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 Conferenc Boston, MA, USA, Aug Distributed Computing Group
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A TCP With Guaranteed Performance in Networks with Dynamic Congestion and Random Wireless Losses
A TCP With Guaranteed Performance in Networks with Dynamic Congestion and Random Wireless Losses. Stefan Schmid, ETH Zurich Roger Wattenhofer, ETH Zurich. D istributed C omputing G roup. 2nd Annual International Wireless Internet Conference (WICON) Boston, MA, USA, August 2006. - PowerPoint PPT Presentation
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A TCP With Guaranteed Performance in Networks with Dynamic Congestion and Random
Wireless Losses
Stefan Schmid, ETH ZurichRoger 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
Stefan Schmid, ETH Zurich @ WICON 2006 5
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)
Stefan Schmid, ETH Zurich @ WICON 2006 6
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!
Stefan Schmid, ETH Zurich @ WICON 2006 7
TCP Congestion Control (3)Lecture
E = mc2
Wasted throughput: student may seek to increase her download bandwidth (selfishly)!
Stefan Schmid, ETH Zurich @ WICON 2006 8
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).
Stefan Schmid, ETH Zurich @ WICON 2006 9
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)
Stefan Schmid, ETH Zurich @ WICON 2006 16
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
Stefan Schmid, ETH Zurich @ WICON 2006 27
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.?