

- But: TCPW does not miss much gain! - TCPW reduces its rate gemoetrically- TCPW never overshoots much

TCP Wichita is 4(μ+Bμ2)-competitive against a bursty adversary if μ<(1-p)/4Bp.

• The following result can be shown:

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Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 25

Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 26

Simulation

• Random bandwidth changes with bursts

- Random changes smaller than μ, until enough is accumulated for burst

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Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 28

Talk Overview

• Basic Model

• Extending the Model to Incorporate Bursts

• TCP Wichita and Analysis

• Simulation

• Conclusion

Stefan Schmid, ETH Zurich @ WICON 2006 29

Conclusion (1)

• A framework which allows for formal protocol analysis and incorporates dynamic congestion and random losses

• We believe that there is still little algorithmic research on the transport layer!

• Network calculus may be a good model for dynamics in various settings!

• Selfish throughput maximization- Really a threat? Experiences?

- Security: Routers often drop UDP packets first in case

of congestion! Cheating possible?

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Conclusion (2)

• Open research questions:

- Better / tight bound for competitive ratio?

- Randomized online algorithms?

- Impact on stability in case of multiple flows?

- Model extensions: buffers, varying round trip times, etc.?

- Model verification & real implementation?

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Questions and Comments?

Stefan SchmidDistributed Computing Group

ETH Zurich, Switzerland

schmiste@ethz.ch

http://dcg.ethz.ch/members/stefan.html

Thank you for your attention!